BitmapData masking: how to merge 2 alpha channels?

How can I apply an alpha mask from one bitmapdata to another? In other words, I want to merge the alphas, not simply copy one alpha channel to another bitmap.
Here's an illustration:
Notice that both the triangle and the ellipse have alpha channels, and the result is an intersection of these alpha channels. The RGB channels from the second image are used, and discarded from the first.
I've looked into using BitmapData/merge(), draw(), copyChannel(), but they don't seem to do what I want. Anyone?
Cheers.
-Aaron

Aha, looks like the answer was right under my nose: copyPixels() can do exactly this:
If you include the alphaBitmap and alphaPoint parameters, you can use a secondary image as an alpha source for the source image. If the source image has alpha data, both sets of alpha data are used to composite pixels from the source image to the destination image. ThealphaPoint parameter is the point in the alpha image that corresponds to the upper-left corner of the source rectangle. Any pixels outside the intersection of the source image and alpha image are not copied to the destination image.
Cheers.
-Aaron

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<bump>

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kopo88 wrote:
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How should I edit an alpha channel after saving?

I have a series of files in which I improperly modified the selection set prior to saving the selection, and now around the intended viewable area there is a hazy artifact that seems to correspond to where I manually edited the selection using Quick Mask (prior to saving).
In other words, within the area of the alpha channel that should be invisible, there is actually a visible, unwanted grey-ish artifact, and I think it's related to how I was using the brush to edit the selection in Quick Mask before saving.
Now I am unable to edit the alpha channel, although Adobe's own website says that in order to edit an alpha channel, all I need to do is select the alpha channel (which I am doing) and then paint at 100% black or white with a brush to either include or exclude parts of selection set.
When I choose the brush and start painting at 100% black or white, nothing happens and I can't figure out what is happening.
Any help would be greatly appreciated.
The exact steps that I'm taking are below:
1) click on alpha channel (visible and ready for editing), all other channels only visible not active.
2) selecting the brush at either 100% black or white
3) attempting to paint to alter the alpha channel (I've tried painting on the actual alpha channel in Channels, and I've tried painting on the alpha channel as it appears in Layers, and neither works for me).
CLARIFICATION:
I actually am able to "edit" the alpha channel HOWEVER I cannot remove the hazy artifact. I can use 100% black or white to remove the pink/light red areas on either the actual alpha channel (in the Channels area) or as it appears in Layers, but I cannot remove the areas that are grey-ish (well I can paint them white but I can't paint them black, i.e. fully pink).
A teacher showed me a quick fix process that basically involves activating the alpha channel, inverting the selection, and then deleting everything outside of the alpha channel, and then resaving the file. That worked at one point, but now that quick fix is not working either (I am just ending up with a completely visible, white space around the intended visible area, rather than an invsible area).
EDIT (07:22 3.20.13) Also, if you look at what Photoshop is registering as the actual alpha channle, it doesn't match with what I'm seeing above, that is the black and white alpha channel appears to be as expected, and there is not the weird distortion that I'm seeing above in the pink/grey image.
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I greatly appreciate your help with this.
However, after adjustting the black from 99 to 100%, and double checking that the white is fully zero (right now I can't find any files with the white set to anything but "0," so I don't know why some of my screenshots showed that), I am still unable to use black to contract a selection, or white to erase unwanted areas of the alpha channel (this specific post's problem).
So, I'm completely baffeled and am wondering if there is anything else that could be accounting for these results (i.e. the black and white paint not producing the expected results).
Thanks again,
dk
EDIT:
I attempted to make another alpha channel, this time without any manual adjustment of selection via the paint brush. Instead, I used the selection modification option of Contract and used it to contract the selection.
SAME problem. The area that I contracted using Contract, (never painted), is once again displaying a slight grey artifact in the area that is supposed to be completely transparent.

BitmapData masking: how to merge 2 alpha channels?

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