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Old June 17th, 2007, 07:11 PM   #1
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gamma control vs. curves

for very low-light footage that needs to be brightened, why do i get so much better results using the gamma control slider on, say, the primary color corrector plugin, compared to using the curves plugin? (of course i also have to move the offset and gain controls on the color corrector plugin)

when i make the slightest adjustment with the curves control i get discontinuities in the color histogram scope. when i make adjustments using the gamma control in the color corrector i continue to get a clean continuous histogram. what is going on here? is the color corrector plugin using wider bit-depth internally? i see no way of duplicating the effect of the gamma curve slider using curves: besides the smooth histogram and output it produces, i can't seem to replicate it even in an unsmooth way. i'm completely happy using the gamma curve control from now on (i used to use the curves to brighten footage), but i'd like to know what's going on.

thanks!
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Old June 17th, 2007, 07:44 PM   #2
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so i did a quick experiment with various generated color gradients. it turns out that with a white gradient i can get close to matching the effect of the gamma control using curves. but with a solid color, like red, i can't. the gamma control is adding in blue and green when it needs to get brighter. the curves control only applies a simple transfer function to the colors that are already there. i can't add a new color using those. and my luma scope shows it can't get nearly as bright as i can make it with the gamma control.

the channel blend plugin can sort of do that. but the color it's adding is based on a simple multiplier of the other colors, not a curve. perhaps i have just discovered what 3-d LUT's are for? can any color experts chime in?
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Old June 17th, 2007, 07:45 PM   #3
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The gamma control on the color corrector operates in YCbCr color space internally (in 32-bit float). It is likely generating fractional values that round in a way that fills the histogram.

IMO, the curves FX is FAR superior. Adding a curve to the Y' makes the image look funky.

In terms of gaps in the histogram... this doesn't really mean that much. Adding noise to the image will fill in the gaps... but this doesn't exactly improve image quality!! Visually (and this is really what counts), color curves is better than using gamma on the color corrector.
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