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Writing a shell script is like riding a bike. You fall off and scrape your knees a lot at first. With a bit more experience, you become comfortable riding them around town, but also quickly discover why most people drive cars for longer trips.

1. Bicyclism: Art Of Riding Mac Os Catalina
2. Bicyclism: Art Of Riding Mac Os X
3. Bicyclism: Art Of Riding Mac Os 11

Shell scripting is generally considered to be a glue language, ideal for creating small pieces of code that connect other tools together. While shell scripts can be used for more complex tasks, they are usually not the best choice.

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If you have ever successfully trued a bicycle wheel (or paid someone else to do so), that's similar to learning the basics of shell scripting. If you don't true your scripts, they wobble. Put another way, it is often easy to write a script, but it can be more challenging to write a script that consistently works well.

This chapter and the next two chapters introduce the basic concepts of shell scripting. The remaining chapters in this document provide additional breadth and depth. This document is not intended to be a complete reference on writing shell scripts, nor could it be. It does, however, provide a good starting point for beginners first learning this black art.

Shell Script Dialects

There are many different dialects of shell scripts, each with their own quirks, and some with their own syntax entirely. Because of these differences, the road to good shell scripting can be fraught with peril, leading to script failures, misbehavior, and even outright data loss.

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To that end, the first lesson you must learn before writing a shell script is that there are two fundamentally different sets of shell script syntax: the Bourne shell syntax and the C shell syntax. The C shell syntax is more comfortable to many C programmers because the syntax is somewhat similar. However, the Bourne shell syntax is significantly more flexible and thus more widely used. For this reason, this document only covers the Bourne shell syntax.

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The second hard lesson you will invariably learn is that each dialect of Bourne shell syntax differs slightly. This document includes only pure Bourne shell syntax and a few BASH-specific extensions. Where BASH-specific syntax is used, it is clearly noted.

The terminology and subtle syntactic differences can be confusing—even a bit overwhelming at times; had Dorothy in The Wizard of Oz been a programmer, you might have heard them exclaim, 'BASH and ZSH and CSH, Oh My!' Fortunately, once you get the basics, things generally fall into place as long as you avoid using shell-specific features. Stay on the narrow road and your code will be portable.

Some common shells are listed below, grouped by script syntax:

Bourne-compatible shells

• sh

• bash

• zsh

• ksh

C-shell-compatible shells

• csh

• tcsh

• bcsh (C shell to Bourne shell translator/emulator) Duck life: adventure mac os.

Many of these shells have more than one variation. Most of these variations are denoted by prefixing the name of an existing shell with additional letters that are short for whatever differentiates them from the original shell. For example:

• The shell pdksh is a variant of ksh. Being a public domain rewrite of AT&T's ksh, it stands for 'Public Domain Korn SHell.' (This is a bit of a misnomer, as a few bits are under a BSD-like open source license. However, the name remains.)

• The shell tcsh is an extension of csh. It stands for the TENEX C SHell, as some of its enhancements were inspired by the TENEX operating system.

• The shell bash is an extension of sh. It stands for the Bourne Again SHell. (Oddly enough, it is not a variation of ash, the Almquist SHell, though both are Bourne shell variants. This should not be confused with the dash shell—an ash-derived shell used in some Linux distributions—whose name stands for the Debian Almquist SHell.)

And so on. In general, with the exception of csh and tcsh, it is usually safe to assume that any modern login shell is compatible with Bourne shell syntax.

Note: Because the C shell syntax is not well suited to scripting beyond a very basic level, this document does not cover C shell variants in depth. For more information, see About the C Shell.

About the C Shell

The C shell is popular among some users as a shell for interacting with the computer because it allows simple scripts to be written more easily. However, the C shell scripting language is limited in a number of ways, many of which are hard to work around. For this reason, use of the C shell scripting language for writing complex scripts is not recommended. For more information, read 'CSH Programming Considered Harmful' at http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/. Although many of the language flaws it describes are fixed by some modern C shells, if you are writing a script that must work on multiple computers across different operating systems, you cannot always guarantee that the installed C shell will support those extensions.

However, the C shell scripting language has its uses, particularly for writing scripts that set up environment variables for interactive shell environments, execute a handful of commands in order, or perform other relatively lightweight chores. To support such uses, the C shell syntax is presented alongside the Bourne shell syntax within this 'basics' chapter where possible.

Outside of this chapter, this document does not generally cover the C shell syntax. If after reading this, you still want to write a more complex script using the C shell programming language, you can find more information in on the C shell in the manual page for csh.

Shell Variables and Printing

What follows is a very basic shell script that prints 'Hello, world!' to the screen:

The first thing you should notice is that the script starts with ‘#!'. This is known as an interpreter line. If you don't specify an interpreter line, the default is usually the Bourne shell (/bin/sh). However, it is best to specify this line anyway for consistency.

The second thing you should notice is the echo command. The echo command is nearly universal in shell scripting as a means for printing something to the user's screen. (Technically speaking, echo is generally a shell builtin, but it also exists as as standalone command, /bin/echo. You can read more about the difference between the builtin version and the standalone version in echo and Use Shell Builtins Wherever Possible.)

