I just discovered the semi-flatten filter and wondered if something similar could be made for rgb images, or maybe I've overlooked another filter or technique that does the same.

I just thought this could be incredibly handy when rendering images from a solid colored background.

ie..
1. color pick background
2. color to alpha (picked color)
3. new layer underneath current (picked color)
4. semi-flatten

I understand that any fully transparent areas within the render created from the "color to alpha" filter will need to be filled in manually, but it still seems like a better alternative than some other techniques I've tried.

I don't really understand what problem you are trying to solve. Semi-flatten exists because GIF has no partial transparency, so border pixels that would use partial transparency are created as non transparent but with the right color mix (for the intended background color...). In other formats, either there is partial transparency support, so this isn't needed or there is no transparency at all and this doesn't even make sense.

Do not semiflatten an image that you've rendered after getting rid of the color in the bg by using color to alpha if you plan to put a color background behind the rendered image.

If you want the bg colored with a solid color, you can flatten the image rendered and the background will be the color of the BG color in the color tool.

The only time you need to semi-flatten image is if you want to make a gif file and have a transparent background.

You can't have any transparency in a jpg image.

Check my tutorial here about semi-flattening. viewtopic.php?f=10&t=214

Read your tutorial a couple of months ago. A must know thing, O.

Thanks for the replies.

I don't doubt that I'm missing something obvious, but I'm not sure I explained it properly either. I fully understand that semi-flatten is for the semi transparency problem of a gif file. When I discovered it, I read the manual page of what it does. I was thinking of a different application for the general idea of "Replace partial transparency with current background color".

I'm always looking for quicker ways to render and probably was over simplified in my thinking here. When I hovered over the semi-flatten filter and saw the tooltip "Replace partial transparency with current background color", it made me think I could restore the semi transparent areas in the render with the color that was removed with "color to alpha", without restoring the background as it is now fully transparent.

Unfortunately I didn't give enough thought to the edges. If they contained any of the color I removed, they would be restored with full opacity as well and would give a jagged look. I realized this after trying to manually create the steps by using the "Threshold Alpha" filter.

So I think I jumped the gun a bit and apologize for not thinking it through completely before posting.