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The Morehead Planetarium & Science Center Production Blog

Eureka! How to stitch alphas

by Peter Althoff

August 19, 2009 | No comments | Production


We’ve been running into some issues stitching together frames that have varying opacity. Namely, clouds and particle systems. Originally when using a sequence of PNG’s we’d find ourselves having a seam around the stitched boarder. This was due to the alpha being added together at the seam line creating a 1-2 pixel boarder that had a combined opacity greater than the pixels around it.

badseams2

I realized the problem came with having the stitching software not being able to understand the alpha channel, and that if I controlled that myself rather than leaving it to the code I could remove this variable from the equation. So by out putting an opaque color pass and an opaque alpha pass I could use one to cut out the other as a luma matte in after effects.

opaque_color1

opaque_alpha1

aftereffects_menu1










 

Thus, removing the seem issues, and having an alpha channel that could be independently manipulated.

noseams1

True this creates more files, but really doesn’t increase render time, as the alpha information is calculated in a render anyway and either mixed into a 32 bit frame, or simply discarded in a 24 bit frame. Though if you select Alpha Split in the .tga File set up when outputting, rather than discard the information it will save it as “A_[filename].tga” giving you the two opaque frames you need for stitching.

alpha_splitsetup

 

Hope this is helpful, I know for us this is a great discovery, and kind of a “why didn’t I think of that before”, moment. I also realize that stitching isn’t the best solution, but sometimes is necessary.

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