If you'd like, you can try this script by saving those lines in a text file (say 'hello_world.sh') in your home directory. Then, in Terminal, type:

Of course, this script isn't particularly useful. It just prints the words 'Hello, world!' to your screen. To make this more interesting, the next script throws in a few variables.

Type or paste this script into the text editor of your choice (see Creating Text Files in Your Home Directory for help creating a text file) and save the file in your home directory in a file called test.sh.

Once you have saved the file in your home directory, type ‘chmod a+x test.sh' in Terminal to make it executable. Finally, run it with ‘./test.sh leaders'. You should see 'Hello, world leaders!' printed to your screen.

This script provides an example of a variable assignment. The variable $1 contains the first argument passed to the shell script. In this example, the script makes a copy and stores it into a variable called FIRST_ARGUMENT, then prints that variable.

You should immediately notice that variables may or may not begin with a dollar sign, depending on how you are using them. If you want to dereference a variable, you precede it with a dollar sign. The shell then inserts the contents of the variable at that point in the script. For all other uses, you do not precede it with a dollar sign.

Important: You generally do not want to prefix the variable on the left side of an assignment statement with a dollar sign. Because FIRST_ARGUMENT starts out empty, if you used a dollar sign, the first line:

You should also notice that the argument to echo is surrounded by double quotation marks. This is explained further in the next section, Using Arguments And Variables That Contain Spaces.

C Shell Note: The syntax for assignment statements in the C shell is rather different. Instead of an assignment statement, the C shell uses the set and setenv builtins to set variables as shown below:

Using Arguments And Variables That Contain Spaces

Take a second look at the script from the previous section:

Notice that the echo statement is followed by a string surrounded by quotation marks. Normally, the shell uses spaces to separate arguments to commands. Outside of quotation marks, the shell would treat 'Hello,' and 'world' as separate arguments to echo.

By surrounding the string with double quote marks, the shell treats the entire string as a single argument to echo even though it contains spaces.

To see how this works, save the script above as test.sh (if you haven't already), then type the following commands:

The first line above prints 'Hello, world leaders!' because the space after 'leaders' ends the first argument ($1). Inside the script, the variable $1 contains 'leaders', $2 contains 'and', and $3 contains 'citizens'.

The second line above prints 'Hello, world leaders and citizens!' because the quotation marks on the command line cause everything within them to be grouped as a single argument.

Notice also that there are similar quotation marks on the right side of the assignment statement:

With most modern shells, these double quotation marks are not required for this particular assignment statement (because there are no literal spaces on the right side), but they are a good idea for maximum compatibility. See Historical String Parsing in Historical Footnotes and Arcana to learn why.

When assigning literal strings (rather than variables containing strings) to a variable, however, you must surround any spaces with quotation marks. For example, the following statement does not do what you might initially suspect:

If you type this statement, the Bourne shell gives you an error like this:

The reason for this seemingly odd error is that the assignment statement ends at the first space, so the next word after that statement is interpreted as a command to execute. See Overriding Environment Variables for Child Processes (Bourne Shell) for more details.

Instead, write this statement as:

Using quotation marks is particularly important when working with variables that contain filenames or paths. For example, type the following commands:

The above example creates a directory in /tmp called 'My Folder'. (Don't worry about deleting it because /tmp gets wiped every time you reboot.) It then attempts to list the files in that directory. The first time, it uses quotation marks. The second time, it does not. Notice that the shell misinterprets the command the second time as being an attempt to list the files in /tmp/My and the files in Folder.

Handling Quotation Marks in Strings

In modern Bourne shells, expansion of variables, occurs after the statement itself is fully parsed by the shell. (See Historical String Parsing in Historical Footnotes and Arcana for more information.) Thus, as long as the variable is enclosed in double quote marks, you do not get any execution errors even if the variable's value contains double-quote marks.

However, if you are using double quote marks within a literal string, you must quote that string properly. For example:

C Shell Note: The C shell handling of backslashes within double-quoted strings is different. In the C shell, the previous example should be changed to:

This quoting technique also applies to literal strings within commands entered on the command line. For example, using the script from earlier in Shell Variables and Printing, the command:

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prints the phrase 'Hello, world 'leaders'!'

The details of quotes as they apply to variable expansion are explained in Parsing, Variable Expansion, and Quoting. (Variable safety with shells that predate this behavior is generally impractical. Fortunately, the modern behavior has been the norm since the mid-1990s.)

Shell scripts also allow the use of single quote marks. Variables between single quotes are not replaced by their contents. Be sure to use double quotes unless you are intentionally trying to display the actual name of the variable. You can also use single quotes as a way to avoid the shell interpreting the contents of the string in any way. These differences are described further in Parsing, Variable Expansion, and Quoting.

