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VSE: Clips with Alpha Over against a transparent background fail to reflect opacity changes
Closed, ResolvedPublic

Description

System Information
Observed on Mac OS X 10.6 and 10.8

Blender Version
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

  1. Get a PNG with transparency. I am providing one below.
  2. In the Video Sequence Editor timeline choose Add > Image > and add the PNG.
  3. With the clip selected, under 'Edit Strip' change the Blend mode to 'Alpha Over'.
  4. 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)

Event Timeline

Seth L. (quantumanomaly) set Type to Bug.
Seth L. (quantumanomaly) created this task.
Seth L. (quantumanomaly) raised the priority of this task from to Needs Triage by Developer.
Sergey Sharybin (sergey) triaged this task as Confirmed, Medium priority.

Thank you for the prompt fix, sergey! You're awesome!