Alpha Over/Under in Sequencer, plus Sky/Premul/Key options

Support Compositing and Post Processing
1

Today I was using the Blender Sequencer to add some text to a video, and I got problems with Alpha Over/Under effect strips. I’m not an advanced user, but I feel there is something wrong with these effects. I made a .blend file to explain better. (using Blender 2.45 on 32-bit Linux)

http://denilsonsa.selfip.org/~denilson/alpha_over_under.blend (144K)
(I would attach it to this forum, but looks like it only accepts images)

I’m not sure this is really a bug in blender, or just I haven’t understood how these effects work (in this case, the lack a of a good documentation and the non-intuitive interface/names are the culprits). Anyway, I wanted to post here before submitting a bug report.

Below is a copy of the text I’ve added to the .blend file.

"Alpha Over" and "Alpha Under" are too confusing and not intuitive. In other
words, they do not behave as the user expect (or at least as I expect). Their
behavior is different and is also dependent on Sky/Premul/Key setting.

With "Sky" selected:
	"Alpha Over"  -> The text is anti-aliased. The blue background from the
	scene has been blended with the color from the other strip.

	"Alpha Under" -> The text is anti-aliased, but has a dark outline at the
	anti-aliased pixels. The blue background from the scene has been completely
	replaced by the color from the other strip.

With "Premul" selected:
	"Alpha Over"  -> The text is anti-aliased. The color from the other strip
	appears to be correctly blended with the anti-aliased text.

	"Alpha Under" -> The text is anti-aliased, but has a dark outline at the
	anti-aliased pixels. The background is the color from the other strip.

With "Key" selected:
	"Alpha Over"  -> The text is NOT anti-aliased. The background is the color
	from the other strip.

	"Alpha Under" -> The text is anti-aliased. The color from the other strip
	appears to be correctly blended with the anti-aliased text.


Note that "RGB" is selected, instead of "RGBA". That's correct, since the alpha
blending will be done before exporting to the final format, which will not have
any alpha information (e.g. an mpeg video).


In my humble opinion, since I have 2 strips (or 2 layers, for people used to
Gimp/Photoshop) and only one of them has Alpha channel, I think there are only
two ways to mix them:
1) get the non-transparent strip over (above) the other one, completely hiding
   it;
2) get the transparent strip over (above) the other one, and thus alpha-blending
   them.

Somehow, Blender has many different ways to combine them, and they are not
intuitive.
2

Hmm…

I’m not sure what the problem is?

The Sky-PreMul-Key buttons will only effect the scene strip. They are part of the internal renderer. They present a final image to the sequencer prior to sequencer compositing

The color strip is only RGB, there is no alpha implied in the strip or a blending control in it’s properties panel. It is like an after effects solid with no opacity or modes.

3

I understand your both points (about Sky/Premul/Key affecting only the scene; and the color strip having only RGB). I don’t understand why Alpha Over/Under give different results.