Exporting Shell Variables

One key feature of shell scripts is that variables are typically limited in their scope to the currently running script. The scoping of variables is described in more detail in Subroutines, Scoping, and Sourcing. For now, though, it suffices to say that variables generally do not get passed on to scripts or tools that they execute.

Normally, this is what you want. Most variables in a shell script do not have any meaning to the tools that they execute, and thus represent clutter and the potential for variable namespace collisions if they are exported. Occasionally, however, you will find it necessary to make a variable's value available to an outside tool. To do this, you must export the variable. These exported variables are commonly known as environment variables because they affect the execution of every script or tool that runs but are not part of those scripts or tools themselves.

A classic example of an environment variable that is significant to scripts and tools is the PATH variable. This variable specifies a list of locations that the shell searches when executing programs by name (without specifying a complete path). For example, when you type ls on the command line, the shell searches in the locations specified in PATH (in the order specified) until it finds an executable called ls (or runs out of locations, whichever comes first).

The details of exporting shell variables differ considerably between the Bourne shell and the C shell. Thus, the following sections explain these details in a shell-specific fashion.

Using the export Builtin (Bourne Shell)

Generally speaking, the first time you assign a value to an environment variable such as the PATH variable, the Bourne shell creates a new, local copy of this shell variable that is specific to your script. Any tool executed from your script is passed the original value of PATH inherited from whatever script, tool, or shell that launched it.

With the BASH shell, however, any variable inherited from the environment is automatically exported by the shell. Thus, in some versions of OS X, if you modify inherited environment variables (such as PATH) in a script, your local changes will be seen automatically by any tool or script that your script executes. Thus, in these versions of OS X, you do not have to explicitly use the export statement when modifying the PATH variable.

Because different Bourne shell variants handle these external environment variables differently (even among different versions of OS X), this creates two minor portability problems:

• A script written without the export statement may work on some versions of OS X, but will fail on others. You can solve this portability problem by using the export builtin, as described in this section.

• A shell script that changes variables such as PATH will alter the behavior of any script that it executes, which may or may not be desirable. You can solve this problem by overriding the PATH environment variable when you execute each individual tool, as described in Overriding Environment Variables for Child Processes (Bourne Shell).

To guarantee that your modifications to a shell variable are passed to any script or tool that your shell script calls, you must use the export builtin. You do not have to use this command every time you change the value; the variable remains exported until the shell script exits.

For example:

Either of these statements has the same effect—specifically, they export the local notion of the PATH environment variable to any command that your script executes from now on. There is a small catch, however. You cannot later undo this export to restore the original global declaration. Thus, if you need to retain the original value, you must store it somewhere yourself.

In the following example, the script stores the original value of the PATH environment variable, exports an altered version, executes a command, and restores the old version.

If you need to find out whether an environment variable (whether inherited by your script or explicitly set with the export directive) was set to empty or was never set in the first place, you can use the printenv command to obtain a complete list of defined variables and use grep to see if it is in the list. (You should note that although printenv is a csh builtin, it is also a standalone command in /usr/bin.)

For example:

The resulting variable will contain 1 if the variable is defined in the environment or 0 if it is not.

Overriding Environment Variables for Child Processes (Bourne Shell)

Because the BASH Bourne shell variant automatically exports all variables inherited from its environment, any changes you make to preexisting environment variables such as PATH are automatically inherited by any tool or script that your script executes. (This is not true for other Bourne shell variants; see Using the export Builtin (Bourne Shell) for further explanation.)

While automatic export is usually convenient, you may sometimes wish to change a preexisting environment variable without modifying the environment of any script or tool that your script executes. For example, if your script executes a number of tools in /usr/local/bin, it may be convenient to change the value of PATH to include /usr/local/bin. However, you may not want child processes to also look in /usr/local/bin.

This problem is easily solved by overriding the environment variable PATH on a per-execution basis. Consider the following script:

This script prints the value of the variable MYVAR. Normally, this variable is empty, so this script just prints a blank line. Save the script as printmyvar.sh, then type the following commands:

Notice that the assignment statement MYVAR=7 applies only to the command that follows it. The value of MYVAR is altered in the environment of the command ./printmyvar.sh, so the script prints the number 7. However, the original (empty) value is restored after executing that command, so the echo statement afterwards prints an empty string for the value of MYVAR.

Thus, to modify the PATH variable locally but execute a command with the original PATH value, you can write a script like this:

Using the setenv Builtin (C shell)

In the C shell, variables are exported if you set them with setenv, but not if you set them with set. Thus, if you want your shell variable modifications to be seen by any tool or script that you call, you should use the setenv builtin. This builtin is the C shell equivalent to issuing an assignment statement with the export https://pv-free.weebly.com/movie-database-1-2.html. builtin in the Bourne shell.

If you want your shell variables to only be available to your script, you should use the set builtin (described in Shell Variables and Printing). The set builtin is equivalent to a simple assignment statement in the Bourne shell.

Notice that the local variable version requires an equals sign (=), but the exported environment version does not (and produces an error if you put one in).

