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 Author Topic: Does a raw file have a color space?  (Read 103607 times)
mrgalleta
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Thanks a lot for your replies,

What I dont really understand is how to make use of those matrices to make this kind of  conversion. I mean, if you take two pictures at different shutter speed values of the exactly same scene, then the values of a little area of the picture would have the same XYZ values but the RAW data of the camera should be different, so this matrix should only be useful for specific shutter speed, and so, isnt it ?

Thanks again and excuse me if say anything that makes no sense, I'm just learning in this extremely interesting subject.

Regards,

Carlos.
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What I dont really understand is how to make use of those matrices to make this kind of  conversion. I mean, if you take two pictures at different shutter speed values of the exactly same scene, then the values of a little area of the picture would have the same XYZ values but the RAW data of the camera should be different, so this matrix should only be useful for specific shutter speed, and so, isnt it ?
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No, the XYZ values would not be the same in the 2 cases. Why should they be? Both the XYZ (D50) and camera RGB values would be lower in the case where the exposure time is longer (i.e., slower shutter speed).

Eric
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But yes, the matrix is very likely a best fit linear map from the camera's spectral response curves to those of the XYZ color matching functions (minimizing the difference between the mapped curves and the XYZ cmf's), along the lines that I posted above in this thread.
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More precisely, the matrices map (non-white-balanced) linear camera coordinates to XYZ with a D50 white point.
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mrgalleta
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No, the XYZ values would not be the same in the 2 cases. Why should they be? Both the XYZ (D50) and camera RGB values would be lower in the case where the exposure time is longer (i.e., slower shutter speed).

Eric
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Sorry, maybe I didnt explain myself properly, Lets say that I have an spectroradimeter and I point it to a place that doesnt change in illumination, the spectroradiometer would give me the XYZ values. Then, i put a camera on the same place pointing at the same direction and I take de bayer array values of the place where the spectroradiometer was pointing at. If I take to pictures at different exposures, is there a way of getting with the camera approximately the same XYZ values obtained with the spectroradiometer independently of the exposure?

Thanks a lot.
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I thought white was X=Y=Z; or is there a distinction between the terms "white" and "white point"?

And BTW Bill, I don't have any special qualifications or claim to authority for this discussion.  Just eager to learn.
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Just in case it wasn't already answered, the white coordinates in XYZ depend on the illuminant. Y is 1 but X and Z are generally not. For example, the D50 white point is about X = 0.9642, Y = 1.0, Z = 0.82521. You can see this page for the XYZ coordinates of various common white points:

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ejmartin
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Thanks a lot for your replies,

What I dont really understand is how to make use of those matrices to make this kind of  conversion. I mean, if you take two pictures at different shutter speed values of the exactly same scene, then the values of a little area of the picture would have the same XYZ values but the RAW data of the camera should be different, so this matrix should only be useful for specific shutter speed, and so, isnt it ?

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Changing the exposure time by a factor X linearly rescales the raw data by X.  In order not to screw up this linearity, any attempt to fit camera raw values to XYZ values should be linear (XYZ values are also linear wrt change of luminosity).   So the general map is

"X" = a1 R +a2 G + a3 B
"Y" = b1 R +b2 G + b3 B
"Z" = c1 R +c2 G + c3 B

where "X", "Y" and"Z" are the best approximation the camera can make to XYZ values of the recorded scene, RGB are the camera raw values for the three color channels (suitably interpolated, and ignoring any issues stemming from such interpolation), and the coefficients ai, bi, and ci (i=1,2,3) are determined by fitting the spectral response functions of the camera to the XYZ spectral response functions as best as possible via a linear map.
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emil
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Yes, it's just a linear scaling, and hence will be preserved with a linear transform such as Adobe's camera/XYZ matrices.
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zcream
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I am trying to convert RAW image data with no header info into a usable DNG. ATM, I am using this color matrix
Color Matrix values 1 to 3...: 1.0048828125 -0.27294921875 -0.04931640625
Color Matrix values 4 to 6...: -0.56689453125 1.34936523437 0.20776367188
Color Matrix values 7 to 9...: -0.2919921875 0.34936523438 0.63916015625

I am getting gaps in the picture after conversion. Stippling?

https://dl.dropboxusercontent.com/u/9906333/fc2_save-0000.jpg
I used a jpg here but can see the same issue in the DNG file in preview or Irfan View.
There seems to a multicolor bands when zooming in on the darker areas.

I was wondering if this could be due to the color matrix ?

I saw other matrices here - http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
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