beginning_gimp.md

Beginning GIMP

An introduction to the GNU Image Manipulation Program or GIMP by Guillermo S. Romero.

Getting to Know GIMP

The Toolbox window is GIMP's main window and includes most of the functions you'll be using. Its menu bar isn't tied to any particular image. When you activate a tool, its options will be displayed below. If you accidentally remove it, get it back with File -> Dialogs -> Tools.

When you edit an image, you'll get an Image window. Near the bottom left, there's a QuickMask tool and the bottom right has the Navigation control.

GIMP usually starts with one dialog - the Layers dialog. This lets you edit, re-arrange, create, or delete layers on your workspace. You can view more dialogs from File -> Dialogs. Every dialog is movable and dockable. Menus also have a "tear-off", which lets you grab any menu and turn it into a window of its own.

You can toggle the Menu Bar via View -> Hide Menu Bar. Just right-click for a context menu to get the same menu.

Under Tool Options, be sure to change the Scaling to Cubic or Sinc. These use more CPU power, but have higher quality.

You can customize your keyboard shortcuts with Edit -> Keyboard Shortcuts....

Improving Digital Photos

This chapter covers some basics like opening files, scaling, cropping, and saving files. I'll note them very quickly:

1. To open a file, use File -> Open
2. Use Image -> Scale Image to scale
3. Use the Cropping tool in the toolbox for cropping
4. Use File -> Save to save. GIMP's file type is XCF

You've got a lot of options for formats. For JPEGs, there are a few quality settings with trade-offs. Make sure you have Show Preview checked. Remember JPEG has lossy compression so it loses quality every time you save. Some of the advanced options are:

• Optimize reduces file size without reducing quality, no reason not to use it
• Progressive lets users see a poor quality version when downloading the image before the high quality replaces it
• Smoothing, use restart markers, subsampling, and DCT method have good defaults that you shouldn't need to adjust
• Save EXIF data will save metadata about the image like camera information
• XMP is Adobe's metadata just like EXIF
• Comment is where you can include plaintext like your name

The Crop tool is relatively straightforward to use. Just select it from the toolbox and drag the area to keep. There are also some additional options like cropping the current layer only, fixed aspect ratios, or using numerical fields for cropping.

To control brightness or darkness, use Tools -> Color Tools. You'll have access to brightness/contrast dialogs and the levels dialog. The levels dialog lets you choose different channels - initially it's brightness but RGB channels are also available.

The Curves dialog is another way to adjust brightness, contrast, and colors. The horizontal bottom bar represents input and vertical left bar represents output. Click on any spot on the line and drag to adjust.

Some other adjustments you can make are Threshold (turns image into two tones with a given threshold), Posterize (turns into an artistic poster), Desaturate (removes color), Invert, and Auto (automated tools for photos). The Auto dialog can be broken down further:

• Equalize spreads the image colors out
• White Balance corrects color casts
• Color Enhance makes the colors more intense
• Normalize adjusts the exposure on underexposed images
• Stretch Contrast and Stretch HSV are like Normalize, but operate on the three color channels independently

The Image -> Transform menu lets you rotate or flip your image.

Sharpening can be done via the Sharpen or Unsharp Mask actions, both under Filters -> Enhance. Unsharp Mask is very simple to use:

• Radius effects distance that sharpening works
• Amount is how strong it is
• Threshold controls smoothness

Use Filters -> Enhance -> Red Eye Removal to remove red eyes.

Introduction to Layers

An image is made up of multiple layers, each on top of another. If a layer is completely opaque, it will cover all layers underneath it. Use the Layers Dialog at File -> Dialogs -> Layers or Ctrl-L to access them.

Some tools automatically create a new layer for you to work with, like the Text tool. Some tools require you to select a layer before interacting with your image, like the Move tool.

GIMP comes with a few effects for layers. Drop shadows are available at Filters -> Light and Shadow -> Drop Shadow.... GIMP creates a whole new layer that contains the drop shadow. This is useful, because you can re-run the effect with different variables. Use the eye button in the Layers dialog to toggle visibility and try out different parameters.

