R' G' B' vs R G B

[wswartzendruber]

I'm seeing a lot of stuff out of the ITU refer to R' G' and B' values.

What does the apostrophe mean?

[Sharc]

R'G'B' are "Gamma corrected" values of R,G,B.

[wswartzendruber]

So...linear brightness?

[Sharc]

Simply speaking a pre-distortion to to linearize (compensate) a non-linear display characteristic.

[wswartzendruber]

I went through BT.2100 and anything with an apostrophe after it seems to reference non-linear.

[Sharc]

Yes, in order to compensate the display non-linearity one has to inject a signal with the 'inverse' non-linearity to make the result look 'linear'.

R'G'B' is gamma corrected RGB.
The gamma correction adjusts the overall brightness of an RGB image by normalizing the pixel's RGB color components from its current integer values of 0 through 255 (for 8bit) to real numbers between 0.0 and 1.0.
It then raises the normalized value of the pixel to a specified power, which is otherwise known as the gamma value. It will then transform the new value back to an integer component between 0 and 255 (for 8bit).

Gamma corrected color is properly noted using the ' symbol, as in Y'U'V' or R'G'B', but generally speaking all color on a computer should be assumed to be gamma corrected unless you have reason to believe otherwise.

More info from here:
http://poynton.ca/notes/colour_and_gamma/

[Balling]

"So...linear brightness?"

What? Brightness is Y'. No, R' in non linear. BT.2100 is HDR. There PQ is the nonlinearity.

[DTL]

Simply speaking a pre-distortion to to linearize (compensate) a non-linear display characteristic.

It was far at the past. Nowdays it is used for bitcompression (small for SDR and about 3:1 for HDR).

Apostrof mean data is converted into system's transfer function domain and may be (or not) in bit-reduced form with small enough visible banding. Or invisible at all.

For example linear PQ HDR encoded linear range about 1..10 milion and need about 30 bits in linear encoding and only about 10 bits in PQ transfer domain.

Also for some 'perfect world rule' - apostrof mark mean data is in the compressed form and no processing is possible before decompressing because it will damage data. Unfortunately in real world even old ITU standards (and current) still directs to make processing with compressed data and it creates irreversibly damaged content.

The Ponyton's site (still working) have good FQA on non-linear transfer: http://poynton.ca/notes/color/GammaFQA.html
Unfortunately the bit-reduced 'compressed' form of data is not completely loss-less so for best results for moving pictures it still recommended to use linear-domain data. As noted in FQA the linear SDR require only about 12..14 bit and with 'compressed' form to 8..10 bit user get only 12:8 .. 14:10 datarate reduction while already having some distortions (in spatial frequency domain that is about levels transients). It may be not any critical for static pictures but visible at motion pictures. The transform to system-Transfer_Function and back is lossless only if it keep full bitnumber in_between but it is useless for data rate reduction in this way. Though it is useful for lossy compression like MPEG/JPEG operating for even spreading of perceptive encoding errors over the perceptive luminance range.