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Alpha Skills
Lesson 3

Adding and removing to/from the current selection

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Sometimes Faster: Invert alpha

In some cases, it may be faster to select something else and then invert the selection to actually get to the selections you want.

For example, in this case, instead of selecting the two eyes and the mouth, you could have done a single click on the rest, i.e. the white background.

After selecting the background, you'll see the marching ants around each eye and the mouth, but also around the outer edges of the entire image.

<< click to enlarge

After this one-click selection of the background, you have just one thing left to do: to invert the selection, i.e. to get it to select the counterparts: Invert alpha

Drawing on Alpha: beyond the magic wand

The magic wand is certainly by far not the only way to make selections. The magic wand depends on the colors in the selected area, as it looks for same or similar colors to where the search started when you clicked a pixel with it. Any neighboring pixels with same or close-enough similar coloring (based on tolerance and anti-aliasing checkbox options) will be included by the magic wand's selection.  However, you can also use many other ways to make a selection: for example, you can simply paint the selection, by painting on alpha. Or you can use selection tools like rectangle to alpha or oval to alpha, which draw their shapes into alpha, hence into the selection.

There again, you'll want to use the SHIFT key to add to the current selection rather than replace and loose the prior selection.

For example, start with a black background. Click the alpha Rectangle tool. Note the tooltip: Using Shift to add or subtract. That indicates that you can use shift for either. (adding or subtracting). You then also need to choose between drawing the shape with the left mouse button or the right mouse button. When doing the selection with the left button (while SHift is down), it gets added to the existing selection. When doing the selection with the right button (while Shift is down), it gets subtracted (removed) from the current selection.

Draw a first rectangle, and then press a hold shift, and draw another next to it, with the left button.

Let's store the current alpha channel's content:

menu: Alpha > Store alpha...

The stored alpha buffer shows the eyes (two squares) as fully white, i.e. fully selected, whereas the rest is black and thus those pixels there are not selected.

Now draw another shape, i.e. a smaller rectangle, inside one of the two bigger squares. DO THIS WITH THE SHIFT KEY DOWN, AND USING THE MOUSE'S RIGHT BUTTON!

If you use the right button and Shift key was down, it will remove the selection from the bigger shape. You can see that in the stored alpha window after clicking 'Get alpha':

Repeat this for the other eye: Right-click-and-draw while SHIFT is down, to also remove a section of that other eye.

(click to enlarge)

Notice that we also can remove an overlapping portion: For example, draw your rectangle inside the mouth to extend below the bottom edge of the mouth.

You can now for example erase the content of the selected pixels to a different color. Set the primary color for example to orange, and right-click the erase tool (the big X), then select "Clear selected to primary"


  part 1  -  part 2  -  part 3