To remove variables in the C shell, you can use the unsetenv or unset builtin. For example:

This will generate an error message. In the C shell, it is not possible to print the value of an undefined variable, so if you think you may need to print the value later, you should set it to an empty string rather than using unset or unsetenv.

If you need to test an environment variable (not a shell-local variable) that may or may not be part of your environment (a variable set by whatever process called your script), you can use the printenv builtin. This prints the value of a variable if set, but prints nothing if the variable is not set, and thus behaves just like the variable behaves in the Bourne shell.

For example:

This prints X is ' if the variable is either empty or undefined. Otherwise, it prints the value of the variable between the quotation marks.

If you need to find out if a variable is simply empty or is actually not set, you can also use printenv to obtain a complete list of defined variables and use grep to see if it is in the list. For example:

The resulting variable will contain 1 if the variable is defined in the environment or 0 if it is not.

Overriding Environment Variables for Child Processes (C Shell)

Unlike the Bourne shell, the C shell does not provide a built-in syntax for overriding environment variables when executing external commands. However, it is possible to simulate this either by using the env command.

The best and simplest way to do this is with the env command. For example:

As an alternative, you can use the set builtin to make a temporary copy of any variable you need to override, change the value, execute the command, and restore the value from the temporary copy.

You should notice, however, that whether you use the env command or manually make a copy, the PATH variable is altered prior to searching for the command. Because the PATH variable controls where the shell looks for programs to execute, you must therefore explicitly provide a complete path to the ls command or it will not be found (unless you have a copy in /usr/local/bin, of course). The PATH environment variable is explained in Special Shell Variables.

As a workaround, you can determine the path of the executable using the which command prior to altering the PATH environment variable.

Or, using env:

The use of the backtick (`) operator in this fashion is described in Inline Execution.

Security Note:If your purpose for overriding an environment variable is to prevent disclosure of sensitive information to a potentially untrusted process, you should be aware that if you use setenv for the copy, the called process has access to that temporary copy just as it had access to the original variable. To avoid this, be sure to create the temporary copy using the set builtin instead of setenv.

Deleting Shell Variables

For the most part, in Bourne shell scripts, when you need to get rid of a variable, setting it to an empty string is sufficient. However, in long-running scripts that might encounter memory pressure, it can be marginally useful to delete the variable entirely. To do this, use the unset builtin.

For example:

The unset builtin can also be used to delete environment variables.

C Shell Note: The C shell unset builtin is identical except that it cannot be used to delete environment variables. Use unsetenv instead, as shown in Overriding Environment Variables for Child Processes (C Shell).

Also, in C shell, if you try to use a deleted variable, it is considered an error. (In Bourne shell, an unset variable is treated like an empty string.)

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Table of contents

• What is gamma and why do we need it?
• Effects of gamma-incorrectness
• References & further reading

A short quiz

If you have ever written, or are planning to write, any kind of code thatdeals with image processing, you should complete the below quiz. If you have answeredone or more questions with a yes, there's a high chance that your code isdoing the wrong thing and will produce incorrect results. This might not beimmediately obvious to you because these issues can be subtle and they'reeasier to spot in some problem domains than in others.

So here's the quiz:

• I don't know what gamma correction is (duh!)
• Gamma is a relic from the CRT display era; now that almost everyone usesLCDs, it's safe to ignore it.
• Gamma is only relevant for graphics professionals working in the printindustry where accurate colour reproduction is of greatimportance—for general image processing, it's safe to ignore it.
• I'm a game developer, I don't need to know about gamma.
• The graphics libraries of my operating system handle gamma correctly.¹
• The popular graphics library I'm using handles gamma correctly.
• Pixels with RGB values of (128, 128, 128) emit about half as much light aspixels with RGB values of (255, 255, 255).
• It is okay to just load pixel data from a popular image format (JPEG, PNG,GIF etc.) into a buffer using some random library and run image processingalgorithms on the raw data directly.

Bicyclism: Art Of Riding Mac Os X

Don't feel bad if you have answered most with a yes! I would have givena yes to most of these questions a week ago myself too. Somehow, the topicof gamma is just under most computer users' radar (including programmerswriting commercial graphics software!), to the extent that most graphicslibraries, image viewers, photo editors and drawing software of today stilldon't get gamma right and produce incorrect results.

So keep on reading, and by the end of this article you'll be moreknowledgeable about gamma than the vast majority of programmers!

The arcane art of gamma-correctness

Given that vision is arguably the most important sensory input channel forhuman-computer interaction, it is quite surprising that gamma correction isone of the least talked about subjects among programmers and it's mentioned intechnical literature rather infrequently, including computer graphics texts.The fact that most computer graphics textbooks don't explicitly mention theimportance of correct gamma handling, or discuss it in practical terms, doesnot help matters at all (my CG textbook fromuni falls squarely into thiscategory, I've just checked). Some books mention gamma correction in passingin somewhat vague and abstract terms, but then provide neither concretereal-world examples on how to do it properly, nor explain what theimplications of not doing it properly are, nor show image examples ofincorrect gamma handling.

