Updated to GIMP 2.2

• 1. Introduction
  • 1.1. History of the GIMP
  • 1.2. Future Directions
  • 1.3. Conventions Used in this Lecture
• 2. Overview of Features
  • 2.1. Colour Modes
  • 2.2. 8 bits per colour value
  • 2.3. RGB and HSV modes
    • 2.3.1. Demo
    • 2.3.2. What are RGB and HSV?
    • 2.3.3. What about CMYK?
  • 2.4. Basic Effects using the Toolbox
  • 2.5. Plug-ins
  • 2.6. Selection and its Usage
    • 2.6.1. Using the Red "Paint Selection" Button
    • 2.6.2. The "<Image> → Select" Menu
  • 2.7. Layers
    • 2.7.1. Alpha
      • 2.7.1.1. Demo 1
      • 2.7.1.2. Demo : Using a Gradient as a Layer Mask
    • 2.7.2. Filters and Layers
    • 2.7.3. Layer Modes
    • 2.7.4. Gradients
• 3. LUT (Look-Up Tables Effects)
  • 3.1. Brightness and Contrast
    • 3.1.1. Answer
    • 3.1.2. Demonstration
  • 3.2. Gamma Correction
    • 3.2.1. Concept
    • 3.2.2. Demonstration
  • 3.3. Equalize
  • 3.4. Color Map Rotation
  • 3.5. Color Balance
  • 3.6. More LUTs
• 4. Basic Areal Effects
  • 4.1. Blur
    • 4.1.1. Answer to how one can blur an image
    • 4.1.2. Demo of a simple blur
    • 4.1.3. Gaussian Blur
      • 4.1.3.1. IIR vs. RLE Gaussian Blurs
      • 4.1.3.2. Demo of IIR Gauss. Blur
      • 4.1.3.3. Demo of RLE Gauss. Blur
  • 4.2. Sharpening
    • 4.2.1. Answer to how one can sharpen
    • 4.2.2. Sharpening Demo
  • 4.3. Motion Blur
    • 4.3.1. Answer to how one can motion blur
    • 4.3.2. Demo
• 5. Basic Areal Transformations
  • 5.1. Scaling an Image
    • 5.1.1. Interpolation
  • 5.2. The Rotate, Scale and Perspective Tools
    • 5.2.1. Demo
• 6. Some Cool Effects
  • 6.1. Gradient Map
    • 6.1.1. Demo 1 : Real-life picture
    • 6.1.2. Demo 2 : Text Caption
  • 6.2. Fractal Explorer
• 7. Turning a Photograph into a Painting
  • 7.1. Oilify
  • 7.2. Gimpressionist
• 8. Scripting the GIMP
  • 8.1. The Procedural Database
  • 8.2. Script-Fu Scripting
    • 8.2.1. Interactive Demo
    • 8.2.2. Generating a Logo
  • 8.3. Gimp-Perl Scripting
  • 8.4. GIMP (C-based) Plug-ins
• 9. Finale
  • 9.1. References and Links
  • 9.2. Origins and Copyrights of the Images

• The GIMP - The GNU Image Manipulation Program
• A sophisticated image processing and composition program, up-to-par with commercial offerings.
• Available for UNIXes and other platforms.

• This Lecture aims to give a taste of the GIMP's capabilities.
• It is not a comprehensive coverage of them.
• I will also try to explain some of the algorithms that are used behind the scenes.

• A document that details the GIMP's history can be found here:

  http://docs.gimp.org/en/ch01s03.html

• GIMP was started as a project by two Berkeley students called Spencer Kimball and Peter Mattis. It was first meant as a university project that they received credit for.
• GIMP used Motif at first but Kimball and Mattis later developed Gtk+, so it can be used as its underlying GUI library instead. Since then, Gtk+ has become the basis for many other projects including GNOME.
• GIMP 1.0 was released several years after the switch to Gtk+, and GIMP 1.2 (which already used Gtk+ 1.2) was released three years afterwards.
• Since then, the GIMP has acclaimed many fans and it is considered the most complete and active image manipulation programs for UNIXes to-date.
• GIMP 2.0.0 was released in April 2004, featuring a port to Gtk+ 2.0, an improved user-interface, a general code cleanup and modularization and many other improvements.
• GIMP 2.2.0 was released in December 2004. It is backwards compatible to GIMP 2.0.x, and featured various improvements, additions and cleanups.

