BGR <-> L*ab accuracy

C++
3

I gave this problem a break, and came back to it later. There were two problems, and it’s solved.

  1. I’m pretty sure the CvtColor function is not fully 32F compatible, despite what the manual says. My guess is that the data is at some point rounded into a reduced number of quantization levels introducing quantization error. I haven’t counted them, but if I convert my data into L*ab / HSV / HLS and directly back again, it has lost precision, and therefore, a lot of detail. I have re-implemented these functions maintaining floating-point arithmetic and 32F precision, it is seems to be fine now.

  2. My BGR2Lab function in my last post missed a key step that is not documented in the manual. I now understand that the RGB bands need inverse companding before they can be converted to XYZ and then to L*ab. I’ve not been interested in the details of color space conversion before, and so this detail had escaped me until now.