[sugar] how to make a grayscale image?

[Albert Cahalan]

Eben Eliason writes:

> Roughly speaking, you can calculate a colored pixel's effective
> luminance by:
>
> Y = 0.3*R+0.59*G+0.11*B

To be clear on why this is rough: it performs an operation
on non-linear data which is only valid on linear data.
That is, it ignores gamma.