[SDL] How to gray out a button image?

[Andreas Schiffler]

You may want to use integer math ...
http://en.wikipedia.org/wiki/YUV#Numerical_approximations

The "brightness" value is the Y component. See also the blurb on MPEG 
compatibility (re: range of Y) and note that the input RGB values need 
to be uint32's to avoid overflows.

Will Langford wrote:
>>  grayed = (source_pixel_red * 0.10) + (source_pixel_green * 0.61) +
>>  (source_pixel_blue * 0.19);
>>     
>
> Doh, mistyped math in the line,
>
> grayed = (source_pixel_red * 0.30) + (source_pixel_green * 0.59) +
> (source_pixel_blue * 0.11);
>
> -Will
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