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Is actual transparency in video layers just impossible?

lastfirst
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I am trying to create a video where text is animated over a background. I was thinking I could have multiple layers of transparency-enabled text and just apply transitions as I like, but that does not work.
I have read up on applying Affine and Composite transitions, but those only seem to act as transitions between a layer and the layer below it; I will be working with several layers at a time and need transparency to apply all the way through the layers. In other words, I need the layer to actually BE transparent.

Is this possible at all? In the threads I have found where this is discussed it /seems/ that the answer is no, but I wanted to ask the question specifically so that I can stop frustrating myself.
capslock
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By today, I have never seen transparent tracks (layers) in kdenlive.

But you can use transparency over multiple tracks by using the "with track" option in the transition dialog (top right). If you want to have three transparent tracks (V1...V3), you add the transition to track V2 (with track "auto", which is V3) and then add another transition to track V1 "with track" V3 and you should have transparency from V1 to V3.
User avatar TheDiveO
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To add to capslock's fine answer: to my limited understanding of Kdenlive's (and thus mlt's) data model, there cannot be such thing as transparency of a track. This is in contrast to effects which you can apply to a track as a whole so they cling to any clip placed on this particular track.

It may help to think of transparency as the third channel alpha, next to red, green, and blue channels. For instance, an ordinary video clip comes without any transparency information, thus the image in the alpha channel is constantly 0.

When you use title clips, then these will be generated with the appropriate alpha channel image. As capslock said, you can compose multiple such title clips from different tracks onto each other. You need to take care of the compositing sequence, though. If you place a title on t1, another on t2, and some video on t3 and compose them with track auto, then Kdenlive/mlt first compose t1 on top of t2. Afterwards, the result is then composed on t3.
Of course, you will now need to tell Kdenlive how to fuse the two alpha channels from t1 and t2 and thus also how to fuse the other three channels for red, green, and blue. This is where the over, xor, and, ... alpha operations come into play. With the default setting of alpha operation over, you will end up with something undesired for this particular title stacking example. Play around with the alpha ops to get a feeling for how they behave. (memo to self: need to write some small doc on this topics...)

Beware of throwing in affine translations and rotations, as there seems to be a bug in the pipeline that causes incorrect behavior. You can use affine nevertheless when you don't do any translation or rotation, as pure compositions works all the time.

If you need affine with translation, rotation and resize, then carefully set up your composition pipeline. Going back go my example this would mean to compose t1 onto t3 by setting with track 3. Similar, compose t2 onto t3 by either keeping with track auto or setting with track 3. Kdenlive then will compose first t2 onto t3, then t1 on t3 on top. (Hopefully I didn't do any gross mistake in my explanation).


TheDiveO on YouTube: diving videos and aerial (quadcopter) videos, all made with Kdenlive.
TheDiveO Blog: know-how about Protune-grading in Kdenlive, GoPro HD Hero 3, DJI Phantom Quadcopter, full face mask diving, and some more...
ttguy
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You can also get some milage by having an empty video track at the bottom of the stack and setting up transitions between higher tracks and the empty track.
See http://userbase.kde.org/Kdenlive/Manual ... quentially for and example.
User avatar TheDiveO
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Ah, thanks for reminding me of this trick, ttguy. Do you know which implicit transition is used? Or does Kdenlive translate it to compositing with the last used track instead?


TheDiveO on YouTube: diving videos and aerial (quadcopter) videos, all made with Kdenlive.
TheDiveO Blog: know-how about Protune-grading in Kdenlive, GoPro HD Hero 3, DJI Phantom Quadcopter, full face mask diving, and some more...
lastfirst
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Thank you guys very much for the help! Not just the workarounds, but the general information as well. I work with audio programs constantly (Ardour, Pro Tools, etc) so I sometimes get overconfident when jumping into other media programs and miss some of the basic principles of the medium, which of course can be a bit of a headache.
Next time I am at my workstation I will try these workarounds and see what results I can get. Thanks again!
ttguy
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TheDiveO wrote: Do you know which implicit transition is used?


You explicitly supply a transition.
http://userbase.kde.org/File:Kdenlive_3_fade_in_titles.png
User avatar TheDiveO
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I'm still wondering what the advantage of compositing with a bottom, empty track is? Wouldn't compositing with the bottommost clip on track 4 yield the same result? What am I missing here?


TheDiveO on YouTube: diving videos and aerial (quadcopter) videos, all made with Kdenlive.
TheDiveO Blog: know-how about Protune-grading in Kdenlive, GoPro HD Hero 3, DJI Phantom Quadcopter, full face mask diving, and some more...
ttguy
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TheDiveO wrote: Wouldn't compositing with the bottommost clip on track 4 yield the same result?

I don't think it does yield the same result.Maybe you could test your theory and report the results. I actually forget what the difference is. But I believe that to get the effect of the 3 titles fading in sequentially you need to use the empty track method. I can't say I understand it.
User avatar TheDiveO
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In this case I don't need to recheck, as I just did some checking these days in preparation of a new blog post. I didn't checked a three title setup but a two title setup instead. One clip contains a Kdenlive-made title clip, the second clip is an svg file created with Inkscape. I'm then composing them separately onto my background clip. This works as expected. No need for another, empty track.

There is one situation though I'm now remembering where your trick becomes necessary: I'm overlaying all my projects with a watermark. Since there is a varying amount of clips stacked onto each other in different parts of the time line, or there are clips alternating between tracks with transitions, it is then impossible to overlay the watermark with a concrete, constant track. In this case, a final bottom empty track then does the trick.


TheDiveO on YouTube: diving videos and aerial (quadcopter) videos, all made with Kdenlive.
TheDiveO Blog: know-how about Protune-grading in Kdenlive, GoPro HD Hero 3, DJI Phantom Quadcopter, full face mask diving, and some more...
ttguy
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Since there is a varying amount of clips stacked onto each other in different parts of the time line, or there are clips alternating between tracks with transitions, it is then impossible to overlay the watermark with a concrete, constant track. In this case, a final bottom empty track then does the trick.


Ahh. Yes. This would be same reason why I needed that track because with 3 titles coming in at different time you have the same situation.

 
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