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Alpha Skills
Lesson 2

Selecting multiple areas (with Shift key)

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Now we're ready to select multiple parts.

As before in lesson 1, click the Magic Wand selection tool.

Then click the middle oval (yellow). Immediately you'll see the marching ants which show the limits of the selected region.

So now, what if we wanted to also select another circle, such as one of the purple ones?

Simply hold the SHIFT key down, and click the next desired color region.

Nte: if you left-click while holding Shift, the selection is added to the current selection. If instead you right-click with shift down, the selection is removed from the current selection.

Thus, if you accidentally shift-click on the wrong shape, or you change your mind, you can shift-click with the right mouse button on the same item to get it removed from the resulting overall selection. We'll explore this in further detail through lesson 3.

Storing a snapshot of the Alpha channel

You probably heard this expression before: "save often". And when you work with very complex selection masks you probably should save them to file. But even without going to that length, you will do yourself a great favor if you at least occasionally store a snapshot of the current selection mask seen in the alpha channel. As in the earlier lesson, we might want to pick these selected colored regions up into a custom brush. Before that however, we might want to keep a safe copy or at least a stored copy of this current selection, in case we mess it up and need to revert back to it.

One way to store this current selection (and only the selected regions, not the color of the pixels they contain!), is to use the 'Store Alpha' option from the alpha menu:

Alpha > Store Alpha...

A floating 'Alpha buffer' window appears, showing the selected pixels.

The selected pixels are showin in white.

The deselected pixels (i.e. those which are not chosen) are black.

This shows a mask of the innermost region and one of the outer concentric oval regions.  This stored alpha image only cares to show what level of selection each pixel has - it doesn't show what the current color of each pixel is. It doesn't really care. The only thing that the Alpha channel cares about is its own content, the 8-bit value representing the selection value.

And yes indeed, it is not a 1-bit value, which would only mean one of two options: either selected or not. With 8 bits (1 byte) per alpha value, each pixel actually can be more than either fully in or out of the selections. The alpha value can represent a selected pixel which is only partly selected. The mask can essentially be a gradient, shown as a grey value, anywhere from black to white. It just so happens that in this example, we used a tool and images which game us either fully selected or non-selected pixels only, so they're truly at their possible extremes: black or white.

Use the SHIFT-click trick again for yet another ring to be added to the selection. In this case the purple one just before the lastr (blue) outermost ring.

If you want to save this new resulting combined selection you can do so the same way as before, from the Alpha>Store alpha menu. Or, if you don't need the original stored mask anymore, use the prior floating window of the prior stored alpha mask, and click Get Alpha.

<< click to enlarge

This will grab a snapshot of the current alpha channel and store it into this stored  alpha buffer window.

What if you want to quickly select all ovals, without the white background?

You could of course SHIFT-click the ovals, one by one, all of them, until the only marching ants would show around the outer blue ovals.

Another and probably faster way is to think outside the box: Select the white background first, and then invert your selection.

menu: Alpha > Invert alpha

This is a commonly used trick, so there's a keyboard shortcut for it too: Shift-\ which is the vertical bar: "|"

Each time that you select the desired set of ovals, you may want to pick it into the custom brush and store that brush:

menu: Brush > Use selected as brush (i.e. turn the selection into a custom brush with transparency derived from the selection mask)


menu: Brush > Store/manage...

You might also want to store a snapshot of the original image which contains all concentric ovals, just in case you want to make other selections later.

menu: Image > Store image...

<< click to enlarge


>> next: part 3