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Use the Alpha Add layer blending mode.
Thank you for your answer. It solved my problem.
I'm having the same problem. I created a background with text in a pre comp. The Pre comp has 2 layers, a background layer and the text layer. I placed the pre comp in the main comp and in the main comp I drew a mask on the pre comp that is the seperating line of a broken image (ie. like glass breaking into 2 jagged pieces) I duplicated the pre comp and inverted the mask. I changed the top pre comp to Alpha Add mode but I still have a seam where the 2 pre comps meet. What am I doing wrong???
If you're using alpha add on both layers then that's as good as it gets without shifting. I'd probably just do a transition at the point where the pieces come together.
I had not looked at his picture blown up as he suggested in the original post. My situation is different from what he is experiencing. I had one layer that I placed a mask on, duplicated the layer and inverted the mask. Where the two mask meet there is a halo. According to the procedure in Mark Christiansan book, setting the layer on top to Alpha Add blending mode is supposed to make the two layer appear as one, without any indication of the mask edge. When there is nothing below the masked layers you don't see the fracture line, but if there is something below, you do. The only way I was able to overcome this was to nudge one of the masks towards the other by one pixel, then I did not see any fracture where they met. However, this nudging workaround is only good if you have 2 layers that are next to each other. In my project it was a Heart breaking in half.
On a mac CS 5.5
Alpha add must be set on both layers.
If it were my project I would have 3 layers. The first with the complete hart, then when it's time to break it in half I would split the layer and draw the mask, then duplicate the masked layer, invert the mask, then animate position of the two masked layers. All done, all thru, no problems.
You're just not thinking about the easiest way to solve the problem. Visual effects is a problem solving business. Those that are the best at it figure out the easiest solutions.