• The development of GIMP 2.4 as part of the 2.3.x series has already began. It will be mostly backwards compatible to GIMP 2.2, and will feature some improvements. It is scheduled to a few months in the future.
• For the more distant future, the GIMP developers are planning on working on the GEGL library, and making the GIMP use it as its core. This will allow support for advanced features such as CMYK support, 16-bits per color value, and color-spaces.

• Demos will be placed inside a box in smaller letters.
• Menu commands will be coloured in red and green.
  • <Toolbox> refers to the menubar in the GIMP's tool-box.
  • <Image> refers to the menu of each image that is present below the title of the image, or alternatively can be invoked with a right mouse button click.

• Throughout the lecture I will turn to the audience and ask them to come up with the algorithms that are used behind the scenes.

  Those that have technical knowledge in image processing, please don't answer.

• 2.1. Colour Modes
• 2.2. 8 bits per colour value
• 2.3. RGB and HSV modes
  • 2.3.1. Demo
  • 2.3.2. What are RGB and HSV?
  • 2.3.3. What about CMYK?
• 2.4. Basic Effects using the Toolbox
• 2.5. Plug-ins
• 2.6. Selection and its Usage
  • 2.6.1. Using the Red "Paint Selection" Button
  • 2.6.2. The "<Image> → Select" Menu
• 2.7. Layers
  • 2.7.1. Alpha
    • 2.7.1.1. Demo 1
    • 2.7.1.2. Demo : Using a Gradient as a Layer Mask
  • 2.7.2. Filters and Layers
  • 2.7.3. Layer Modes
  • 2.7.4. Gradients

• Gimp supports RGB, Grayscale and Indexed (= palette based) Images
• It can convert between them, but conversion from RGB to something else is not reversible.

• Some effects are not available in all modes
• It is recommended to work in RGB or in Grayscale and convert it to be a palette-based one only before publishing it. (and keep a copy of the RGB source)

• The GIMP supports 8-bits per RGB colour value and 8-bit grayscale images.
• That means pixel values in the range 0 to 255.

• There's a fork of the GIMP called CinePaint that adds 16-bit and 32-bit color-value precision to.
• The core GIMP version will provide such functionality in the future, once GEGL is integrated.

• 2.3.1. Demo
• 2.3.2. What are RGB and HSV?
• 2.3.3. What about CMYK?

RGB

• RGB stands for Red-Green-Blue.
• They are additive light components from which one can form every visible shade.

HSV

• H = Hue : A unit that specifies the "colour" of the colour
• S = Saturation : Specifies how much black, white or gray is mixed with the colour.
• V = Value : Corresponds to Brightness.

• CMYK stands for Cyan, Magenta, Yellow and blacK.
• It is a subtractive method that is akin to mixing paint colours.
• One can select colours using it by using the Printer-iconed tab of the colour selector.
• Nevertheless, GIMP 2.2 lacks full support for CMYK.

• Many effects can be performed by using the tools in the Toolbox.
• Most tools can be double-clicked to display their preferences.
• Experiencing is the key to success here, as there are many tools.

• In the GIMP, effects and filters are implemented using plug-ins.
• Plug-ins are separate programs that communicate with the main GIMP program using pipes.
• Most of the plug-ins that are distributed with the GIMP are GPLed, but it is legal and possible to write proprietary plugins.
• Except for plug-ins written in C, GIMP has extensions for writing filters and scripts in Scheme, Perl, Python and other languages.

• If you define a selection, then only the pixels of the selection are affected by tools or filters.
• GIMP Selections can be any subset of pixels.
• The toolbox contains numerous selection tools that can be used to shape the selection.
• Pressing Ctrl, Alt and Shift before pressing the button and while releasing it modifies the behaviour of the selection.
• Just experience to see how everything works!

• An image has a mode that enables you to "paint" the selection using existing tools.
• In this mode colouring with black adds pixels to the selection.
• And colouring with white excludes them.
• To access this mode press the button in the lower-left corner of the image, right to the left of the bottom scrool-bar.
• Note: this mode has a meta-selection, but there aren't any meta^N-selections.