I came across the need for correct gamma handling during writing my raytracer and I had to admit that my understanding of thetopic was rather superficial and incomplete. So I had spent a few days readingup on it online, but it turned out that many articles about gamma are not muchhelp either, as many of them are too abstract and confusing, some contain toomany interesting but otherwise irrelevant details, and then some others lackimage examples or are just simply incorrect or hard to understand. Gamma isnot a terribly difficult concept to begin with, but for some mysterious reasonit's not that trivial to find articles on it that are correct, complete andexplain the topic in a clear language. Theia mac os.

What is gamma and why do we need it?

Alright, so this is my attempt to offer a comprehensive explanation of gamma,focusing just on the most important aspects and assuming no prior knowledge ofit.

The image examples in this article assume that you are viewing this web page ina modern browser on a computer monitor (CRT or LCD, doesn't matter). Tabletsand phones are generally quite inaccurate compared to monitors, so try toavoid those. You should be viewing the images in a dimly lit room, so nodirect lights or flare on your screen please.

Light emission vs perceptual brightness

Believe it or not, the difference of light energy emission between any twoneighbouring vertical bars in the below image is a constant. In other words,the amount of light energy emitted by your screen increases by a constantamount from bar to bar, left to right.

Now consider the following image:

Bicyclism: Art Of Riding Mac Os 11

On which image does the gradation appear more even? It's the second one! Butwhy is that so? We have just established that in the first image the bars areevenly (linearly) spaced in terms of emitted light intensity between thedarkest black and brightest white your monitor is capable of reproducing. Butwhy don't we see that as a nice even gradation from black to white then? Andwhat is being displayed on the second image that we perceive as a lineargradation?

The answer lies in the response of the human eye to light intensity, which isnon-linear. One the first image, the difference between the nominal lightintensity of any two neighbouring bars is constant:

$$Δ_{linear} = I_n-I_{n-1}$$

On the second image, however, this difference is not constant but changes frombar to bar; it follows a power law relationship, to be exact. All humansensory perception follows a similar power lawrelationship in terms ofthe magnitude of stimulus and its perceived intensity.

Because of this, we say that there is a power law relationship betweennominal physical light intensity and perceptual brightness.

Physical vs perceptual linearity

Let's say we wanted to store a representation of the following real-worldobject as an image file on the computer (let's pretend for a momentthat perfect greyscale gradients exist in the real world, okay?) Here's howthe 'real world object' looks like:

Now, let's pretend that we can only store 5-bit greyscale images on thisparticular computer system, which gives us 32 distinct shades of grey rangingfrom absolute black to absolute white. Also, on this computer, greyscalevalues are proportional with their corresponding physical light intensities,which will result in a 32-element greyscale as shown on Figure 1. We can saythat this greyscale is linear in terms of light emission betweensuccessive values.

If we encoded our smooth gradient using only these 32 grey values, we would getsomething like this (let's just ignore dither for now to keep things simple):

Well, the transitions are rather abrupt, especially on the left side, becausewe only had 32 grey values to work with. Intern purgatory: limbo mac os. If we squint a little, it's easy toconvince ourselves that this is a more or less 'accurate' representation ofthe smooth gradient, as far as our limited bit-depth allows it. But note howthe steps are much larger on the left side than on the right—this is becausewe are using a greyscale that is linear in terms of emitted lightintensity, but as we have mentioned before, our eyes don't perceive lightintensity in a linear way!

This observation has some interesting implications. The error between theoriginal and the 5-bit encoded version is uneven across the image; it's muchlarger for dark values than for light ones. In other words, we are losingrepresentational precision for dark values and are using relatively too muchprecision for lighter shades. Clearly, we'd be better off choosinga different set of 32 greys for our limited palette of shades that would makethis error evenly distributed across the whole range, so both dark and lightshades would be represented with the same precision. If we encoded ouroriginal image with such a greyscale that is perceptually linear, butconsequently non-linear in terms of emitted light intensity, and thatnon-linearity would match that of the human vision, we'd get the exact samegreyscale image we have already seen in Figure 2:

The non-linearity we're talking about here is the power law relationshipwe mentioned before, and the non-linear transformation we need to apply to ourphysically linear greyscale values to transform them into perceptuallylinear values is called gamma correction.

Efficient image encoding

Why is the all the above important? Colour data in so-called 'true colour' or'24-bit' bitmap images is stored as three 8-bit integers per pixel. With8 bits, 256 distinct intensity levels can be represented, and if the spacingof these levels were physically linear, we would be losing a lot of precisionon dark shades while being unnecessarily precise on light shades (relativelyspeaking), as shown above.

Clearly, this is not ideal. One solution would be to simply keep using thephysically linear scale and increase the bit depth per channel to 16 (ormore). This would double the storage requirements (or worse), which was notan option when most common image formats were invented. Therefore, a differentapproach was taken. The idea was to let the 256 distinct levels representintensity values on a perceptually linear scale instead, in which case thevast majority of images could be adequately represented on just 8 bits percolour channel.

