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rendering & shades of gray in a feathered vignet

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Alexandre Brandt
rendering & shades of gray in a feathered vignet
on Sep 3, 2012 at 4:11:35 pm

Hello all,

I've been making some simple titles in AE. My titles on white background have a mask creating a vignette of light in the center and gently moving to gray in the outer ellipse (the very classical feathered vignette effect).

I render my video in AppleProRes 422HQ. When I look at the output in QuickTime, I can clearly notice the different layers of shades of gray, instead of a real gradient not noticeable. I was wondering if this is due to the power of processor of the computer or something that can't render as much detail in the gradient. (I'm working on a MacBook pro 2.22Ghz Intel Core 2 Duo). Or maybe it's only the video card that can't show detail but the detail is their in deed ?

Thanks for advice,
Alex


motion pictures + storytelling = ?
-> FILMMAKING !


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Thomas Hannen
Re: rendering & shades of gray in a feathered vignet
on Sep 3, 2012 at 4:44:16 pm

I've had this problem before too. I don't know why it happens.. One possible workaround is to deliberately add a tiny amount of noise to areas within the image where the banding is occurring. It's not a great idea, as it increases the file-size, and effectively reduces the available bandwidth for the rest of the image, but sometimes it looks better than the alternative.


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Erik Waluska
Re: rendering & shades of gray in a feathered vignet
on Sep 3, 2012 at 5:55:31 pm

That's a well-known problem called "banding", which you can search in the forums here and elsewhere and find a lot of info. Like Thomas said, adding noise helps but with the caveats he mentioned. It occurs because of the limited color range of the 8-bit format.

Search "banding", "gradient banding", and "color banding" for more info. Here are a couple of threads that might help:

http://forums.creativecow.net/readpost/2/1011726

http://forums.creativecow.net/thread/2/907947#907949


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Walter Soyka
Re: rendering & shades of gray in a feathered vignet
on Sep 3, 2012 at 8:22:01 pm

Think of a gradient or feathered vignette like a ramp -- a smooth surface rising from low to high over a certain run. However, computers, being digital instead of analog, don't do smooth. Think of a computer's representation of that gradient as a staircase, with a series of discrete steps from low to high over that same distance.

If you use only a few steps, they must be very deep to get you all the way across the run, and they must be very steep you get you all the way up the rise. As a result, you get big bumps going from one deep and steep step to another.

If you use many, many steps, they must each be shallower to fit within the run and shorter to fit within the height. A great many very shallow, short steps approximates a smooth ramp.

"Bit depth" or "color depth" is the number of steps. Because of the way computers count, using 8 bits gets you only 256 separate shades of gray between black and white, but using 16 bits gets you 32,768 separate shades of gray.

Practically speaking, just bumping your project's color depth from 8bpc to 16bpc should improve the banding, as AE will then attempt to dither the gradient (adding noise to smooth it out) when outputting to an 8-bit format.

ProRes 422 (HQ) is a 10-bit format, so it can accommodate much smoother gradients than 8-bit video can (1024 shades of gray from black to white versus 256). Make sure to set your output module's depth to "Trillions of Colors" to preserve this color depth.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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Erik Waluska
Re: rendering & shades of gray in a feathered vignet
on Sep 3, 2012 at 8:37:49 pm

Wow, Walter, that is by far the best explanation I've seen for the banding issue!


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Walter Soyka
Re: rendering & shades of gray in a feathered vignet
on Sep 3, 2012 at 10:32:39 pm

[Erik Waluska] "Wow, Walter, that is by far the best explanation I've seen for the banding issue!"

Thank you, Erik -- much appreciated!

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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Alexandre Brandt
Re: rendering & shades of gray in a feathered vignet
on Sep 4, 2012 at 6:45:03 pm

Hello all,
Thank you very much for your explanations and advice !
Alex


motion pictures + storytelling = ?
-> FILMMAKING !


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