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Painting on Alpha

intro - part 1 - part 2

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Part2: Get buffer (into stored alpha)

When you click the "Get buffer" button in the stored alpha panel, it grabs a copy of the current image buffer. It then converts this to a greyscale and makes it the stored alpha content. In other words, it's not in the active alpha channel yet, but it is in the stored location in memory, as a greyscale image.

Here's an example: a few lines were drawn with a simple brush, using various colors. Some of these colors appear dark as a greyscale, others are pretty light.

Notice that the white color from the image buffer sets the white color in the stored alpha's greyscale image.

Given that white indicates 'selected' whereas black indicates 'not selected', here's what we can expect when you now click on 'Replace':

Since the marching ants (dashed lines) around the image's border is showing there, indeed, the white area is within the selection. The dashed lines around the dark painted lines represent another border region of the selection. Thus, the lines themselves are not selected, onl the while background appears to be selected.

If you don't want the background selected, but instead want the colored fat lines selected, just inverse your selection, then click replace.

Now the colored lines are selected, not the white background.

Notice however that it appears as if the lighter colors such as cyan did not get selected. They don't appear to show the marching ants dashed lines.

Keep in mind that the marching ants doesn't designate all pixels that are selected, because a selection is not just a yes or no thing. You can have a pixel that is fully selected, or one that is not selected at all, but it can also be many things in-between. The selection value is not a single bit of yes or no. it is an 8-bit value, a byte. The alpha channel is 8 bits deep, just like each of the R, G and B color channels. Thus, a selection is a weight factor, from 0% selected to 100% selected. (in terms of byte vaues: 0 to 255, or in terms of normaloize value: 0.0 to 1.0)

So what does the marching ants show, which value is it there along the dashed lines? It is the threshold between 0% and 100%, right in the middle, where the alpha value is at 50%

Here's a way to visualize it: use a large smooth airbrush, and paint with it, in various colors, some dark and others light. Then grab the image into the stored alpha, and replace alpha with that. You will see the marching ants 'inside' the colored tube, because that's where the transition of 50% selection is found, not at the outer-most region of the tube.

You will also notice that in some cases, the marching ants appear broken up (lite gren example), or may be even completely missing (yellow example). This makes sense since these colors are very light and when converted to greyscale they may never reach 50% grey darkness.(medium grey). Thus, while they're partly selected based on the lite grey, they're not selected enough to reveal a marching ant at the 50% limit.

To adjust the selection so that the lite areas are also fully selected, you can adjust alpha, which means changing the contrast in its greyscale representation.

Select:  Alpha -> Adjust alpha...  to work with COntrast, Value, Brightness or Gamma and find a better selection mask.

Here's what you may get:

While you're at it, try the other alpha manipulation tools:

  • Blur alpha
  • Grow alpha
  • Shrink alpha
  • Transform alpha
  • Flip alpha

These can be useful in creating the right selections.

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