• The <Image> → Select sub-menu is handy in performing several common-operations on selections.
• Among the tools that can be found are:
  
  1. Select None
  2. Select All
  3. Invert the Selection
  4. Sharpen the Selection

A GIMP image is composed of layers. They can be viewed and manipulated using the Layers Dialog ( <Image> → Dialogs → Layers ). These layers can be duplicated, deleted, moved, etc.

• 2.7.1. Alpha
  • 2.7.1.1. Demo 1
  • 2.7.1.2. Demo : Using a Gradient as a Layer Mask
• 2.7.2. Filters and Layers
• 2.7.3. Layer Modes
• 2.7.4. Gradients

Every layer contains a special layer called the "alpha layer" that specifies its opacity. This layer is treated as a black and white bitmap that can be manipulated with all the normal tools, and in which black is considered full transparency (zero opacity) and white is full opacity between this layer and the layer below it.

The Resultant pixel of the two layers is calculated using the formula:

Result = (1-alpha) * Pixel_Below + alpha * Pixel_Above

We can also use gradients as layer masks to create all kinds of fading effects.

Most filters affect only the active layer, so if you wish them to have a cumulative effect on a combination of layers you'll need to merge them first ( <Layers> → "Merge Visible Layers..." or <Layers> → "Flatten Image" )

Layers can be merged using other methods except their Alpha. These are available in the "Mode" combo-box of the layers dialog.

The operations that are used to merge the various layers are described the in the Gimp User's Manual.

• Gradients can be drawn in various forms - linear, radial, conical, etc.
• A basic gradient can be created from the foreground colour through the background colour with either RGB or HSV interpolation.
• More complex gradients can be constructed using the Gradient Editor. To access it select <Toolbox> → File → Dialogs → Gradients and then press the Edit button.

Explanation

Look-up Tables (or LUT for short) are transformations that act on every pixel based on its value alone and not on the values of neighbouring pixels.

• 3.1. Brightness and Contrast
  • 3.1.1. Answer
  • 3.1.2. Demonstration
• 3.2. Gamma Correction
  • 3.2.1. Concept
  • 3.2.2. Demonstration
• 3.3. Equalize
• 3.4. Color Map Rotation
• 3.5. Color Balance
• 3.6. More LUTs

Question

How can we increase/decrease the brightness and contrast of an image?

Increasing or decreasing the brightness involves adding or subtracting a certain amount from the value of each pixel.

The contrast can be increased by plotting an y=mx graph according to the value, where m is greater than 1 to increase the contrast, and less than 1 to decrease it.

Question

How can we brighten or darken an image without causing colour values to overflow?

If we assume that the maximal colour value is 1 and the minimal is 0, then by applying a function of x ** (1/gamma) to every pixel we will modify the histogram while not overflowing from the image boundaries (which cannot be said on brightness and contrast manipulations).

The more positive the Gamma is the brighter the resultant image will be.

The "Equalize" operation constructs an appropriate Look-Up Table so that the histogram of the resultant image will be more or less a constant function.

Low colour values will remain lower than higher ones, but the spread of them will change.

So far we have seen LUTs from the V value to the V value. But there can also be RGB to RGB LUTs. An example of it is the Color Map Rotation which enables us to swap a range of colours with other colours in the Hue spectrum.

Another Useful LUT is the Color Balance. It enables one to increase or decrease the individual components of Red, Green and Blue for either shadows, midtones or highlights.

There are more LUTs present in the GIMP. And if that's not enough you may wish to play with the Gimp::Pixel PDL bindings. (which is the GIMP's answer to the Image Toolbox of Matlab).

• As opposed to Look-Up Tables, areal filters may take into account the values of the neighbouring pixels and use them to calculate the value of the resultant pixel.

• 4.1. Blur
  • 4.1.1. Answer to how one can blur an image
  • 4.1.2. Demo of a simple blur
  • 4.1.3. Gaussian Blur
    • 4.1.3.1. IIR vs. RLE Gaussian Blurs
    • 4.1.3.2. Demo of IIR Gauss. Blur
    • 4.1.3.3. Demo of RLE Gauss. Blur
• 4.2. Sharpening
  • 4.2.1. Answer to how one can sharpen
  • 4.2.2. Sharpening Demo
• 4.3. Motion Blur
  • 4.3.1. Answer to how one can motion blur
  • 4.3.2. Demo

Question: How can we blur?

