Quote:
Originally Posted by e-t172
- You say "use 2.4 if you have high contrast, BT.1886 otherwise". Thing is, BT.1886 uses a gamma function with… 2.4 as the exponent. What's the difference? Is it because the BT.1884 function has "black compensation" (the "+ b" term)? As far as I'm aware, most calibration software (at the very least HCFR) displays black compensated gamma, i.e. when calculating gamma, L' is used with L' = L - L(black). That would mean that, within such software, the "+ b" is already accounted for and thus your target really is 2.4 as calculated by the software.
- Appendix 2 indicates that "This Recommendation does NOT change any signal parameters defined in Recommendation ITU-R BT.709". The way I understand it, this only makes sense assuming that BT.709 defines the optical → electrical transfer function, while BT.1884 defines the electrical → optical transfer function. However, having different transfer functions for "input" and "output" strikes me as odd, as that would mean that when capturing something using an ideal reference camera and then playing it back on an ideal reference monitor, the display will not be consistent with the reality of what has been captured, because of the difference between the transfer functions used for capture and playback. What gives?
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Okay, so, thinking about it some more, I arrive at the following conclusions:
- BT.709 very clearly uses the term "
opto-electronic conversion", while BT.1886 uses the term "
electro-optical transfer function". This does seem to indicate that BT.709 defines the parameters for the capture/transfer side, while BT.1886 defines the parameter for the playback side. I am still unsure why they are different, but I assume there's a good reason.
- BT.1884 describes "black-compensated gamma", which, as far as calibration software and procedures are concerned, is basically what is referred to as simply "gamma". Black-compensated gamma is equal to simple gamma when the display contrast is infinite (e.g. CRT).
- According to BT.1884, the One True Display Gamma©®™ is
2.4.
One very interesting thing is that display gamma being 2.4 and not 2.2, this means it is actually different from the sRGB gamma, which is 2.2 (well, approximately). This brings me to an interesting thought regarding madVR: considering that madVR is running on a Windows PC, the defaults should assume that the display is calibrated for sRGB, which is the standard for this platform, and should by default convert the content from 2.4 to 2.2.
In other words, for maximum compliance with current specifications, madVR should default "the display is calibrated to the following transfer function / gamma" to "pure power curve 2.20" (IIRC it already does), because sRGB, and more importantly, it should default "enable gamma processing" to "enabled, pure power curve 2.40", because BT.1884.
@madshi: I'm advocating a change in the default settings. What do you think of this reasoning?
On a related note, it would be useful to have a "sRGB curve" option under the "calibration" tab. It should be extremely easy to implement as it is exactly the same curve as BT.709 but with different coefficients for the linear part. If this is to be implemented then the default setting would be "sRGB curve 2.40" (as 2.2 is just the pure power curve approximation).