Masking with alpha + alpha over = strange edge colors #35716

New Issue

Tee Vee · 2013-06-12T09:04:19+02:00

Tee Vee commented

2013-06-12 09:04:19 +02:00

%%%--- Windows 7, 64 bit, 560Ti ---

- Blender 2.67b, others unknown ---


- ---

Convert premult makes no difference (on or off).

%%%

%%%--- Windows 7, 64 bit, 560Ti --- - Blender 2.67b, others unknown --- - --- --- Convert premult makes no difference (on or off). %%%

Tee Vee commented

2013-06-12 09:04:19 +02:00

Changed status to: 'Open'

Tee Vee commented

2013-06-12 09:09:30 +02:00

%%%Sorry, accidently pressed submit...

Description:
See attached images...
This node-setup with masks and alpha creates colored edges, but shouldn't alpha + alpha stay alpha?
Now the top background images is used as the alpha background of the bottom mask.
This causes edge anomalies in similar setup. Using an extra alpha over node, just switches the problem (other mask gets colored edge).

Convert premult makes no difference (on or off).
Render result is the same

%%%

%%%Sorry, accidently pressed submit... Description: See attached images... This node-setup with masks and alpha creates colored edges, but shouldn't alpha + alpha stay alpha? Now the top background images is used as the alpha background of the bottom mask. This causes edge anomalies in similar setup. Using an extra alpha over node, just switches the problem (other mask gets colored edge). - Convert premult makes no difference (on or off). - Render result is the same %%%

Sergey Sharybin commented

2013-06-12 12:54:43 +02:00

%%%Please do not attach srceenshots, they are not as much helpful for troubleshooting. .blend files are much more descriptable and saves tons of time reproducing the issue.

In your case it's not actually a bug, just a specific of how set alpha works: it replaces alpha channel. And here's the trick. Images in compositor are expected to have premultiplied alpha and Set Alpha node leaves images in a wrong multiplied state (well, not strictly wrong, just not usable for alpha-over an some other operations). So after setting alpha channel you need to convert alpha from key to premul using converter node. Using Convert Premul in alpha over node could also help.

Thanks for the report, but it's not a bug.%%%

%%%Please do not attach srceenshots, they are not as much helpful for troubleshooting. .blend files are much more descriptable and saves tons of time reproducing the issue. In your case it's not actually a bug, just a specific of how set alpha works: it replaces alpha channel. And here's the trick. Images in compositor are expected to have premultiplied alpha and Set Alpha node leaves images in a wrong multiplied state (well, not strictly wrong, just not usable for alpha-over an some other operations). So after setting alpha channel you need to convert alpha from key to premul using converter node. Using Convert Premul in alpha over node could also help. Thanks for the report, but it's not a bug.%%%

Sergey Sharybin commented

2013-06-12 12:54:43 +02:00

Changed status from 'Open' to: 'Archived'

Sergey Sharybin closed this issue

2013-06-12 12:54:43 +02:00

Tee Vee commented

2013-06-12 13:15:18 +02:00

%%%Sergey, thanks for clearing this up.
Couldn't find a solution for this anywhere.
Will try what you suggest.
%%%

%%%Sergey, thanks for clearing this up. Couldn't find a solution for this anywhere. Will try what you suggest. %%%

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Masking with alpha + alpha over = strange edge colors #35716