• Can you come up with a methodical filter that when applied on an image, the image will appear blurred?

• A blurred image has less abrupt areal changes than the original.
• Thus, we can blur by averaging every pixel with some of the energy of its neighbours.
• A blurring acts as a low-pass filter by removing high areal frequencies which cause abrupt changes.

• Gaussian Blur is a blur that uses an elliptical Gaussian as its kernel.
• Why Gaussian?
• If we apply a finite blur enough times, then by virtue of the Central Limit Theorem, we will get a Gaussian-like blur.

• GIMP supports two implementations of Gaussian Blur: IIR G.B. and RLE G.B.
• They both produce the same results, but each one can be faster in some cases.

RLE Gaussian Blur

• RLE stands for run-length encoding
• RLE Gaussian Blur is best used on computer-generated images or those with large areas of constant intensity.

IIR Gaussian Blur

• IIR stands for "Infinite Impulse Response".
• This blur works best for large radius values and for images which are not computer generated.

• Sharpening is the process of making the edges of an image look clearer.

Question: How can we sharpen?

• Sharpen is in effect the reverse of blurring.
• We sharpen by negating some of the energy of the surrounding pixels from the pixel at the processed position.

• Motion blur makes the picture seem like it is moving in respect to the camera.

Question: How can we create a motion blur effect?

• We can perform a motion blur by blurring with a one-sided kernel.
• We still have to make sure the energy of the kernel is 1.

• 5.1. Scaling an Image
  • 5.1.1. Interpolation
• 5.2. The Rotate, Scale and Perspective Tools
  • 5.2.1. Demo

• In order to scale an image use the <Image> → Image → "Scale Image..." menu option.

• When Enlarging, GIMP interpolates between the pixels.
• That way, the picture does not look blocky, but it is still not as good as the original.

Question: How can interpolation be done?

• When having an active selection one can use the Transform tools to transform it in several different ways.
• Select a transform tool from the toolbox, and you can get some of its options, by double-clicking it.

• 6.1. Gradient Map
  • 6.1.1. Demo 1 : Real-life picture
  • 6.1.2. Demo 2 : Text Caption
• 6.2. Fractal Explorer

• A gradient map maps each pixel to an RGB triad found in the active gradient in proportion to its brightness level.
• It is in fact a V → RGB LUT in which the resultant colors are chosen arbitrarily according to the gradient.

• The fractal explorer is a GIMP plug-in that can be used to render some fractals.
• It supports only a subset of all the known two-dimensional fractals, but it is quite powerful at rendering them.

• Using the GIMP, it is possible to convert a photo into something that resembles a painting.
• If you have a special printer that will paint it for you on canvas, you can use it to sell your "original" artwork and become filthy rich. :-)

• The "Oilify" tool enables one to turn a photograph (or something similar) into an oil painting-like image.

• Oilify works by replacing the pixel at (x,y) with the value that occurs most often in its N x N neighbourhood.
• It does so separately for each RGB Channel.

• The Gimpressionist takes an image and paints it for you using a specified brush and many other parameters.

• Gimp was designed from the ground's up to be modular and extensible.
• Therefore, it has an extensive support for scripting.
• There are in fact several options to script it. Some of them will only be reviewed here and will not be demonstrated.

• Gimp contains a database of named procedures.
• This database is flat, so care must be taken to maintain namespace integrity
• Scripting extensions can interact with the GIMP by accessing the functions through their names in the database.
• This list can be viewed by accessing: <Toolbox> → Xtns → "DB Browser..."
• It is evident that there are several classes of functions which are distinguished by their prefix.

• Script-Fu enables to write GIMP Scripts and PDB entries in Scheme.
• The Scheme scripts reside in ~/.gimp-2.2/scripts. (Replace 2.2 with the current version of GIMP)
• Whenever changed they need to be refreshed by selecting <Toolbox> → Xtns → Script-Fu → Refresh
• Otherwise, GIMP sports an interactive Script-Fu Console.

• Now, let's see some Scheme Scripts in action.

