A720 IS and its overexposure-behaviour - page 6 - General Discussion and Assistance - CHDK Forum

A720 IS and its overexposure-behaviour

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Offline LjL

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Re: A720 IS and its overexposure-behaviour
« Reply #50 on: 22 / July / 2008, 13:30:28 »
I think I understand what you're saying, and I understand why you make a difference between the non-linear "RGB" space and the linear sensor space.

But I was simply nitpicking on the usage of the term "RGB space" in this sense.

RGB space simply mean a color space made up of three values, one for red, one for green and one for blue. That's all it means. It says nothing about linearity, so you could have a linear "RGB space" or a non-linear "RGB space".

For that matter, it doesn't even say what "red", "green" and "blue" are. It's a pretty generic term.

See how Wikipedia defines RGB space: "An RGB color space is any additive color space based on the RGB color model. [...] The complete specification of an RGB color space also requires a white point chromaticity and a gamma correction curve." (emphasis mine).

So, "an" RGB space can very well have gamma=1.0, and be linear with the input data - if the input data were produced using a sensor that, in turn, has a linear output, of course.

It says then: "As of 2007, sRGB is by far the most commonly used RGB color space". Now this is "an" RGB space, and one where the non-linearity is present very well defined: "The overall gamma is approximately 2.2, consisting of a linear (gamma 1.0) section near black, and a non-linear section elsewhere involving a 2.4 exponent and a gamma (slope of log output versus log input) changing from 1.0 through about 2.3".

Can you have linear RGB? Of course you can, just use a linear sensor and then apply no gamma. Or use a non-linear sensor and then appy gamma to compensate the non-linearity. My scanner natively outputs something that very closely approximates linear RGB, although the driver software then converts it to something closer to sRGB.


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