The chain link icon in the Layers dialog lets you link several layers together. This is useful if layers need to stick together - like a drop shadow and its subject.

When you copy/paste, GIMP creates a new "Floating Layer". This is a historical artifact that will be removed in future versions. Click the "New Layer" button to create a normal layer from the floating one.

You can select multiple layers by holding down the Shift key while clicking them. Use the Align tool to align multiple layers.

The Layer Dialog has many useful features:

• Layer Mode - the blending mode for the current layer
• Opacity - what % is transparent
• Keep Transparent - lock transparency so you can't draw on transparent portions of the layer
• Layers List - contains each layer
• Visibility Eye - toggles visibility
• Chain Link - locks two layers together
• Preview - preview of each layer
• Masks - hide portions of the layer

You can even right-click a layer for a context menu and more actions. For example, a useful one is "Add Alpha Channel" which adds a transparency channel to the current layer. That way, erasing leaves transparency instead of just plain white.

Drawing

The first rule of drawing is to create a new layer. Add one with a name, width/height, and transparent fill type.

The Pencil tool lets you draw sharp-edged lines. You can even set a brush size so the stroke is thicker. Use File -> Dialogs -> Brushes for more options. If you edit a brush, it becomes a parametric brush. You can adjust its radius, hardness, aspect ratio, angle, and spacing. Drawing also lets you set various modes: dissolve adds randomness, behind draws behind anything already present, color erase matches the current brush color and erases it. Use the brush tool for a soft-edged stroke. The Airbrush tool is for fuzzier edges. The Ink Pen tool imitates a fountain pen.

The Eraser tool is like the Pencil tool, except it erases what's on the current layer. You can configure it to be soft or hard edged.

GIMP doesn't have shape tools, but you can make any selection and fill it or stroke it. After making a selection, choose Edit -> Stroke Selection. You can also use any line drawing tool. Use Edit -> Fill with FG Color to fill your selection. For complicated selections, use the Bucket Fill tool.

You can even fill with patterns. A pattern is just a special type of image that's usually tile-able. GIMP can help with this via Filters -> Map -> Make Seamless. An even more advanced plugin called Resynthesizer does an even better job.

You can also fill with gradients using the Gradient Fill tool.

Selection

To toggle your selection, use View -> Show/Hide Selection or Ctrl-T. The Select menu has a lot of actions, like All or None (Ctrl-A and Shift-Ctrl-A) that are useful. Select -> Invert to invert your selection. There's also Select -> Float which is similar to copy/paste, except it keeps the pasted layer in the exact same location.

Also in the Select menu, you can:

• By Color - create a selection by color matching
• From Path - turn a vector path into a selection
• Selection Editor - edits your current selection
• Feather - makes edges fuzzier
• Sharpen - does the reverse
• Shrink and Grow
• Border - replaces a selection with a new one that follows the border
• Rounded Rectangle - to round off selections
• To Path - converts a selection to a path

Moving from the center of a selection will move the selection boundaries. Moving from the edge will change the boundaries. Dragging with Ctrl and Alt pressed will float and move the selection.

The Magic Wand tool lets you select by similarly colored regions that are touching.

You can create a path and turn it into a selection using the Bezier Path tool. It works best if you're zoomed in a lot. Keep adding points by clicking, when you're done Ctrl-Click on the initial point to close the path. Hit Enter to create a selection. When you're creating a path, click and drag a point to create a curve.

There are three modes with the Bezier Path tool: Design, Edit, and Move. By default you're in Design

• which is for adding new nodes. Switch to Edit to add/delete points. Check the Polygonal box if you want to toggle the handles on new points. Remove a point by Shift-clicking on it in Edit mode.

The Intelligent Scissors tool works like Bezier Path tool, except it uses the image's colors as a guide similar to the Magic Wand.

Every selection tool has four modes: Replace, Add, Intersect, Subtract. Use these to add/delete from your selection.

