VSE Bug in Alpha Over

Got a PNG -> it’s just a white-to-transparent gradient. I want to alpha that over some footage.

For some reason the ‘Alpha Over’ option behaves just like an ‘Add’ option.

I’m on Windows XP, Blender 2.49a. Will see if I can run home and try the same thing on an Ubuntu box.

Does anyone else have this problem?

Confirmed it also happens on Ubuntu 9.04.

Can anyone reproduce this? More importantly, can anyone suggest a workaround?

AlphaOver uses the alpha channel of the image, it doesn’t build one from the colors. Could that be the problem?

I’ve no idea. I’m not particularly well up on PNG file format specifications.

I saved the first PNG from GIMP and when that went wrong I tried another one from Photoshop. So I 'm reasonably sure it’s Blender. I just have no idea what to do about it.

I’d still wonder about the alpha channel. That isn’t something that is just created when you save an image; you have to explicitly make it. If you didn’t do that, then I think the problem is with the image, not Blender.

In the same sitting, I’d made two more images - some text and a logo in the corner, and everything else was just transparency. The logo had semitransparent parts, but no gradients. This displayed fine, including the semitransparent parts.

It’s only on gradients that it seems to stumble. I don’t know how PNGs store their gradients, but I know it’s not the file format that’s to blame: Both Photoshop and Gimp can open each other’s files, and they display fine - a white gradient on a transparent background.

The 'Add’ing is the thing that confuses me. I can think of no reason for it to do that.
I’m sure it’s being added, as when I replace the white gradient with a black one, it displays fine. As soon as I start adding colour, it goes to hell.

But does noone else get this? If you make a PNG with a gradient, put it over another image in the Sequencer, and set it to ‘Alpha Over’, what do you see?

I tried to reproduce your problem, but everything seemed to be normal when I just remembered to activate the ConvertPremul-button in the AlphaOver-node.

But I DID have a same sort of problem much earlier with some previous version of Blender, and if I recall correctly in my case it was solved by using TIF instead of PNG in the gradient image.

No, it’s not a bug, and actually, AlterEgo kind of backed into the correct answer - even though he was referring to nodes

…everything seemed to be normal when I just remembered to activate the ConvertPremul-button in the AlphaOver-node
The VSE alpha over effect also depends on premultiplied alpha to work correctly and I believe that gimp does not premul it’s output. It’s an easy fix though, select your gradient image strip and then go to the sequencer buttons -> filter tab and toggle the premul button on and you should be good to go :wink:

Ah wonderful. Thanks very much.