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Banding with PPro composite in linear color

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Oliver PetersBanding with PPro composite in linear color
by on Oct 31, 2015 at 12:08:15 am
Last Edited By Oliver Peters on Oct 31, 2015 at 1:53:54 am

Can someone explain Adobe's concept of the "composite in linear color" setting that's part of the sequence settings of Premiere Pro CC? The default for a new sequence is to have that checked. However, it appears to cause banding.

In the example I've uploaded, the test graphic was created in After Effects as 720p and rendered out as Animation codec with an alpha. This is then composited in Premiere Pro over a ProRes4444 1080p clip on a 1080p timeline. The test clip is set to "scale to frame size". The reason for these settings is that I'm trying to reproduce a problem I encountered today in updating a legacy project.

Enabling the Max Bit Depth and Max Render Quality makes no difference. Trying different sequence presets also makes no difference. It's the same when it's 8-bit or 10-bit. It seems consistent in my tests on both Macs (with Sapphire card) and PCs (with Nvidia cards). When I composite in After Effects sticking with all the defaults, it looks smooth like the image on the right.

Edit: I have checked these same clips in FCPX and when I change the alpha on the graphic clip from "straight" (the default) to "pre-multiply" I get this same banded look in X. When I ran my tests our of AE, I rendered the alpha as both straight and pre-multiplied and it didn't seem to make any difference in how Premiere reacted. There doesn't appear to be a similar toggle within Premiere, unless I'm missing something.

This is running the CC2015.0.2 version of the apps.

- Oliver


Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Chris WrightRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 6:48:40 am

if you render out a 8 bit codec, you're going to have 8 bit quality in your final composite even in 32bpc. There's just no data there.

Does it band in AE? import it back into AE and try grading. It will fall apart in linear, most likely.

Let us know if you have any luck with 16bit(trillions) or 32bpc(float) files.


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Oliver PetersRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 1:42:48 pm

[Chris Wright] "if you render out a 8 bit codec, you're going to have 8 bit quality in your final composite even in 32bpc. There's just no data there.
Does it band in AE? import it back into AE and try grading. It will fall apart in linear, most likely."


This is not a render - except to generate the original graphic from AE. This is how it appears in the Premiere Pro viewer and video outputs before any rendering there. The screen grab is from a live comp. I have rendered it out in various ways and it still exhibits the problem. AE and FCPX both do it correctly at their defaults and do not exhibit this issue. It appears to be some sort of pre-multiply issue in Premiere in how the alpha is interpreted.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Oliver PetersRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 2:11:59 pm

PS: Premiere does it correctly as well when I uncheck the default setting "composite in linear color". The 8-bit codec thought makes a lot of sense, but that doesn't seem to be the issue. For example, I can uncheck that setting and render the final out as MPEG2 and it's clean. It's really a question of what is this "composite in linear color" setting really doing? There doesn't seem to be an equivalent in AE or other apps.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Chris WrightRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 2:31:13 pm
Last Edited By Chris Wright on Oct 31, 2015 at 2:34:10 pm

I think you're missing my point. Linear light is a completely different way to map the gamma curve. Linear, itself requires at LEAST 14bits/pixel to not create banding. It's a math quantize thing. It's why most people don't use it. It's used primarily in the log and float land for blending film with effects.

also, as a side note, Dolby Vision's new color profile needs at least 12 bits due to pixels requiring enough data to store luminescence relative to color values. Its pretty neat stuff in white papers.


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Oliver PetersRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 2:49:48 pm

[Chris Wright] "I think you're missing my point. Linear light is a completely different way to map the gamma curve. Linear, itself requires at LEAST 14bits/pixel to not create banding....
...also, as a side note, Dolby Vision's new color profile needs at least 12 bits"


Aah, OK. That starts to explain it. And it would also explain why I haven't seen this issue before as it seems part of what Adobe is adding to deal with HDR. So then, I would presume that this check box should be OFF by default, not ON, with most standard video sequences. Thanks.

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Oliver PetersRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 3:00:00 pm

Here's an additional explanation:

http://www.lynda.com/Premiere-Pro-tutorials/Composite-Linear-Color-setting/...

- Oliver

Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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Oliver PetersRe: Banding with PPro composite in linear color
by on Oct 31, 2015 at 4:00:18 pm
Last Edited By Oliver Peters on Oct 31, 2015 at 4:02:06 pm

Actually in doing a little deeper digging, I don't think the issue I'm having is related to bit depth. It's purely a multiply/straight alpha issue.



I went back in PPro and Modified the clip setting to Interpret the alpha as "Conform Alpha Premultiplication to:" (leaving the next box unchecked). Then I changed the sequence setting back to "Composite in linear color" turned on (the default). The result is a smooth gradient in the white glow as I would have expected. (see image below).

- Oliver



Oliver Peters Post Production Services, LLC
Orlando, FL
http://www.oliverpeters.com


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