GIMP Usage

Interface

• If you run the GIMP under KDE, start it in a new desktop: that's currently the only way I found to get all the GIMP windows to reappear when working with multiple applications.
• All tools have simple key shortcuts that are very useful, try to remember them.

Saving

• PNG format allows you to choose the compression level. The default maximal compression level acts strangely (artifacts on the image), so if that happens reduce the compression level to 5-6.

• Saving in PNG format from within a Python script has a risk of croping the image to the layer boundary. To avoid this, just resize the layer to the image first like this:

pdb.gimp_layer_resize_to_image_size(finalLayer)

• Saving in XCF format (or any other format) loses all your history / undo information, so be careful.

Exporting for the Web

• Firefox and other browsers apply overlays (a block with a color and an opacity) in a way incompatible with Gimp. The solution is to make sure in Gimp that there are no partially transparent pixels (eg, all the alphas are set to 100%). Else it can lead to subtle differences that will be visible.

Selection

• When making a selection, for example with the rectangle tool, you have to confirm it by clicking on it. Until this click, the selection is easily modifiable by using the sub-rectangles appearing
• A selection also usually shows the layer boundaries (which is a good thing).
• You can create a new selection based on a layer "contents" (parts that are not transparent). This is done by right-clicking on the layer and choosing "Alpha to selection".
• If you create a anti-aliased selection, this will apply to future operations. For example, a fill on an anti-aliased rectangle will not fill uniformly the rectangle with the color; the edges will get a smooth transition.
• Showing grids (and activating "Snap to Grid") will greatly help with the creation of rectangles, exact circles, etc. If you want to create a shape, the best is to save a temporary selection to a path, then add or remove new selections to this path.
• There is a shortcut for clearing the selection, which can be very handy.

Selections as masks

• In Gimp, a selection does not contain selected pixels and unselected ones. Pixels can be partially selected; usually most selection tools will create antialiased selections. This effects operations such as the paint tool and so on. It is best to think of the selection as a mask with multiple possible values. It's not a binary mask.

• To get a "binary selection", use Selection->Sharpen command.

Gradients

• One useful gradient is the FG to Transparent one. With it you can create glossy effects easily (picking white as foreground color).

Colors

• Some color operations (like color balance) won't work on desaturated components (black or white parts of the layer).

Layers

General

• To copy a layer from an image to another one, you can drag and drop it from the old image to the new one. You can also do a copy and paste (but then you need to anchor the new selection that you just pasted - this will create a new layer).

• You can obtain a layer offset with the attribute myLayer.offsets (which is a Python tuple). You can move the layer by setting the offset with the set_offsets method.

• An layer with the overlay type will modify the color of other layers beneath. Usually for the effect to work you must have a white background or layer below. One solution if you need a transparent background is to just duplicate the elements for which you need the overlay effect applied, and paint the copy in white (keeping alpha values intact).

Text Layers

• You can obtain the size of a text layer bounding box programmatically by calling gimp-text-get-extents-fontname. This won't actually create the layer.

Layer Masks

• Layer masks represent the opacity of the layer. Layer masks have their color values coded in gray tones (256 I think). White means full opacity, black means deletion (full transparency). Using layer masks you can for example easily create a fading effect. Just create a layer mask (initialized to white), paint in black the area you want to remove, and use a gaussian blur to make a smooth transition.

• One common effect is to select all the image, shrink it by a certain amount, invert the selection and then paint the selection in black before blurring it.

Tools

• The bucket fill tool, called via the PDB interface, does not seem to work without a selection, so make a selection first. If it is a newly created layer that you want to fill entirely, it is better to use:

pdb.gimp_drawable_fill(layer, FOREGROUND_FILL)

• There is an align tool that allows you to precisely move objects (enter their positions with numerical values). Unfortunately, the units are always in pixels which can be troublesome.