Transparent clip problem

@shotcut, @brian I believe there is a bug in the way transparency works. Below are the steps to demonstrate the problem.

@Chevee_Dodd posted an issue a week or so ago and since I had 5 minutes to spare I thought I’d try to experiment.

  1. Put a clip on track V1.
  2. Create track V2.
  3. Use Open Other to create a TRANSPARENT clip and put it on V2 and SELECT it.
  4. Use the 3-D TEXT filter.

Nothing appears on the clip. My understanding is that the 3D-TEXT should appear on a transparent background. This has to be a bug otherwise what use is there for a transparent clip?

I also tried the following, putting a non-transparent BLACK background instead for step 3. then:

  1. Use the Opacity filter and change it 0%.
  2. Use the 3-D TEXT filter.

That didn’t work either (it used to since that is how I used Overlay HTML filters a year or so ago).

What did work, sort of, was to increase the opacity back to 100% and change the track’s BLEND mode to ‘Subtract’, though this of course also made the text (as well as the background) partially transparent, apart of course for the white color.

As far as I can remember 3D text never worked on transparent clips. It needs to be applied to a video or an image clip. If 3D text is put on a color clip a chroma key filter needs to be used to key the color.

It;s not just 3D text that doesn’t work on transparent, Overlay HTML filters don’t work either. Yes, I know that 3D-text is implemented as an Overlay HTML.

So the question is “Why doesn’t the Overlay HTML filter work on transparent clips”? Is this a restriction or a bug?

It is not a bug; it is a limitation. The use of a transparent clip is for other filters like Text and Timer. To answer your question why, it is because of the way WebVfx works: it returns an opaque image. Like I’ve said before, I will try to take a look into it eventually, but it is not a prority for me now.

Not a problem, since these days I apply the Overlay HTML filter to the track and that works fine. Thanks for the info.

In the original thread, I talked about my workaround:

I rendered a mostly transparent PNG with a white background directly where my overlay would appear. This works for statically-placed WebFX.

For a scrolling news ticker type effect you can try this.
3 Video tracks.
Video or image on the bottom track. Color clip sized with SP filter on middle track. Color clip with simple scroll overlay HTML on the upper track. Chroma key the color on the upper track. The overlay will display and scroll.


This bug has been fixed for the next release v19.06. It includes a new version of WebVfx where I have added full support for MLT alpha channel for input and output.

Nice addition. being able to put an overlay HTML filter (or 3D text) on a transparent clip will be useful. Thanks @shotcut, keep up the good work :wink:

It’ll make it easier to do HTML overlays.
One can still use KFs and conventional filters to make reasonable titles.


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