Re: [boost] [GIL] color convert and color tranfer characteristics

Subject: Re: [boost] [GIL] color convert and color tranfer characteristics
From: Christian Henning (chhenning_at_[hidden])
Date: 2010-08-05 13:47:35

Hi Fabien, good conversations! Been a while I was thinking about gil so much.

> I have a problem with the distinction between colorspaces and color characteristics.
> I'm writing a basic transfer characteristics between lin, log, sRGB, rec601, rec709...

Interesting! I would be interested in those implementation for the
toolbox extension. Let me know if you're interested submitting. I can
help out when needed.

>
> I'm writing something like I talked for colorspaces, in the mail about "color_correct":
> * a specialization between each color characteristic and lin
> * a generic conversion to a templated type using an intermediate lin conversion
>
>
> I have two problems:
> 1) I have written a per channel functor, which seems to be the right way.
> But I don't know how to not apply it on the alpha channel if there is an alpha channel... without loosing performances because my common usage is rgba :).

I'm not sure I follow here. Small code sample possible?

>
> 2) I don't know how we can connect this with color_convert, because if we want to make a conversion from RGB to gray,
> we have to assume that RGB is linear (?) and we need to precise if we want a rec709 or rec601 luminosity...
>
> GIL code use an approximation of Rec601 to compute the luminance.
> Y' = 0.299 R' + 0.587 G' + 0.114 B' (Rec. 601 coefficients)
> Y' = 0.2126 R' + 0.7152 G' + 0.0722 B' (Rec. 709 coefficients)

Right now color_convert doesn't supply passing the rgb_to_luminance
algorithm. But I 'm not sure if that's needed for your case. When you
use the algorithm copy_and_convert_pixels algorithm you pass as the
third parameter the color converter. In your case just define your own
color converter.

Regards,
Christian