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More GIMP Tricks for Doctoring Images (part 2)
Many Ways to Make Selections
November 11, 2010
In a previous article, I wrote about some basic tricks for cutting objects out of your photos. But you can get a lot more flexibility and control by adding a few advanced techniques.
Suppose you've spent a lot of time carefully selecting a figure in a photo (Figure 1).
You use the Quickmask to paint carefully over those thin legs and antennae. And then you start wondering -- would it look better to include the flower when I paste it onto my stationary? How do you find out?
Saving your selection
The first goal is save your selection so you don't lose it: Select->Save to channel. Once it's saved, you can see it in the Channels dialog, Windows->Dockable Dialogs->Channels (Figure 2).
Initially the channel is named "Selection Mask copy". Double-click the name and change it to something more descriptive, like "butterfly selection". If you save the image in GIMP's native XCF format now, you'll save that channel along with the current selection. You can get the butterfly selection back at any time by clicking on it in the Channels dialog, then clicking the red rectangle button with the tooltip "Replace the selection with this channel".
Now it's safe to start changing the selection, knowing you won't lose all your earlier work.
Adding to and subtracting from selections
How do you take your current selection and extend it to include the flower?
Of course, you could use the Quickmask, painting white where you want more selection, black where you want less. But what if you want to use another tool, like Select by Color, to add the purple flowers without losing the white butterfly?
For that job, each of GIMP's selection tools includes a set of Mode buttons in their tool options (Figure 3).
The default mode is Replace, to replace the current selection with the new one you're about to make; the other buttons let you Add to the selection, Subtract from it, or Intersect with it.
So choose Add mode in Select by Color, then click on one of the purple flowers and drag down-right until the outline seems right. You can even repeat the process: once for the light purple flower tops, a second time for the dark purple stems (Figure 4), and repeat as necessary.
Clean up the details afterward with the Quickmask.
You probably won't use Intersect very often, but Add and Subtract are very handy. Just one problem with them: it's easy to forget you're in Add or Subtract mode, then come back later and wonder why your selection tool isn't working quite right. So get in the habit of setting Mode back to Replace when you finish with a selection tool.
Select by Hue, Saturation or Other Attributes
Sometimes you try to use Select by Color or Fuzzy Select, but it's just not picking the right part of the image. Recent versions of GIMP added a wonderful feature: you can select by Hue, Saturation or Value, or by the amount of Red, Green or Blue in the image (Figure 5).
The default, Composite, tries to "do the right thing", but for some images you can get a much better result by being more specific.