GIMP Tricks for Isolating Parts of Images - page 2
Erasing to Alpha
When you click in the image, don't release the mouse button right away. Instead, take a look at the dotted outline that shows the selection boundary (Figure 3).
Drag the mouse down and to the right to select more, or up and left to select less. In Figure 3 I'm trying to select the mostly-white background so I can delete everything but the feathers, and I'd need to drag right and down to select more of the white background.
Tune selections with the Quickmask
Most selection tools will get you close, but they aren't perfect. For instance, using Select by Color on the feather photo, if I want to select all the white background, I'll also select some of the white parts of the feather quills.
That's where the Quickmask comes in. Click on that unobtrusive little button at the lower left of the image window (Figure 4) and GIMP switches to a mode where everything that isn't selected is red.
Now you can paint with tools like the Paintbrush tool. Paint white to add to the selection -- like that upper-right corner that should be selected and isn't. Paint black to subtract from the selection, like those white (and therefore selected) parts of the feather shafts. When you're happy, click on the Quickmask button again to go back to normal mode and see the "marching ants" selection outline.
Whatever method you use to make your initial selection, once you've
tried the Quickmask you'll probably use it to fine-tune every
selection you make.