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mahmoudnafifi / C5

Reference code for the paper "Cross-Camera Convolutional Color Constancy" (ICCV 2021)
Apache License 2.0
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Question about your data augmentation method and CIE XYZ color space #9

Closed DY112 closed 2 years ago

DY112 commented 2 years ago

Hi, @mahmoudnafifi , I have a question about your data augmentation method.

I think that I have a little confusion about the color space transform process in general ISP, which converts WB applied raw image into CIE XYZ color space. (I referenced the 2019 ICCV tutorial by your supervisor, Professor Michael Brown)

As far as I know, CST only changes the axis(or basis) representing the color, it doesn't change the unique color itself. So from what I understand, the CIE XYZ images (with WB applied) for the same scene on two different devices are different because they represent colors (unique colors, which are different from each other) observed by different sensors in the canonical color space (axis, CIE XYZ space).

However, according to the data augmentation method presented in the paper, the above sentence I said is wrong. According to the method used in your paper, since images in CIE XYZ space are device-independent, data augmentation in RAW corresponding to each device is possible using conversion/inverse transformation to CIE XYZ space.

I'd appreciate it if you could let me know which of the two is correct in the part where I'm mistaken.

mahmoudnafifi commented 2 years ago

Hi @DY112,

Thanks for your question. I think there is some misunderstanding. Suppose we have an object with uniform reflectance properties with a single diffuse color and it is let by a uniform light. If this object was captured by 2 different camera sensors which have different sensitivities, the captured images may have two different RGB values of this object's pixels. If camera calibration perfectly projects colors from the sensor space to the CIE XYZ space, we should have precisely the same chromaticity values of this object's pixels. That is, the color of this object should be represented by the same values in the CIE XYZ space. However, this projection to CIE XYZ is not always perfect and practically we may have some color differences even after this projection. In the paper, we followed the theory and assumed the calibration 3x3 matrix accurately maps to the CIE XYZ space. This assumption is valid only if the sensor satisfied the Luther condition.

Hope this helps.

Thanks!

DY112 commented 2 years ago

Thanks for the detailed information. As I expected, I misunderstood, and the conversion to CIE XYZ color space is a calibration operation that corrects the difference between the two camera sensors.

Thans!!