Author Topic: Green/purple fringe on high contrast edges  (Read 7783 times)

avasarin

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Green/purple fringe on high contrast edges
« on: January 17, 2014, 03:50:25 PM »
Hi there,
I'm dealing with this bad artifact for a while, but I haven't got any useful result.
This green/purple fringe is present in high contrast edges, in my case I have a girl standing in front of an overexposed window. The only way I find to reduce it (in ACR) is to play with the chromatic aberration value, trying to isolate those colors. The downside is that you can easily erase colors somewhere else the frame, and are still visible the pixels fringing and moving randomly.
I have some reference images here.

Frame without CA reduction(just WB adjustment)
https://www.dropbox.com/s/h6g2fzuortm019o/M30-1058_01193-2.png

Frame with CA reduction
https://www.dropbox.com/s/r3qq4b46qo0mo7r/M30-1058_01193.png

Some details(Leg and blanket) with CA reduction
https://www.dropbox.com/s/ior3y1bgbri35fi/Irin-fringe-dett.png
https://www.dropbox.com/s/8gtnlv7xbpg42ky/Irin-fringe-dett-2.png


Have somebody find a better way to eliminate it? Have somebody understand what is causing this artifacts?

cheers

Alessio
5dmk2 - Nikkor set(20-28-35-50-105mm), Vivitar 85-105 - BMPCC
http://www.frabiatofilm.com
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Kharak

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Re: Green/purple fringe on high contrast edges
« Reply #1 on: January 17, 2014, 05:03:48 PM »
Those anomalies look minimal to me after CA reduction. Sure video/cinematographers might notice it if they zoom in 300%, but the normal viewer will not offer those edges any thought. Keep the story interesting and people won't have time to think of those things.

And as you are on the 5dII, isn't it more likely its aliasing?
once you go raw you never go back

avasarin

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Re: Green/purple fringe on high contrast edges
« Reply #2 on: January 17, 2014, 05:30:34 PM »
Thanks Kharak,
luckily I have the VAF filter, so aliasing and moire don't appear at all.
Of course this is a compromise we have to take while working with dslr in RAW mode. I'm getting still amazing results, this is just a very little cons.
By the way I'm playing around with CA value and unsharp mask value: I notice that if you disable sharpening in ACR module and then apply two unsharp mask(one 100 quantity and 0,5 ray and the other 50 quantity and 0,7 ray) the problem is almost gone.
And for the CA modules, the values I find work best are: factor between 2-3 and tonality it really depends on the color of the footage(avoid orange at the very end because this is a common skin color).

I keep on working with it.
5dmk2 - Nikkor set(20-28-35-50-105mm), Vivitar 85-105 - BMPCC
http://www.frabiatofilm.com
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engardeknave

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Re: Green/purple fringe on high contrast edges
« Reply #3 on: January 17, 2014, 06:05:01 PM »
Things that contribute to CA: certain specific lenses, shooting at larger apertures, high-contrast areas that are out of focus.

Note that the CA here occurs in slightly out of focus areas. If you get these areas in focus, there will be no CA.

I'm not sure what version of ACR you're using, but the older versions don't have the defringe function, which is very effective.

avasarin

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Re: Green/purple fringe on high contrast edges
« Reply #4 on: January 17, 2014, 06:26:39 PM »
Hi engardeknave,
my footage is not affected by CA, which is simply gone with the CA removal (I work with ACR 8.2). This fringe is very different, I'm just saying I try to fight it with CA module.
5dmk2 - Nikkor set(20-28-35-50-105mm), Vivitar 85-105 - BMPCC
http://www.frabiatofilm.com
https://www.facebook.com/stringssenzatempo

dmilligan

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Re: Green/purple fringe on high contrast edges
« Reply #5 on: January 17, 2014, 10:40:33 PM »
Hi engardeknave,
my footage is not affected by CA, which is simply gone with the CA removal (I work with ACR 8.2). This fringe is very different, I'm just saying I try to fight it with CA module.
No, your artifact is CA, the green/purple fringing in your image is caused by aberrations that are the result of the optical system, that is the definition of CA. There are different types of CA and software tools cannot remove all forms of them, especially the more severe it is. The reason you have to play with the CA settings in ACR to get rid of it, is because your CA is rather severe and the default settings aren't removing it completely (overly heavy CA correction can result in negative side effects in other parts of the image, so Adobe isn't going to make the defaults extremely aggressive). If you do what engardeknave says, it will greatly reduce your problem.

Different wavelengths of light travel through glass at different speeds, so the glass focuses them to different points, this is what is causing the color fringing. The lens is designed to minimize this effect for areas that are in focus, but out of focus areas are not corrected for by the lens design (b/c it is not entirely possible or prohibitively expensive/complex). This is why CA is worse in out of focus areas like in your scene. Do what engardeknave says to correct it.

http://en.wikipedia.org/wiki/Chromatic_aberration

engardeknave

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Re: Green/purple fringe on high contrast edges
« Reply #6 on: January 18, 2014, 07:38:45 AM »
Trust, bro. I shoot girls in front of windows errday. And rooms with windows. I basically shoot windows.

avasarin

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Re: Green/purple fringe on high contrast edges
« Reply #7 on: January 18, 2014, 08:19:31 AM »
I understand.
But is the first time that I see CA only affecting a couple of pixels that are moving randomly. And The girl in front of the window is in focus.
BTW is almost gone with CA removal in ACR.

Thanks
5dmk2 - Nikkor set(20-28-35-50-105mm), Vivitar 85-105 - BMPCC
http://www.frabiatofilm.com
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engardeknave

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Re: Green/purple fringe on high contrast edges
« Reply #8 on: January 18, 2014, 08:37:28 AM »
It could just be the contrast and your lens, but the girl in front of the window represents like a foot of depth at least. Maybe two with that back pack. We're talking about differences of inches. CA lives in just slightly out of focus areas.

What lens and aperture was that shot with?

avasarin

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Re: Green/purple fringe on high contrast edges
« Reply #9 on: January 18, 2014, 12:28:25 PM »
Engardeknave, the lens is Nikkor Ai-s 35mm f/2.8 @ f4.

This morning I tried a couple of workflow for resolving this thing.
The simplest is to manage DNGs and color grading inside AE, using CA module in ACR.
If you want to use the power of Davinci Resolve you can import DNGs in AE, set just WB and remove CA, and then render a Prores 422 HQ and grade in Davinci.
The last one is to grade the DNGs directly in Davinci Resolve, using the full power of RAW color depth and latitude, and then processing just the "artifacted" clips in Photoshop, because is the only way to use ACR as a filter with a video layer.

Do you have better ideas?
5dmk2 - Nikkor set(20-28-35-50-105mm), Vivitar 85-105 - BMPCC
http://www.frabiatofilm.com
https://www.facebook.com/stringssenzatempo