[SDL] How to gray out a button image?

[Erik]

Will Langford skrev:
> The formula for converting something to grey scale is something similar to
>
> G * 59%
> B * 11 %
> R * 30 %
>
> Those are pulled off my head from a project about 7 years ago, so they
> might be wrong.... but they should get ya darn close :).
>   

Thanks for the information. I had no idea that there should be such 
magic numbers in the calculation. I would have just used the 
(unweighted) average, like David Olsen wrote. Is there a link with the 
rationale for those particular numbers?

I was actually expecting to not do the gray scale conversion myself. I 
was thinking that we would not be the first SDL based project that uses 
pictorial buttons that can be disabled. I was hoping that there was some 
library function that could do it and take advantage of special hardware 
features on different platforms (such as MMX).