• The source code of this script can be seen in /usr/share/gimp/2.0/scripts/bovinated-logo.scm. (where /usr is your GIMP installation directory.)
• You can play with the other scripts in the Xtns → Script-Fu → Logos menu and view their source code. (My personal favourite is Frosty, which may take quite a while to render on a slow computer.)

• Gimp-Perl scripts are technically plug-ins, which are written in Perl.
• Inside such a plug-in one can do everything that can be done in Perl - e.g: opening files, sockets, fork()ing and executing other programs and GUI code.
• Gimp-Perl is not available for Win32 at the time of this writing, and also has a more picky installation process.
• Gimp-Perl has an extension called Gimp::Pixel that enables to write plug-ins using the Perl Data Language (=PDL). This is, in fact, the GIMP's answer to the Image Toolbox of Matlab.

• If all else fail, one can write a plug-in in C.
• You can find some documentation about it here: http://gimp-plug-ins.sourceforge.net/
• Plug-ins can access the GIMP's PDB, so they can invoke other plug-ins (and scripts !).

• This lecture covered only the tip of the iceberg of the GIMP's functionality.
• To learn more, consult the resources I link to, or experience on your own.
• I hope you enjoyed the lecture.
• Happy Gimping, and may the Wilber (that's the GIMP's mascot) be with you!

General GIMP Links

http://www.gimp.org/ - The GIMP's Homepage. Contains a lot of material and links.

http://gug.sunsite.dk/ - The GIMP Users' Group.

Books about the GIMP

http://manual.gimp.org/ - The Gimp User's Manual. A complete book that covers GIMP 1.0.x with some plug-ins that as of then, were not distributed with it. Still Pertinent as it is very detailed and explains a lot of good techniques.

http://gimp-savvy.com/BOOK/ - "Grokking the GIMP" by Carey Bunks. This book aims to explain the "10% of the tools that are used 90% of the time." I did not read it thoroughly but it looks very nice.

http://www.xach.com/gimp/books/ - Xach's GIMP Books List. Lists, Reviews and links to other books, some of them available only in paper.

GIMP Ports

http://www.gimp.org/win32/ - GIMP for "the other" operating system which is sold by a large Redmond-based company. (All right - it's GIMP for Win32).

http://www.macgimp.org/ - MacGIMP - Gimp for Mac OS X.

Gimp Development

http://www.gtk.org/ - The Homepage of the Gtk+ Toolkit which is the GUI toolkit used by the GIMP

RFC - The Future of the GIMP - This document explains the purpose of the 1.3.x and 1.9.x branches, and what is the workplan for the next releases of the GIMP.

http://www.gegl.org/ - The GIMP 'E' Graphics Library. A sophisticated image representation C library that will be used as the basis for GIMP 2.0.

Miscellaneous Links

The Lena Image (Yahoo) - contains links to information about the neophyte picture that became a standard in the image processing world.

The images used for this lecture can be found in this lecture images directory. This page is meant to detail their origins and their copyright notices.

lena.png

This image is available from this page:

http://www.eso.org/~ndevilla/lena.html - The Lena Story

It is part of a Playboy centerfold and copyrighted by Playboy corporation. The latter, however has given permission to use this particular portion freely for image processing research.

The image there is available in GIF format, but I converted it to PNG.

lena_color.jpg

I found this image here:

http://www.lenna.org/

It seems to be the colour original of the Grayscale one. Usually, the grayscale one is used for image processing research (probably because scientific journals are usually not printed in colour), but I decided to use the colour version here as well.

tiger_sitting.jpg

karakal.jpg

These images were found on the image archive of Gimp-Savvy:

http://gimp-savvy.com/PHOTO-ARCHIVE/

It is copyright-free, so you can do with it as you please.

g_anwar1.jpg

This is a photograph of Gabrielle Anwar, which can be found here:

http://www.candy-fans.com/anwar/index.html

It looks like it was scanned from a magazine, so it's almost surely copyrighted. However, distribution of copyrighted images on the Internet is so common, that I don't think one has anything to afraid of by using this image. Publishing it is a different story, though.

A-Little-Exercise-3.jpg

Fifteen-Cent-Ramen-1.jpg

These images are part of the Propaganda collection of desktop backgrounds.

They are distributed under the GNU General Public License (GPL), which I'm not sure if makes too much sense for images. They were created using the GIMP, but the scripts or the temporary images used to create them are not available as far as I know.