The transformation used to represent the physically linear intensity dataeither generated synthetically via an algorithm or captured by a linear device(such as a CMOS of a digital camera or a scanner) with the discrete values ofthe perceptually linear scale is called gamma encoding.

The 24-bit RGB colourmodel(RGB24) used on virtually all consumer level electronic devices uses 8-bitgamma encodedvalues perchannel to represent light intensities. If you recall what we discussedearlier, this means that pixels with RGB(128, 128, 128) will not emitapproximately 50% the light energy of pixels with RGB(255, 255, 255), but onlyabout 22%! That makes perfect sense! Because of the non-linear nature of humanvision, a light source needs to be attenuated to about 22% of its originallight intensity to appear half as bright to humans. RGB(128, 128, 128)appears to be half as bright as RGB(255, 255, 255) to us! If you find thisconfusing, reflect a bit on it because it's crucial to have a solidunderstanding of what has been discussed so far (trust me, it will only getmore confusing).

Of course, gamma encoding is always done with the assumption that the image isultimately meant to be viewed by humans on computer screens. In some way, youcan think of it as a lossy MP3 like compression but for images. For otherpurposes (e.g. scientific analysis or images meant for furtherpost-processing), using floats and sticking with the linear scale is oftena much better choice, as we'll later see.

The gamma transfer function

The process of converting values from linear space to gamma space is calledgamma encoding (or gamma compression), and the reverse gammadecoding (or gamma expansion).

The formulas for these two operations are very simple, we only need to use theaforementioned power law function:

$$V_{encoded} = V_{linear} ^ {1/γ}$$

$$V_{linear} = V_{encoded} ^ {γ}$$

The standard gamma (γ) value to use in computer display systems is2.2. The main reason for this is because a gamma of 2.2 approximatelymatches the power law sensitivity of human vision. The exact value that shouldbe used varies from person to person and also depends on the lightingconditions and other factors, but a standard value had to be chosen and 2.2was good enough. Don't be too hung up on this.

Now, a very important point that many texts fail to mention is that the inputvalues have to be in the 0 to 1 range and the output will be consequentlymapped to the same range too. From this follows the slightly counter-productivefact that gamma values between 0 and 1 are used for encoding(compression) and greater than 1 for decoding (expansion).The below charts demonstrate the gamma transfer functions for encoding anddecoding, plus the trivial linear gamma (γ=1.0) case:

We have only seen greyscale examples so far, but there's nothing special aboutRGB images—we just simply need to encode or decode each colour channelindividually using the same method.

Gamma vs sRGB

sRGB is a colour space that is thede-facto standard for consumer electronic devices nowadays, includingmonitors, digital cameras, scanners, printers and handheld devices. It isalso the standard colour space for images on the Internet. https://free-bet-hobbyrisejetyoutubeskillet.peatix.com.

The sRGB specification defines what gamma to use for encoding and decodingsRGB images (among other things such as colour gamut, but these are notrelevant to our current discussion). sRGB gamma is very close to a standardgamma of 2.2, but it has a short linear segment in the very dark range toavoid a slope of infinity at zero (this is more convenient in numericcalculations). The formulas to convert from linear to sRGB and back can befoundhere.

You don't actually need to understand all these finer details; the importantthing to know is that in 99% of the cases you'll want to use sRGB instead ofplain gamma. The reason for this is that all graphics cards have hardware sRGBsupport since 2005 or so, so decoding and encoding is virtually for free mostof the time. The native colour space of your monitor is most likely sRGB(unless it's a professional monitor for graphics, photo or video work) so ifyou just chuck an sRGB encoded pixel data into the framebuffer, the resultingimage will look correct on the screen (given the monitor is properlycalibrated). Popular image formats such as JPEG and PNG can store colour spaceinformation, but very often images don't contain such data, in which casevirtually all image viewers and browsers will interpret them as sRGB byconvention.

Gamma calibration

We have talked about gamma encoding and decoding so far, but what is gammacalibration then? I found this bit slightly confusing too, so let me clearit up.

As mentioned, 99% of all monitors today use the sRGB colour space natively,but due to manufacturing inaccuracies most monitors would benefit from someadditional gamma calibration to achieve the best results. Now, if you nevercalibrated your monitor, that doesn't mean that it will not use gamma! That issimply impossible, most CRT and LCD displays in the past and present have beendesigned and manufactured to operate in sRGB.

Think of gamma calibration as fine tuning. Your monitor will always operate insRGB, but by calibrating it (either in the video card driver or on the OSlevel) the monitor's gamma transfer curve will more closely match the idealgamma transfer function we discussed earlier. Also, years ago it was possibleto shoot yourself in the foot in various creative ways by applying multiplegamma correction stages in the graphics pipeline (e.g. video card, OS andapplication level), but fortunately this is handled more intelligentlynowadays. For example, on my Windows 7 box, if I turn on gamma calibration inthe NVIDIA Control Panel then the OS level calibration will be disabled andvice versa.

