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选题[tech]: 20210126 Write GIMP scripts to make image processing faster
sources/tech/20210126 Write GIMP scripts to make image processing faster.md
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[#]: collector: (lujun9972)
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[#]: translator: ( )
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Write GIMP scripts to make image processing faster)
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[#]: via: (https://opensource.com/article/21/1/gimp-scripting)
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[#]: author: (Cristiano L. Fontana https://opensource.com/users/cristianofontana)
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Write GIMP scripts to make image processing faster
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======
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Learn GIMP's scripting language Script-Fu by adding an effect to a batch
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of images.
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![Painting art on a computer screen][1]
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Some time ago, I wanted to give a blackboard-style look to a typeset equation. I started playing around with the [GNU Image Manipulation Program (GIMP)][2] and was satisfied with the result. The problem was that I had to perform several actions on the image, I wanted to use this style again, and I did not want to repeat the steps for all the images. Besides, I was sure that I would forget them in no time.
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![Fourier transform equations][3]
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Fourier transform equations (Cristiano Fontana, [CC BY-SA 4.0][4])
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GIMP is a great open source image editor. Although I have been using it for years, I had never investigated its batch-processing abilities nor its [Script-Fu menu][5]. This was the perfect chance to explore them.
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### What is Script-Fu?
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[Script-Fu][6] is the scripting language built into GIMP. It is an implementation of the [Scheme][7] programming language. If you have never used Scheme, give it a try, as it can be very useful. I think Script-Fu is a great way to start because it has an immediate effect on image processing, so you can feel productive very quickly. You can also write scripts in [Python][8], but Script-Fu is the default option.
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To help you get acquainted with Scheme, GIMP's documentation offers an [in-depth tutorial][9]. Scheme is a [Lisp][10]-like language, so a major characteristic is that it uses a [prefix notation][11] and a [lot of parentheses][12]. Functions and operators are applied to a list of operands by prefixing them:
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```
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(function-name operand operand ...)
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(+ 2 3)
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↳ Returns 5
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(list 1 2 3 5)
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↳ Returns a list containing 1, 2, 3, and 5
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```
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It took me a while to find the documentation for the full list of GIMP's functions, but it was actually straightforward. In the **Help** menu, there is a **Procedure Browser** with very extensive and detailed documentation about all the possible functions.
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![GIMP Procedure Browser][13]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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### Accessing GIMP's batch mode
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You can run GIMP with batch mode enabled by using the `-b` option. The `-b` option's argument can be the script you want to run or a dash (`-`) that makes GIMP launch in an interactive mode instead of the command line. Normally when you start GIMP, it loads its graphical user interface (GUI), but you can disable that with the `-i` option.
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### Writing your first script
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Create a file called `chalk.scm` and save it to the `scripts` folder found in the **Preferences** window under **Folders → Scripts**. In my case, it is at `$HOME/.config/GIMP/2.10/scripts`.
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Inside the `chalk.scm` file, write your first script with:
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```
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(define (chalk filename grow-pixels spread-amount percentage)
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(let* ((image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
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(drawable (car (gimp-image-get-active-layer image)))
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(new-filename (string-append "modified_" filename)))
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(gimp-image-select-color image CHANNEL-OP-REPLACE drawable '(0 0 0))
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(gimp-selection-grow image grow-pixels)
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(gimp-context-set-foreground '(0 0 0))
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(gimp-edit-bucket-fill drawable BUCKET-FILL-FG LAYER-MODE-NORMAL 100 255 TRUE 0 0)
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(gimp-selection-none image)
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(plug-in-spread RUN-NONINTERACTIVE image drawable spread-amount spread-amount)
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(gimp-drawable-invert drawable TRUE)
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(plug-in-randomize-hurl RUN-NONINTERACTIVE image drawable percentage 1 TRUE 0)
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(gimp-file-save RUN-NONINTERACTIVE image drawable new-filename new-filename)
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(gimp-image-delete image)))
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```
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### Defining the script variables
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In the script, the `(define (chalk filename grow-pixels spread-amound percentage) ...)` function defines a new function called `chalk` that accepts the parameters: `filename`, `grow-pixels`, `spread-amound`, and `percentage`. Everything else inside the `define` function is the body of the `chalk` function. You might have noticed that variables with long names are spelled with dashes between the words; this is the idiomatic style of Lisp-like languages.
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The `(let* ...)` function is a special procedure that allows you to define some temporary variables that are valid only inside the body. In this case, the variables are `image`, `drawable`, and `new-filename`. It loads the image with `gimp-file-load`, which returns a list that includes the image, then it selects the first entry with the `car` function. Then, it selects the first active layer and stores its reference in the `drawable` variable. Finally, it defines the string containing the new filename of the resulting image.
