Observed on Mac OS X 10.6 and 10.8
Broken: 2.69.0 r60991 Also confirmed on latest unofficial build 2.69b53f310
Worked: never, as far as I know
Short description of error
In the Video Sequence Editor a clip with its blend mode set to Alpha Over cannot have its Opacity adjusted when over a transparent background. I observed this with PNG image sequences, but it may occur with other file types as well.
If a solid background image is added to a channel beneath the transparent clip the problem disappears.
This leads me to suspect that Alpha Over was coded only to account for transparency over solid - not transparency over tranparency.
This bug prevents image sequences with transparency from being composited correctly and in that regard is a workflow-stopper.
For example if a client wanted an animated logo fading in over a transparent background this bug would stop you from being able to produce it.
Exact steps for others to reproduce the error
- Get a PNG with transparency. I am providing one below.
- In the Video Sequence Editor timeline choose Add > Image > and add the PNG.
- With the clip selected, under 'Edit Strip' change the Blend mode to 'Alpha Over'.
- Lower the clip's Opacity slider and see no effect at all. Even with the slider at 0 the clip will still be fully visible.
Refreshing the Sequencer doesn't help. Neither does reloading the .blend.
A PNG with transparency for testing:
An example .blend (uses above .png file)