Processing gamma-encoded images

So, if virtually the whole world defaults to sRGB, what is exactly theproblem? If our camera writes sRGB JPEG files, we can just decode the JPEGdata, copy it into the framebuffer of the graphics card and the image would bedisplayed correctly on our sRGB LCD monitor (where 'correctly' means it wouldmore or less accurately represent the photographed real-world scene).

The problem will happen in the moment we start running any image processingalgorithms on our sRGB pixel buffer directly. Remember, gamma encoding isa non-linear transformation and sRGB encoding is basically just a funky way ofdoing gamma encoding of around γ=1/2.2. Virtually all image processingalgorithms you will find in any computer graphics text will assume pixel datawith linearly encoded light intensities, which means that feeding thesealgorithms with sRGB encoded data will render the results subtly—or insome cases quite obviously—wrong! This includes resizing, blurring,compositing, interpolating between pixel values, antialiasing and so on, justto name the most common operations!

Effects of gamma-incorrectness

Alright, enough theory talk, show me how these errors actually look like!That's exactly what we'll do in this section; we will examine the most commonscenarios when running image processing algorithms directly on sRGB data wouldmanifest in incorrect results. Apart from illustrative purposes, theseexamples are also useful for spotting gamma-incorrect behaviour or bugs indrawing programs and image processing libraries.

It must be noted that I have chosen examples that clearly demonstrate theproblems with gamma-incorrectness. In most cases, the issues are the mostobvious when using vivid, saturated colours. With more muted colours, thedifferences might be less noticeable or even negligible in some cases.However, the errors are always present, and image processing programs should workcorrectly for all possible inputs, not just okayish for 65.23% of all possibleimages… Also, in the area of physically based rendering gamma correctness isan absolute must, as we'll see.

Gradients

The image below shows the difference between gradients calculated in linear(top gradient) and sRGB space (bottom gradient). Note how direct interpolationon the sRGB values yields much darker and sometimes more saturated lookingimages.

Just going by the looks, one might prefer the look of the sRGB-space versions,especially for the last two. However, that's not how light would behavein the real world (imagine two coloured light sources illuminating a whitewall; the colours would mix as in the linear-space case).

Almost everybody does this the wrong way: CSS gradients and transitions arewrong (see thisthread fordetails), Photoshop is wrong (as of version CS6) and there's not even an optionto fix it.

Two drawing programs that got this (and gamma-correctness in general) rightare Krita and Pixelmator.SVG also let's the user tospecifywhether to use linear or sRGB-space interpolations for gradients, compositingand animations.

Colour blending

Drawing with soft brushes in gamma-incorrect drawing programs can result inweird darkish transition bands with certain vivid colour combinations.This is really a variation of the gradient problem if you think about it (thetransition band of a soft brush is nothing else than a small gradient).

Some random people claimed on the Adobe forums that by doing this Photoshop isreally mimicking how mixing paints would work in real life. Well, no, ithas nothing to do with that. It's just the result of naive programming to workdirectly on the sRGB pixel data and now we're stuck with that as the defaultlegacy behaviour.

Alpha blending / compositing

As another variation on colour blending, let's see how alpha blending holdsup. We'll examine some coloured rectangles first. As expected, thegamma-correct image on the left mimics how light would behave in real life,while the sRGB space blending on the right exhibits some weird hue andbrightness shifts.

The appearance of false colours is also noticeable when blending two photostogether. On the gamma-correct image on the left, the skin tones and the redsand yellows are preserved but faded into the bluish image in a natural way,while on the right image there's a noticeable overall greenish cast. Again,this might be an effect you like, but it's not how accurate alphacompositing should work.

Image resizing

These examples will only work if your browser doesn't do any rescaling on theimages below. Also, note that screens of mobile devices are more inaccuratewith regards to gamma than regular monitors, so for best results try to viewthis on a desktop computer.

The image below contains a simple black and white checkerboard pixel pattern(100% zoom on the left, 400% zoom on the right). The black pixels areRGB(0,0,0), the minimum light intensity your monitor is capable of producing,and the white ones RGB(255,255,255), which is the maximum intensity. Now, ifyou squint a little, your eyes will blur (average) the light coming from theimage, so you will see a grey that's halfway in intensity between absoluteblack and white (therefore it's referred to as 50% grey).

From this follows that if we resized the image by 50%, a similar averagingprocess should happen, but now algorithmically on the pixel data. We expectto get a solid rectangle filled with the same 50% grey that we saw when wesquinted.

Let's try it out! On the image below, A is the checkerboard pattern, B theresult of resizing the pattern by 50% directly in sRGB-space (using bicubicinterpolation), and C https://forum-slotscolorswheelroulettenumbersandxnl.peatix.com. the resizing it in linear space, then converted tosRGB.

