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Offset, Gamma, Gain


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#1 Casper Buijtendijk

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Posted 19 May 2012 - 11:06 AM

An undoubtedly silly question: I've been playing around with SpeedGrade for 2 days now, and I've got three wheels to work with: Offset, Gamma and Gain. Now, I've messed around with the three, but I don't exactly know what they mean, and google doesn't really seem to help. In the past I worked with Premiere's curves, which really worked for me, but I thought I should move to something better. Can anyone tell me what those three things mean? Thank you!

Edited by Casper Buijtendijk, 19 May 2012 - 11:07 AM.

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#2 Phil Rhodes

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Posted 19 May 2012 - 12:44 PM

Offset (sometimes termed lift) is an additive function; it simply adds a fixed amount to all the brightness values of the image. You will notice that when you increase offset, the entire waveform display slides up and down without changing shape. You can do the same thing in curves by moving the two ends of the initial straight line towards the top left, so the bottom end slides straight up and the top end moves left. Offset is most obvious in the shadows.


Gamma is an exponential function; it raises the values of the image to a fixed amount, often in the range 0.5-3. This has the effect of bending the middle of the curve, much as you'd get if you added a single control handle in the middle of a curve and dragged it towards the top left or bottom right of the curve display. This is, obviously, most apparent in the midtones.

Gain is a multiplicative functionl; it multiplies the values of the image by a given amount. This is like moving the top right end of a straight-line curve left, so the entire image gets brighter. Gain is similar, though not identical due to nonlinearity, to opening the iris. Gain is most apparent in the highlights.

These are terms which are left over from video processing amps, where these features were implemented as analog video amplifiers. The first creative colour work was done by using (or misusing) the video processing amps that on telecine outputs, ostensibly to allow the operator to adjust to the individual characteristics of the negative; the terms persist into software to this day. The layout of three trackballs with a surrounding rotary control is based on the three joysticks of early video processing amps.

Modern implementations frequently just call these three "shadow", "midtone" and "highlight", but in any case they may not (in fact usually do not) literally implement the processing-amp functions as literal additive, exponential and multiplicative mathematics. Commonly it's possible to define what each control does, the extent of its effects, somewhere in the software's settings, or even on a per-grade basis.

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#3 Bruce Greene

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Posted 19 May 2012 - 03:12 PM

Good answer Phil.

However, I think offset is more akin to iris or "exposure" and gain is like changing the "ISO" setting in the camera.
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#4 Will Earl

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Posted 24 May 2012 - 12:12 PM

Depends on whether you and the grading tool are working in linear or log.

In log, using an offset is roughly the same as an exposure adjustment - from memory it's something like 0.18 equals one stop, so if you wanted to increase the exposure by 2 stops you'd set the offset to 0.36.

In linear, adding an offset is more like flashing the negative as it's most obvious effect is to lift the shadows in the image - it will lift the entire image but the effect is not the same as an exposure adjustment.

Gain can be made to act like an exposure value adjustment in stop values by setting the gain value to 2^stop (the ^ symbol represents "to the power of" in maths geek). So if you wish to increase the exposure by 3 stops you would set the gain to 8 (ie... 2^3). In order to reduce the exposure by 3 stops you would set the gain to 0.125 (2^-3). Although like any thing done on the computer, it's probably not quite the same as changing the aperture or iso on the lens or camera - it does double/halve the values recorded in the image.
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