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Old December 5th, 2006, 09:49 AM   #1
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Severe blocking artifacts in red colors

Hey all,

This is a problem I have been noticing recently on several projects I have done. If you refer to the attached image, you can see that all the colors in the image appear clear and correct (ignoring any small jpeg artifacts) but that the reds on the woman's shawl are very blocky and messed up.

Like I said, this has happened on several different occasions, and only occours with the color red. I am shooting in 24pa, and no artifacts are visible at the the time of recording, only during playback either on the XL2, in FCP, or on one of my broadcast monitors.

I have been searching the forums for any answers to no avail. Perhaps something to do with DCT compression???? Any ideas would be appreciated!! Thanks!!
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Severe blocking artifacts in RED-blocking.jpg  

Last edited by Ryan Black; December 5th, 2006 at 11:36 AM.
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Old December 5th, 2006, 10:45 AM   #2
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reds are notoriously challenging to DV. if you're working with clients, advise them not to wear it for the shoot. no stripes either. no stripes, no reds. some things that can help mitigate the effects of bouncing reds include de-interlacing and flicker filters....
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Old December 5th, 2006, 11:06 AM   #3
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Quote:
Originally Posted by Meryem Ersoz
reds are notoriously challenging to DV. if you're working with clients, advise them not to wear it for the shoot. no stripes either. no stripes, no reds. some things that can help mitigate the effects of bouncing reds include de-interlacing and flicker filters....
I have to agree, red is also problematic in the world of digital photography and after processing.
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Old December 5th, 2006, 11:20 AM   #4
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I have wondered this too. Why are they a problem? Its seems to me that because of the 4:1:1 compression that both blue and red should be equality as bad but blue seems to be okay most times. I have only had problems with blue when I tried blue screening stuff and learned that it bleeds on some occasions. So I use green now. But why is read a hard color to capture and how can we correct the problem either in camera (maybe some kinda modification) or in post?
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Old December 6th, 2006, 10:42 AM   #5
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If your NLE has a chroma blur effect, try using it when you see chroma blocking like this. It will blur only the color information in the clip (therefore not blurring the b&w luma part at all), effectively "smoothing" out those blocky edges in your reds. Depending on the severity of the problem, which depends mainly on the shade of red in question and the level at which the red is exposed, this approach is often just the little magic trick that everyone is looking for.

If your NLE doesn't have chroma blur--Vegas is the only one I know really well--then you can try the Magic Bullet deartifactor, which is just a fancy name for chroma blur. It comes with Magic Bullet. It's nowhere near as adjustable as Vegas' chroma blur, for example, but it's more or less the same thing.
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Old December 6th, 2006, 11:07 AM   #6
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The problem is that red is a very important colour to the human eye and it can be very difficult to avoid filming a subject that has some red in it, and more often, especially in nature, it is a vitally important colour to include in some scenes or landscapes.

Yes, deep or bright reds have always been the most difficult to capture cleanly, especially with sunsets over water.

The older XL1s was far worse in this respect and I think that the XL2 copes far better with red subjects.
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Old December 6th, 2006, 12:35 PM   #7
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Thanks for your input guys. It is weird how it only seems to happen with specific hues of red. And I know that it is not a defect with my specific camera, because I am running a multicam XL2 setup, and it is affecting all of them. Maybe I just found the "compression sweet spot" with red, where the in-camera compression freaks out.

Jarrod, I tried out the "chroma blur" filter (in FCP 5 its the "Channel Blur" filter) and while smoothing the reds a little, it seemed to just add in more fringing and blocking artifacts around the red on the subject.

The other thing I thought of, is that perhaps it is the contrasting colors of red on blue? They are on opposite ends of the light spectrum if I remember correctly, and that may contribute to the issue. But then again, the blocking is prevalant throughout the red tones, not just the edges.......I give up!!!
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Old December 6th, 2006, 12:38 PM   #8
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Quote:
Originally Posted by Ryan Black
Jarrod, I tried out the "chroma blur" filter (in FCP 5 its the "Channel Blur" filter) and while smoothing the reds a little, it seemed to just add in more fringing and blocking artifacts around the red on the subject.
Sorry that didn't help very much. It usually works quite well in Vegas, if that's helpful to anyone here.
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Old December 9th, 2006, 02:57 PM   #9
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So can someone please explain scientifically why this happens? Like I said above it seems because of the 4:1:1 compression that it would happen equally in the blues, but I dont seem to notice.
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Old December 9th, 2006, 05:15 PM   #10
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I've always thought that it happens with all colors, but that it's just more noticeable with red. If you look very carefully, you can often see the same kinds of blocking on greens and blues--it just doesn't stick out as much.

That's not very scientific, and might possibly be a load of crap. Sorry. :)
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Old December 10th, 2006, 02:02 PM   #11
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Green is the cleanest channel, blue is the sharpest, red is somewhere in the middle. The result, blues get noisy due to the sharpening effect, greens are VERY clean and reds are neither! Chroma blur will only help if you are not recompressing to DV. This is generally a bigger problem when you have warm white balances, the saturation turned up or if the the reds are a little overexposed.

When I shoot something with lots of red I turned down the red gain, turn UP the coring and down the sharpness a little. Master pedestal going down a bit helps as well. I generally am able to avoid this almost every time, it just takes some practice.




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Old December 12th, 2006, 08:52 PM   #12
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Thanks Ash I will try those settings next time I am faced with this problem.
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Old January 6th, 2007, 08:10 AM   #13
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Also can be because of the fact that the ccds have a very non linear spectral sensitivity much higher in the red area (for example look at http://www.ph.tn.tudelft.nl/Courses/...-Spectral.html )
The camera manufacturers try to compensate that curve electronically but i suspect that many times the reds get saturated so the ccd "pass" a clipped information to the electronics. Im a PAL user and I saw many times the blocks in saturated reds.
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