Unsurprisingly, C gives the correct result, but the shade of grey might notbe an exact match for the blurred checkerboard pattern on your monitor ifit's not properly gamma-calibrated. Even the math shows this clearly: a 50%grey pixel that emits half as much light as a white pixel should have a RGBvalue of around (186,186,186), gamma-encoded:

$$0.5^{1/2.2} ≈ 0.72974$$$$0.72974·255 = 186$$

(Don't worry that on the image the 50% grey is RGB(187,187,187). That smalldifference is because the image is sRGB-encoded, but I used the much simplergamma formula for my calculation here.)

Gamma-incorrect resizing can also result in weird hue shifts on some images.For more details, read Eric Brasseur's excellentarticle on the matter.

Antialiasing

I guess it's no surprise at this point that antialiasing is no exception whenit comes to gamma-correctness. Antialiasing in γ=2.2 space results in overlydark 'smoothing pixels' (right image); the text appears too heavy, almost asif it was bold. Running the algorithm in linear space produces much betterresults (left image), although in this case the font looks a bit too thin.Interestingly, Photoshop antialiases text using γ=1.42 by default, and thisindeed seems to yield the best looking results (middle image). The reason forthis is that most fonts have been designed for gamma-incorrect fontrasterizers, hence if you use linear space (correctly), then the fonts willlook thinner than they should.

Physically-based rendering

If there's a single area where gamma-correctness is an absolute must, that'sphysically-based rendering (PBR). To obtain realistic looking results, gammashould be handled correctly throughout the whole graphics pipeline. There'sso many ways to screw this up, but these are the two most common ways:

• Doing the calculations in linear space but failing to convert the finalimage to sRGB and then 'tweaking' various material and lighting parametersto compensate.
• Failing to convert sRGB texture images to linear space (or set the sRGB flagwhen hardware acceleration is used).

These two basic errors are then usually combined in various interesting ways,but the end result would invariably fail to resemble a realistic looking scene(e.g. quadratic light falloff will not appear quadratic anymore, highlightswill be overblown and will exhibit some weird hue and saturation shifts etc.)

To demonstrate the first mistake using my own raytracer, the left image below shows a very simple butotherwise quite natural looking image in terms of physical lighting accuracy.This rendering took place in linear space and then the contents of theframebuffer were converted to sRGB before writing it to disk.

On the right image, however, this last conversion step was omitted and I triedto tweak the light intensities in an attempt to match the overall brightnessof the gamma-correct image. Well, it's quite apparent that this is not goingto work. Everything appears too contrasty and oversaturated, so we'd probablyneed to desaturate all material colours a bit maybe use some more fill lightsto come closer to the look of the left image. But this is a losing battle; noamount of tweaking will make the image correct in the physical sense, and evenif we got it to an acceptable level for one particular scene with a particularlighting setup, any further changes to the scene would potentially necessitateanother round of tweaks to make the result look realistic again. Even moreimportantly, the material and lighting parameters we would need to choosewould be completely devoid of any physical meaning whatsoever; they'll be justa random set of numbers that happen to produce an OK looking image for thatparticular scene, and thus not transferable to other scenes or lightingconditions. It's a lot of wasted energy to work like that.

It's also important to point out that incorrect gamma handling in 3D renderingis one of the main culprits behind the 'fake plasticky CGI look' in some(mostly older) games. As illustrated on the image below, rendering realisticlooking human skin is almost impossible with a gamma-incorrect workflow; thehighlights will just never look right. This gave birth to questionablepractices such as compensating for the wrong highlights in the specular mapswith inverted hues and all sorts of other nastiness instead of fixing theproblem right at the source…

Conclusion

This is pretty much all there is to gamma encoding and decoding.Congratulations for making it so far, now you're an officially certifiedgamma-compliant developer! :)

To recap, the only reason to use gamma encoding for digital images is becauseit allows us to store images more efficiently on a limited bit-length. Ittakes advantage of a characteristic of human vision that we perceivebrightness in a logarithmic way. Most image processing algorithms expect pixeldata with linearly encoded light intensities, therefore gamma-encoded imagesneed to be gamma-decoded (converted to linear space) first before we can runthese algorithms on them. Often the results need to be converted back togamma-space to store them on disk or to display them on graphics hardware thatexpects gamma-encoded values (most consumer-level graphics hardware fall intothis category). The de-facto standard sRGB colourspace uses a gamma ofapproximately 2.2. That's the default colourspace for images on the Internetand for most monitors, scanners and printers. When in doubt, just use sRGB.

From the end-user perspective, keep in mind that most applications andsoftware libraries do not handle gamma correctly, therefore always make sureto do extensive testing before adopting them into your workflow. For a properlinear workflow, all software used in the chain has to be 100%gamma-correct.

And if you're a developer working on graphics software, please make sureyou're doing the correct thing. Be gamma-correct and always explicitly stateyour assumptions about the input and output colour spaces in the software'sdocumentation.

May all your lights be linear! :)

References & further reading

General gamma/sRGB info

Linear lighting & workflow (LWF)

• Jeremy Birn – Top Ten Tips for More Convincing Lighting and Rendering – (Section 1. Use a Linear Workflow)

Bonus stuff

1. Only if your operating system is Mac OS X 10.6 or higher or Linux. ↩

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