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Old 11-05-2018, 07:00 AM   #1
sonicowl
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Default video processor: multiply with alpha, compositing

I'm looking for a way to crop video on one track with black/white image (or video) on a second track. Like this: https://en.wikipedia.org/wiki/Alpha_compositing

Two tracks get combined into one. Result would be partly seethrough video - transparent on black parts and fully visible on white parts. Video editors have this, but I cannot figure out how to do it in Reaper (apart from basic croping to rectangle).

Has anybody already written preset for video processor that can do this, and can share it?

Otherwise, which function in video processor would be appropriate for this task?

Thank you for help.

Last edited by sonicowl; 11-05-2018 at 07:21 AM.
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Old 11-05-2018, 08:43 AM   #2
Eliseat
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This would be a great addition.

I guess it could be kind of a combination off "chroma key" and "image overlay". Also PS layer modes like multiply, darken, lighten etc. would be a great addition. This stuff is standard in every video software. And i guess its not hard to do.

Video processor and video in Reaper has such a huge potential. More people should code presets. I tried to write and change some presets but as a noob its pretty hard to put something together. Even the Reaper internals couldn't help me.
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Old 11-05-2018, 07:05 PM   #3
wwwmaze
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Quote:
Originally Posted by sonicowl View Post
Otherwise, which function in video processor would be appropriate for this task?
I think what you are looking for is "gfx_mode"

This blends the next (or previous, depending what you have set in project settings) 2 video tracks after the track which has this video processor active:
Code:
*snip* see below
From the docs:
Quote:
gfx_mode:
0 = normal
1 = additive
3 = multiply (very different in YUV vs RGBA)
17 = (dest + src*gfx_a)*.5 + .5 (only valid when using YUV colorspaces)
18 = dest + (src-0.5)*gfx_a*2.0 (only valid when using YUV colorspaces)
19 = absolute difference: abs(dest-src)*gfx_a (only valid when using YUV colorspaces)
...

Last edited by wwwmaze; 11-06-2018 at 03:36 AM.
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Old 11-06-2018, 12:43 AM   #4
Eliseat
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Many thanks.

But where are the docs? Didn't even know there are docs for that.
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Old 11-06-2018, 02:57 AM   #5
wwwmaze
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Either press F1 when the mouse cursor is within the editor or use this online documentation: https://mespotin.uber.space/Mespotin...API_Video.html


On another glimpse I'm not even convinced if blending works the way I think (or the linux implementation of video processor maybe has a bug).

2 observations:
1. After I blend-in completely white pixels while adding both images/videos I still can see some non-white pixels. Same problem with subtraction, the picture should be black but I also see non-black pixels.
2. Multiplying makes the resulting image/video lose its color.
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Old 11-06-2018, 03:10 AM   #6
Eliseat
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Quote:
Originally Posted by wwwmaze View Post
Either press F1 when the mouse cursor is within the editor or use this online documentation: https://mespotin.uber.space/Mespotin...API_Video.html


On another glimpse I'm not even convinced if blending works the way I think (or the linux implementation of video processor maybe has a bug).

2 observations:
1. After I blend-in completely white pixels while adding both images/videos I still can see some non-white pixels. Same problem with subtraction, the picture should be black but I also see non-black pixels.
2. Multiplying makes the resulting image/video lose its color.
Many thanks. Unfortunately my F1 key is broken.

And yes, I also discovered a very strange blend mode behavior. It could be because of the different color spaces. Have no idea actually. I only know those Photoshop blend modes and they react much different from that.

It also does not blend drawn colors from other tracks. (Tried to blend in "solid color" from reaper blog.) Only video signals work.
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