QuickMask lets you use painting tools to make your selections. It can be turned on by pressing the small button at the bottom-left corner of the image window. Paint with white to select, paint with black to hide. Use gray for fuzzy selections.

Romero walks us through a neat trick to blur out the background of a photo. He uses Curves to darken the background while keeping the foreground at the same lighting. Then he selects the foreground and Gaussian Blurs the background.

You can save your selection as a channel - channels are just black/white images that represent an aspect like QuickMask. Save a selection with Select -> Save to Channel. Right click a channel and use Channel to Selection to convert it back to a selection.

A Layer Mask is sort of like a permanent QuickMask. It represents the alpha channel of a layer. Right click a layer and use Add Layer Mask to get started. You can use Edit Layer Mask in the context menu to toggle between editing the actual layer and the mask. Show Layer Mask shows you the mask at full size.

SIOX or Simple Interactive Object Extraction is a new tool for automated foreground extraction. First, draw an outline around the object. Next, draw on the object you want to extract. Stay away from the parts you don't want. The tool will create a selection for you, hit Enter to accept.

Erasing and Touching Up

Dodge/burn are terms from film photography. By exposing certain parts for a shorter/longer time, you can get better details in light/dark areas.

The Dodge/Burn tool starts in dodge by default. There are three modes: Shadows, Midtones, and Highlights. By default it's Midtones which works well in most cases. If you want to brighten a very dark area, use Shadows. This lets you be a little sloppy with the brushing (it will only affect very dark areas).

Opacity makes the effect more subtle. Other configurations you can adjust are Fade Out, Jitter, and Exposure (for strength). Make sure you use one giant stroke - the tool won't re-dodge an area if you highlight over it during a single stroke.

Burning is the opposite. Unfortunately for digital cameras, these areas are usually so saturated they only record white. That means Burning can only be used for areas that are a little brighter.

The Smudge Tool drags a color and smudges it across your stroke. This is useful for smudging out unwanted objects in photos.

The Clone Tool can be used for similar cases but gives greater control. Instead of a stroke, you set a clone source and then paint onto the destination. Choose a source by holding down the Ctrl key.

The Heal Tool is similar to cloning, but it combines texture from the source layer with color/lighting of the destination.

The Blur/Sharpen tool is like a mini blur/sharpen filter via brush.

Filters and Effects

GIMP's effects are located in the image window's Filter menu or the Toolbox window's Xtns menu. Filters operate on the existing image and Xtns will create new images.

Most filters are implemented via GIMP plug-ins. Whereas tools can respond to clicks in the editor, plug-ins can only be configured via their own dialog.

Whenever you run a filter, a new action item appears in the Filter menu called Repeat Last. This does exactly what it sounds like. Re-show Last will re-open the filter's dialog. Reset all Filters will reset all dialogs.

Here's a quick overview of some filters:

• Gaussian Blur - simplest blur
• Motion Blur - blurs in a single direction
• Selective Gaussian Blur - gaussian blur that keeps edges sharp
• Tileable Blur - blur that can later be combined for tiles
• Sharpen - for sharpening images
• Unsharp Mask - advanced sharpening
• Antialias - makes jagged edges smoother
• Despeckle - removes noise from images
• Destripe - removes vertical lines left by scanners
• NL Filter - combination of smoothing, despeckle, and destripe
• Distort - category of filters creating fun effects
• Light Effects - category of filters dealing with light and shadows
• Noise - adds noise to image
• Edge Detection - creates outlines from images
• Aristic - category which turns photos into artworks
• GIMPressionist - crown jewel of filters, includes large amount of options for many artistic effects
• Cloud - adds cloud pattern

The Xtns menu has many scripts to create new images. This is especially useful for creating ready-to-use buttons, logos, and images.

Color

This chapter includes a review on color (RGB, CYMK, additive vs subtractive) and color depth (8-bit or 256 colors, 24-bit or 8-bit per channel, 32-bit includes alpha channel).

