Doug Kerr
Well-known member
I have assumed that one of the following must be true for a Canon EOS camera:
A. The camera must embed in a JPG file information on the "color correction vector" used to perform color correction on the image in that file.
The reason is that, when we select a JPG image file to be the "reference frame" for use in setting a custom white balance (CWB), the camera, before evaluating the average chromaticity of the image over the "center spot" must "back out" the color correction that is has been applied, for which it would need to know the vector that had been applied.
Otherwise, the conclusion reached by the evaluation would be irreversibly poisoned by the color correction that had been applied (and we are not advised to set some specific fixed color correction when taking the reference frame to dispose of that uncertainty).
B. The camera must, on every shot, make a determination of the average chromaticity of the central spot (from the raw data, presumably) and record that in the JPG file metadata for use just in case the user later selects that frame for use as a CWB reference frame.
We know that when we take a raw output file, the color correction vector that was (or would have been) used to correct the JPG output is recorded in the metadata. We can read this in various raw processing tools.
Now:
1. Do we know whether A or B above is what actually happens?
2. Do we know how to read the color correction vector from a JPG file? (Perhaps it is read by some well-known image editong or viewing tool.)
Thanks for any insight into this you may have.
A. The camera must embed in a JPG file information on the "color correction vector" used to perform color correction on the image in that file.
The reason is that, when we select a JPG image file to be the "reference frame" for use in setting a custom white balance (CWB), the camera, before evaluating the average chromaticity of the image over the "center spot" must "back out" the color correction that is has been applied, for which it would need to know the vector that had been applied.
Otherwise, the conclusion reached by the evaluation would be irreversibly poisoned by the color correction that had been applied (and we are not advised to set some specific fixed color correction when taking the reference frame to dispose of that uncertainty).
B. The camera must, on every shot, make a determination of the average chromaticity of the central spot (from the raw data, presumably) and record that in the JPG file metadata for use just in case the user later selects that frame for use as a CWB reference frame.
We know that when we take a raw output file, the color correction vector that was (or would have been) used to correct the JPG output is recorded in the metadata. We can read this in various raw processing tools.
Now:
1. Do we know whether A or B above is what actually happens?
2. Do we know how to read the color correction vector from a JPG file? (Perhaps it is read by some well-known image editong or viewing tool.)
Thanks for any insight into this you may have.