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Alpha Matte in a different video

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Ernest Rosado
Alpha Matte in a different video
on Nov 26, 2017 at 2:51:49 am

Hi all,

I have two h.264 videos, one is an animation, the other is the Alpha (Luma) matte for that video.

To make the transparency work, I made the animation layer a child of the matte, used Generate Mask on the matte and set the transfer mode of the matte-layer to multiply.

This worked, but it has the side-effect of making my animation ever-so-slightly transparent. This changes the colors and lets the videos on lower layers shine through just a tiny bit. I have confirmed that the colors in the matte video are correct, pure #000000 and #FFFFFF, so there's something else going on.

How can I mask out my video without the semitransparency on pixels that should be opaque? I am open to using a video codec with the alpha matte built in, but I'm dealing with 4k 60fps footage, so the quicktime Animation codec, PNG etc is too much filesize to accommodate.

Any ideas?

Thanks!


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Graham Bernard
Re: Alpha Matte in a different video
on Nov 26, 2017 at 6:09:35 am
Last Edited By Graham Bernard on Nov 26, 2017 at 6:33:31 am

Track 1 is an Event which has a Transparent Background- Yes?
Track 2 is an Event is just a straight Video Event - Yes?

I don’t understand why you’re needing to utilise any Compositing at all?

* Grazie

Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge


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Ernest Rosado
Re: Alpha Matte in a different video
on Nov 26, 2017 at 6:51:31 am

The alpha is a matte in a different file. Here's a screenshot. You can see how the alpha for the lower-third is in a separate movie, allowing it to animate in over the clip. But you can also see in the preview that the lower-third is semitransparent for some reason.

Any ideas?



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Ernest Rosado
Re: Alpha Matte in a different video
on Nov 26, 2017 at 6:55:06 am

It's tough to see the transparency problem in that jpg. The image compression creativecow put on the image hides it.

Here's a link to the PNG: https://images2.imgbox.com/d8/5f/a2tVY1YY_o.png


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Marco Baer
Re: Alpha Matte in a different video
on Nov 26, 2017 at 12:18:44 pm

Could you share your matte source file?


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Ernest Rosado
Re: Alpha Matte in a different video
on Nov 26, 2017 at 12:40:49 pm

Sure - The lower third itself is at 11908_lower3rd1.mp4.zip
The matte file is at 11907_lower3rdalpha1.mp4.zip

Let me know if you need it on a different host.

Thanks for your help!


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Marco Baer
Re: Alpha Matte in a different video
on Nov 26, 2017 at 12:51:45 pm

The luma values of the MP4 matte file aren't correct if them are meant to be white = opaque and black = transparent, it is not #FFFFFF and #000000. But you could correct the values by adding the Levels FX using the "Studio RGB to Computer RGB" preset.


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Ernest Rosado
Re: Alpha Matte in a different video
on Nov 29, 2017 at 4:34:01 am

Thanks, I was able to fix this using the method you described. But I don't understand why it's not seen as pure white.

If I load the matte clip into After Effects and hover over the white area, you can see it's R: 32768, G: 32768, B: 32768, which is pure white as far as I know. What gives?



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Marco Baer
Re: Alpha Matte in a different video
on Nov 29, 2017 at 10:19:12 am

The R'G'B luma range of this certain matte clip is 16-235 which is also known as "studio swing" or "limited range" and actually in this clip there is a "limited" color range flag.
Now some systems read this limited range flag and will spread the signal from 16-235 to 0-255 and some systems (like Vegas Pro) just ignore this limited range flag and pass the signal as it is.
So sometimes it's worth to take a closer look onto Vegas' waveform monitor or histogram to see the actual color range used in Vegas Pro, especially in cases of compositings where using precise R'G'B' values for doing a correct math are crucial.


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Graham Bernard
Re: Alpha Matte in a different video
on Nov 29, 2017 at 11:32:37 am

Thanks Marco.

You and I have been hanging out on this and other forums for a long time, and when I read the depth of understanding you have for this, at times, complex craft we follow, you always make it clear and understandable. If I see you involved with any post replies I make a bee-line for your comments.

Cheers

Grazie

* Grazie

Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge


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Marco Baer
Re: Alpha Matte in a different video
on Nov 29, 2017 at 2:05:07 pm

Thanks, Grazie. Enjoying your good vibes! :)


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