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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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How to change the brush size

Now that we're going to paint over the stem in order to add it to the selection, we may need a smaller brush size, so let's see how to easily change it.

You can open brush settings by clicking the brush image in the upper left of the Tools panel. (not the brush tool below it, just the brush image, that larger icon in the upper left corner). Or you can hit 'o' for options.

Select the Custom tab and be sure to check the option checkbox: Allow custom brush transform. Indeed, the pre-defined brushes that paint on alpha are 50-pixel large, which is too large for an 'internal' brush type. Those are limited to 31 pixels wide or high. Thus, we need to enable the custom brush transform option. By default, it is disabled because on slower systems it can be prohibitively slow to transform (such as rotate or scale) large images.

Once you have checked this box, it is possible to use the slide to adjust the size. If you enable brush pressure from a tablet, you can also control the size with pressure. The Main tab also has options to map the size to speed, for example to get bigger as you speed up.

With a smaller brush size in effect nwo, you can paint the inside of the stem and gradually paint towards the edge of it.

In no time you'll have the stem selected too.

Note that if there are sudden kins, hard corners or inflection points such as where the stem detaches from the apple, you'll need to briefly make the brush size very small and zoom in. This is where it comes very handy to have practiced your viewing control skills ;-)

Taking a look at the Alpha channel - Store alpha

As usual, you may want to grab a snapshot of the current alpha mask. Select

menu: Alpha > Store alpha...

The alpha channel will look similar to this now. White means 'selected' or 'enabled'.  The background, in black, is not selected.

In the alpha channel, the area outside of the alpha is all black, meaning 'not selected'. However, the RGB channels of the original image are far from black. They still contain the original image.  If you want to erase the background to black, one way to do so is to invert the alpha and clear the selection. Then invert the alpha back to the prior where the apple is selected.

Click on 'Invert' inside the stored alpha image, and click 'Replace' to replace the current alpha with this stored and inverted mask.

The Right-click the erase tool ('X') and select 'Clear selected to black'.

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