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Alpha Skills Lesson #8
Isolating objects from multi-color backgrounds to make a custom brush
Painting on Alpha gives fine-detail control

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Painting on Alpha

To select the apple, including the stem, and isolate it from the photo background, we'll paint the selection on it. Simply select one of the pre-set "Paint on alpha" brushes.

Right-click on the brush tool to select the pre-defined brushes:

< click to enlarge

There are only 3 pre-defined (built-in) brushes set for painting on alpha, but of course you can easily create your own, use your own images or make other custom brushes, and enable the "PostFX" in the brush settings tab. ('o' for options). There you can enable the 'Alpha' mode, so it effectively paints on alpha while showing the current image.

Do a few tests you get an idea of how big the brush will paint and use Ctrl-Z or 'u' to undo. Then draw a brush stroke along the edge of the apple as shown here.

Don't try to cover the whole circle.  Let go after a short brush stroke.

The initial paint color (e.g. black) disappears and the painted brush stroke appears as a selection in the alpha channel. You can see the marching ants along the outline of the selection. Actually, to be precise, the marching ants are not showing where the edge or selection is. They're indicating the 50% selection mark, which is more inward of the selection if you had used the soft brush. It's good to use a hard brush here, so as to control the selection and make it an all-or-nothing selection type.

You can paint another brush stroke nearby, partly overlapping the current selection, extending it to the other side. Don't stop at the stem, just paint over it. Stay inside of the apple though:

Keep adding painted selections along the apple's egde and soon you'll have the whole circle closed. It's still 'hollow inside though, i.e. not selected uey, and the stem is not selected yet either.

Now, paint inside of the paple and select it all.

And here it is a few seconds later, with the whole apple selected, except for the stem.


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