Re: Single window interface
For those not familiar with the GIMP: Imagine clicking on the left menu item and then the first entry in it and a window appears. Do the same for each entry in the left menu so you have lots of separate windows that look like different applications. Then go for the second column in the menu and create more windows. Repeat for each column. You now have some idea of what the GIMP is like when it starts up. It is not that simple as some menu items open multiple windows and if you close some windows it can take ages to find the icon in another window you closed that brings the one you want back again. To make life more fun, when you try to do something impossible a new window pops up with an error message and all the other controls lock up until you get rid of that window. Several years ago GIMP developers made a great effort to not hide error message windows behind other windows.
This utter insanity accounts for much of the steep learning curve for using the GIMP. Years ago I made an effort to actually read the documentation and discovered there is an off switch. The GIMP can behave as if it has one window divided into non-overlapping sub-windows for different tasks. These sub-windows can be closed so that there is a reasonable amount of space to look at the image you are working on. Sometimes you can even find a way to bring them back again when needed.
I am sure some people love the multi-window interface and are sharpening the torches and lighting their pitch-forks ready to lynch me for my heresy. Please just accept that the vast majority of the population find the multi-window interface far more trouble than any possible benefit.
Here are the other major reasons why the GIMP appears not to work:
You are not working on a single picture. Instead you are working on several pictures of different sizes layered one above the other. Most of the GIMP's tool only work on the currently active layer. Find the layers window. Learn how to make a layer visible and active. Layers consist of bands, usually three: red green and blue. The lower layers will be hidden if an upper layer does not have a transparency band (look for the jargon 'alpha channel'). Find out how to add one and how to draw with invisible ink (try the eraser).
Most tools only work on the select area. A selection is another band covering the whole image that decides whether a tool affects each pixel completely, not at all, or only partially. Find the selection window and work out how to change the default behaviour (a new selection replaces the previous one) to one of the other possibilities: unite with the previous selection, select the intersection of two selections, subtract the new selection for the current one. "Select all" does what a programmer would expect when in subtract mode (subtract everything from the selection so nothing is selected and all the tools do not work).
Some tools create a new temporary layer that must be anchored to become functional. I assume this extra step was created to confuse beginners.
The GIMP is an amazing tool that can do all sorts of beautiful things. Budget a of day frustration reading the instructions and trying to get simple things to work and another day to do one tricky thing. Then expect anything new and complicated to take an hour to find a tutorial and another hour to get it to work. Once you get the hang of how the GIMP works, the insane troll logic to the user interface begins to make a kind of sense and the buttons start doing what you expect (or your expectations change to match insane troll logic).