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Degorath

Color Balance

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Degorath    100
A year ago, I started working on a video editing application for an animation company (which I will X). X wanted it to work like Flame (by Discreet but is now probably owned by Autodesk). Has anyone ever heard of it? Or seen it? Apparently it is rather expensive and requires a massive machine to work. So X wanted a cheap, fast solution. Right now, I'm working on the color balance algorithm. Currently I'm using my own algorithm based loosely around a white balance algorithm. This looks for the brightest and whitest patches (But not as white as RGB 255, 255, 255) and finds the dominant color in the bright patches. I have found that this works very good for over-blue and over-green images but not so good for over-red images. Does anyone know of other fast (as it will be used to process WHOLE MOVIES) algorithms I might use?

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Yann L    1802
You can't just find the brightest spots in an image and white-correct towards them. The image you have is already tonemapped, and lacks the full range HDR information. There is no way of knowing the true spectrum of a saturated light patch on a LDR medium such as digital or traditional film. Also, if you correct each frame individually, you'll get a completely messed up colour fading and morphing throughout the entire movie.

In professional movie postproduction and photography, grey cards are used instead. Before a sequence is shot, a camera operator assistant holds a card (or usually multiple cards) infront of the camera. These cards have standarized diffuse emission spectra. The white and colour balance is adjusted onto these cards, once per sequence. Not on individual frames or on unknown materials.

Furthermore, colour correction is much more complex than just a simple linear remap. You need to take the spectral response curves of the CCD or the film into account, the CIE psychovisual models, etc.

If you use a cheap linear algorithm, the results are just not going to be good. Especially under more challenging lighting conditions. Even Flame only offers rather basic colour correction tools. For the real thing, you'd need something like Autodesks Lustre.

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Degorath    100
I checked out Lustre. Thanks!

With regard to single images, what techniques are available?
Do you mean that the frames containing the gray cards are already balanced? Mmm. Perhaps I should try to understand what CAUSES unbalanced frames before I try to go on?

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Yann L    1802
Quote:
Original post by Degorath
With regard to single images, what techniques are available?
Do you mean that the frames containing the gray cards are already balanced?

No, all frames are initially unbalanced. The frames containing the grey cards supply the reference for white and colour balance data applied to the remaining frames of a sequence. A sequence is defined as a set of frames that share the same colorimetric base. Usually that means several takes from a single set with the exact same lighting conditions can reuse colorimetric correction data. As soon as you change the location, lighting, or even something as basic as a colour gel, you need to readjust the colormetry by using greycards before the take. Most directors get pissed at this (because it takes additional time), but not doing it can cause major headaches during post production.

Technically, this means that you will pick the colorimetric reference base from the frames containing the grey cards. Since you know the reflectance spectrum of each card, as well as the spectral transfer curves of the CCD or the film material used during the shot, you can accurately calculate the correction and balance parameters. This data is then applied to all subsequent frames from the take or sequence.

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