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To help you better understand the procedure, I'll break it down. First, start GIMP with the GUI enabled and the Script-Fu console, which is found in **Filters → Script-Fu → Console**. In this case, you cannot use `let*` because the variables must be persistent. Define the `image` variable using the `define` function, and give it the proper path to find the image:
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```
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`(define image (car (gimp-file-load RUN-NONINTERACTIVE "Fourier.png" "Fourier.png")))`
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```
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It appears that nothing has happened in the GUI, but the image is loaded. You need to enable the image display with:
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```
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`(gimp-display-new image)`
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```
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![GUI with the displayed image][14]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Now, get the active layer and store it in the `drawable` variable:
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```
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`(define drawable (car (gimp-image-get-active-layer image)))`
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```
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Finally, define the image's new filename:
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```
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`(define new-filename "modified_Fourier.png")`
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```
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Here is what you should see in the Script-Fu console after running these commands:
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![Script-Fu console][15]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Before acting on the image, you need to define the variables that would be defined as the function arguments in the script:
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```
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(define grow-pixels 2)
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(define spread-amount 4)
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(define percentage 3)
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```
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### Acting on the image
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Now that all the relevant variables are defined, you can act on the image. The script's actions can be executed directly on the console. The first step is to select the color black on the active layer. The color is written as a list of three numbers—either as `(list 0 0 0)` or `'(0 0 0)`:
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```
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`(gimp-image-select-color image CHANNEL-OP-REPLACE drawable '(0 0 0))`
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```
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![Image with the selected color][16]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Grow the selection by two pixels:
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```
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`(gimp-selection-grow image grow-pixels)`
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```
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![Image with the selected color][17]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Set the foreground color to black, and fill the selection with it:
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```
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(gimp-context-set-foreground '(0 0 0))
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(gimp-edit-bucket-fill drawable BUCKET-FILL-FG LAYER-MODE-NORMAL 100 255 TRUE 0 0)
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```
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![Image with the selection filled with black][18]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Delete the selection:
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```
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`(gimp-selection-none image)`
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```
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![Image with no selection][19]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Move the pixels around randomly:
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```
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`(plug-in-spread RUN-NONINTERACTIVE image drawable spread-amount spread-amount)`
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```
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![Image with pixels moved around][20]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Invert the image colors:
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```
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`(gimp-drawable-invert drawable TRUE)`
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```
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![Image with pixels moved around][21]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Randomize the pixels:
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```
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`(plug-in-randomize-hurl RUN-NONINTERACTIVE image drawable percentage 1 TRUE 0)`
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```
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![Image with pixels moved around][22]
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(Cristiano Fontana, [CC BY-SA 4.0][4])
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Save the image to a new file:
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```
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`(gimp-file-save RUN-NONINTERACTIVE image drawable new-filename new-filename)`
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```
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![Equations of the Fourier transform and its inverse][23]
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Fourier transform equations (Cristiano Fontana, [CC BY-SA 4.0][4])
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### Running the script in batch mode
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Now that you know what the script does, you can run it in batch mode:
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```
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`gimp -i -b '(chalk "Fourier.png" 2 4 3)' -b '(gimp-quit 0)'`
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```
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After the `chalk` function runs, it calls a second function with the `-b` option to tell GIMP to quit: `gimp-quit`.
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### Learn more
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This tutorial showed you how to get started with GIMP's built-in scripting features and introduced Script-Fu, GIMP's Scheme implementation. If you want to move forward, I suggest you look at the official documentation and its [tutorial][9]. If you are not familiar with Scheme or Lisp, the syntax could be a little intimidating at first, but I suggest you give it a try anyway. It might be a nice surprise.
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Professional design software like Photoshop is terrific, but it’s also expensive. What do you do...
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/1/gimp-scripting
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作者：[Cristiano L. Fontana][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/cristianofontana
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/painting_computer_screen_art_design_creative.png?itok=LVAeQx3_ (Painting art on a computer screen)
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[2]: https://www.gimp.org/
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[3]: https://opensource.com/sites/default/files/uploads/fourier.png (Fourier transform equations)
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[4]: https://creativecommons.org/licenses/by-sa/4.0/
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[5]: https://docs.gimp.org/en/gimp-filters-script-fu.html
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[6]: https://docs.gimp.org/en/gimp-concepts-script-fu.html
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[7]: https://en.wikipedia.org/wiki/Scheme_(programming_language)
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[8]: https://docs.gimp.org/en/gimp-filters-python-fu.html
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[9]: https://docs.gimp.org/en/gimp-using-script-fu-tutorial.html
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[10]: https://en.wikipedia.org/wiki/Lisp_%28programming_language%29
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[11]: https://en.wikipedia.org/wiki/Polish_notation
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[12]: https://xkcd.com/297/
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[13]: https://opensource.com/sites/default/files/uploads/procedure_browser.png (GIMP Procedure Browser)
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[14]: https://opensource.com/sites/default/files/uploads/gui01_image.png (GUI with the displayed image)
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[15]: https://opensource.com/sites/default/files/uploads/console01_variables.png (Script-Fu console)
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[16]: https://opensource.com/sites/default/files/uploads/gui02_selected.png (Image with the selected color)
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[17]: https://opensource.com/sites/default/files/uploads/gui03_grow.png (Image with the selected color)
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[18]: https://opensource.com/sites/default/files/uploads/gui04_fill.png (Image with the selection filled with black)
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[19]: https://opensource.com/sites/default/files/uploads/gui05_no_selection.png (Image with no selection)
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[20]: https://opensource.com/sites/default/files/uploads/gui06_spread.png (Image with pixels moved around)
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[21]: https://opensource.com/sites/default/files/uploads/gui07_invert.png (Image with pixels moved around)
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[22]: https://opensource.com/sites/default/files/uploads/gui08_hurl.png (Image with pixels moved around)
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[23]: https://opensource.com/sites/default/files/uploads/modified_fourier.png (Equations of the Fourier transform and its inverse)
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