Luma calculation

[trevlac]

Why do filters in Vdub use the HDTV spec to calculate luma?
Greyscale for example. Why not spec 601?

The results are different in the middle of the range.

For 0,191,0

54R + 183G + 19B = 137Y
77R + 150G + 29B = 112Y

I don't know what is correct, I was just trying to figure out why.

[Belgabor]

I have no idea :)
You'd have to ask Avery.

[trevlac]

I asked. He said greyscale is intended for viewing not video conversion. The 709 specs are better for viewing. He referenced Poynton. Here is what Poynton says:


Contemporary CRT phosphors are standardized in Rec. 709 [8], to be
described in section 17. The weights to compute true CIE luminance from
linear red, green and blue (indicated without prime symbols), for the Rec.
709, are these:

Y = 0.212671 * R + 0.715160 * G + 0.072169 * B;

This computation assumes that the luminance spectral weighting can be
formed as a linear combination of the scanner curves, and assumes that the
component signals represent linear-light. Either or both of these
conditions can be relaxed to some extent depending on the application.

Some computer systems have computed brightness using (R+G+B)/3. This is at
odds with the properties of human vision, as will be discussed under What
are HSB and HLS? in section 36.

The coefficients 0.299, 0.587 and 0.114 properly computed luminance for
monitors having phosphors that were contemporary at the introduction of
NTSC television in 1953. They are still appropriate for computing video
luma to be discussed below in section 11. However, these coefficients do
not accurately compute luminance for contemporary monitors.