Figure 3.1.  Layers dialog showing a floating selection.

How to tell: If there is a floating selection, many actions are impossible until it is anchored. To check, look at the Layers dialog (making sure it's set to the image you're working on) and see whether the top layer is called “Floating Selection”.

How to solve: Either anchor the floating selection, or convert it into an ordinary (non-floating) layer. If you need help on how to do this, see Floating Selections .

Figure 3.2.  Unstuck show selection menu

In the View menu, make sure that "Show Selection" is checked.

How to tell: If this is the problem, merely reading this will already have made you realize it, probably, but to explain in any case: sometimes the flickering line that outlines the selection is annoying because it makes it hard to see important details of the image, so GIMP gives you the option of hiding the selection, by unchecking Show Selection in the View menu. It is easy to forget that you have done this, though.

How to fix: If this hasn't rung any bells, it isn't the problem, and if it has, you probably know how to fix it, because it doesn't happen unless you explicitly tell it to; but anyway: just go to the View menu for the image and, if Show Selection is unchecked, click on it..

Figure 3.3.  Unstuck select all

Click “All” in the Select menu to make sure that everything is selected.

How to fix: If doing this has destroyed a selection that you wanted to keep, hit Ctrl-Z (undo) a couple of times to restore it, and then we'll figure out what the problem is. There are a couple of possibilities. If you couldn't see any selection, there may have been a very tiny one, or even one that contained no pixels. If this was the case, it surely is not a selection that you wanted to keep, so why have you gotten this far in the first place? If you can see a selection but thought you were inside it, it might be inverted from what you think. The easiest way to tell is to hit the Quick Mask button: the selected area will be clear and the unselected area will be masked. If this was the problem, then you can solve it by toggling Quick Mask off and choosing Invert in the Select menu.

Figure 3.4.  Unstuck layer invisibility

Layers dialog with visibility off for the active layer.

How to tell: The Layers dialog gives you ability to toggle the visibility of each layer on or off. Look at the Layers dialog, and see whether the layer you are trying to act on is active (i.e., darkened) and has an eye symbol to the left of it. If not, this is your problem.

How to fix: If your intended target layer is not active, click on it in the Layers dialog to activate it. (If none of the layers there is active, the active drawable might be a channel -- you can look at the Channels tab in the Layers dialog to see. This does not change the solution, though.) If the eye symbol does not appear, click in the Layers dialog at the left edge to toggle it: this should make the layer visible. See the Help section for the Layers Dialog if you need more help.

Figure 3.5.  Unstuck layer transparency

Layers dialog with opacity set to zero for the active layer.

How to fix: Move the slider.

How to tell: In GIMP, layers don't need to have the same dimensions as the image: they can be larger or smaller. If you try to paint outside the borders of a layer, nothing happens. To see if this is happening, look for a black-and-yellow dashed rectangle that does not enclose the area you're trying to draw at.

How to fix: You need to enlarge the layer. There are two commands at the bottom of the Layer menu that will let you do this: Layer to Image Size, which sets the layer bounds to match the image borders; and Layer Boundary Size, which brings up a dialog that allows you to set the layer dimensions to whatever you please.

How to tell: GIMP can handle three different color modes: RGB(A), Indexed and Grayscale. The indexed colormode uses a colormap, where all used colors on the image are indexed. The color pickerin GIMP however, let you choose RGB colors. That means, if you try to paint with a different color than it is indexed in the colormap, you end up in very undetermined results (e.g. it paints with the wrong color or you can't paint).

How to fix: Always use the RGB Color mode to paint on images. You can verify and select another color mode from the Mode menuitem in the Image menu.