HSV stands for Hue, Saturation, and Value. GIMP uses 0 to 360 to represent hue for degrees. Saturation ranges from 0 to 100. Value is the brightness.

A quick way to adjust color is via Colors -> Hue-Saturation. Another way which deals with shadows/highlights is Colors -> Color Balance.

For greatest control on adjust color, use Levels or Curves.

To change a photo to black/white, use Image -> Mode or Colors -> Desaturate.

Image -> Mode -> Decompose or Filters -> Colors -> Decompose will let you see your image in different channels. Remember the human eye is most sensitive to the color green. Decomposing is particular useful for creating selections. For example, get the blue channel with a Threshold and QuickMask to select it.

Colors -> Channel Mixer gives complete control over colors and luminosity.

Some popular effects for photos:

• "old style" via Filters -> Decor -> Old Photo or Script-Fu -> Decor -> Old Photo
• Colors -> Colorize then choose a Sepia tone
• Add a new layer with fill color #a28a65, change its mode to Color

If you need to set thresholds (for example, make a picture completely black and white), use Colors -> Threshold. This works great for scanned books where there's usually a shadow near the binding.

Advanced Drawing

Some Layer Mask tricks:

• you can use a gradient from white to black to fade out a layer
• make a selection, feather it, change to a layer mask to get a faded out mask
• make it fuzzier with a Gaussian Blur on the mask

GIMP has several blend modes for layers:

• addition - adds color of both pixels
• subtract - removes color of front pixel from back pixel
• difference - absolute value of subtraction
• multiply - generally makes the image darker
• divide - generally makes the image white out
• burn/overlay - similar to multiply
• dodge/screen - similar to divide
• hard light - combination of multiply and screen
• soft light - tends to soften edges and dim colors
• darken/lighten - shows only the darker/lighter of two pixels
• grain extract - gives desaturated look
• grain merge - adds texture to the image
• hue/saturation/value - only takes one of the HSV from the front pixel, the other two are taken from the back pixel
• color - takes hue/saturation from front, keeps brightness from back

The easiest way to create a brush in GIMP is to copy the image and access it via "Clipboard brush" in the Brush dialog. Brushes can be grayscale or colored. A brush is saved with the .GBR file extension. If you save a GIMP file with multiple layers with the .GIH file extension, it becomes an animated brush with each frame represented by a layer.

GIMP comes with its own built-in patterns. You can create your own patterns the same way you can create a brush. The easiest is to create a "Clipboard Pattern". Filters -> Map -> Make Seamless is a useful filter for creating your own patterns.

Advanced Composition

Romero shows us how to change the color of a car:

1. select the parts of the photo you want to colorize
2. copy the selection and paste into a floating layer
3. now fill the layer with the color you want
4. experiment with layer modes, in this case Color will work nicely
5. this works with two-tone colors or gradients also
6. some modes look strange at first, experiment with the opacity

This technique loses details easily, like the fur on a squirrel. In that case, you can keep the texture by using Grain Extract or Grain Merge modes.

Layer modes can be used to improve photos also. To brighten a dark photo, duplicate it and change the duplicated layer's mode to Screen. Adjust the opacity until it pleases you.

Self-composing with Overlay or Hard Light will turn a flat image into one with more bright colors. Overlay or Burn works well to see through fog or haze. You can even stack multiple layers with Overlay.

If you're compositing two images - like texture onto an original image - Soft Light works very well. Overlay is good if the bottom layer is dark. Use Screen for a lighter effect.

Grain Merge is great for adding texture to an image.

Plug-ins and Scripting

GIMP's architecture makes it easy to add plug-ins and scripts. A plug-in uses the C language and compiles with GIMP whereas scripts don't need to be compiled in.

See Xtns -> Plug-in Browser for the plug-ins already installed. This is also useful to search for functionality you might not already know exists.

To find other plug-ins, you can search through GIMP's official plug-in registry at http://registry.gimp.org or just do a Google search.

You can write GIMP scripts in any scripting language. GIMP has its own language called Script-Fu which is a dialect of Scheme.