Hi All,
I just wanted to open a discussion about something that has been bothering me for years now.
I would like to know how to accurately calibrate photographed texture maps for use in Corona, so that texture behaves physically accurately, as well as just looking correct.
I have been using a color checker to calibrate photographed textures, this works but I have always felt that the colours are maybe not true to life when looking at the rgb values (see image attatched).
The white is 243 or 95.3% reflective, which sounds far too reflective, and most of the coloured swatches are 100% saturation, which is far greater than with the default camera profile.
The following stagement: "The code value of 118 in an 8-bit system corresponds to 18% of the maximum final output for sRGB." suggests that the correct value for 'mid grey card' which reflects 18% of the light hitting it would be rgb 118,118,118, instead of rgb 46,46,46 because of the gamma curve.
I'm assuming Corona offsets the gamma curve, converting textures to linear color space before path tracing.
If this it the case, then a value of 128 in gamma 2.2 would result in rgb 47, or 18% reflectivity (sounds about right...) and a value of 243 in gamma 2.2 would result in rgb 225, or 88% reflectivity, which also sounds.. correct.
This suggests that the X-rite calibration values are in fact accurate to physical reflectance values, when converted to Linear colourspace / Gamma 1.
The question is, if this is correct, why have I been hearing for so many years from almost every source that a white value of 243 would be far too high, and that 225 should be a maximum.
It seems all the references of albedo to rgb value conversions have not been taking the gamma curve into account?
I'd really love to hear peoples thoughts on this because it is something that has never sat well with me.