[cairo] Forcing black and white or grayscale

[Lawrence D'Oliveiro]

On Fri, 22 Apr 2016 18:26:21 +0000 (UTC), Michele Cleary wrote:

> Does anyoneknow how to use cairo to force a job to be black and white
> or grayscale?

It’s quite simple:

1) Draw the full-colour image into the destination image surface
2) Set a source colour RGB = (255, 255, 255) and transfer mode
CAIRO_OPERATOR_HSL_COLOR <http://cairographics.org/operators/#hsl_color>
3) Paint this source all over the destination image surface. That will
replace the hue and saturation (in particular, set the saturation to
zero), leaving the greyscale intensities unchanged.