[Bf-committers] Color to greyscale conversion - confusion

[Jeroen Bakker]

Hi

Well as already said, there is no correct formula. Also if you think 
about what the method does it is basically two things:

1. mimic black and white conversion by multiplying the color channels 
with a certain value. Enhancing a light-channel to influence the BW more.
nR = nG = nB = 0.2126 R + 0.7152 G + 0.0722 B
2. do a conversion to an undefined color space. (if you may call it a 
color space)
Value = (nR+nG+nB)/3 // as nR NG & nB are same, we can optimize to value=nR

There are many aperature plugins only to mimic black and white 
conversion (silver efex pro) in case for the compositor. But Campbell 
has a point that we don't have a clue when to use which and why. There 
are to many assumptions that is not clear in the method name.

Luminance is chosen as it works best in most cases.
see here comparison between the different algoritms 
http://www.tannerhelland.com/3643/grayscale-image-algorithm-vb6/

"A full discussion of which formula is “better” is beyond the scope of 
this article. For further reading, I strongly suggest the work of 
Charles Poynton <http://poynton.com/>. For 99% of programmers, the 
difference between these two formulas is irrelevant."
- But we are the 1% right :) -
Jeroen.


On 06/23/2012 04:27 AM, Troy Sobotka wrote:
> On Jun 22, 2012 11:56 AM, "CoDEmanX" <codemanx
> <codemanx at gmx.de>@<codemanx at gmx.de>
> gmx.de <codemanx at gmx.de>> wrote:
>> but correct seems to be:
>>
>> Y = 0.2126 R + 0.7152 G + 0.0722 B
> Again, there is no singular "correct".
>
> I believe the above is a luminance based conversion from linearized sRGB.
>
> With respect,
> TJS
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-- 

Met vriendelijke groet,

Jeroen Bakker

*At Mind BV
*

Telefoon: 06 50 611 262
E-mail: j.bakker at atmind.nl <mailto:j.bakker at atmind.nl>