[cairo] Color-Tinting a surface with alpha channel

[Uli Schlachter]

Am 24.12.20 um 21:10 schrieb Simon Schneegans:
> Hi Uli,
> 
> thank you for the kind reply!
>> Could you explain a bit more what exactly you want to achieve? E.g.,
>> what should happen when tinting with r=0.5, g=1, b=0, a=0.5 to colors
>> such as full red and... basically all the other colors.
> 
> I think you understood my issue very well, but I will try to explain it
> with more examples.

Thanks a lot. The image helped. My checkerboard pattern does not look as
well as yours, but attached is a program which seems to do what you
want. You want to look at the function tint().

It works like this:

A given surface is split into its color component and alpha channel by
copying it to a color-only (CAIRO_CONTENT_COLOR / CAIRO_FORMAT_RGB24)
and an alpha-only (CAIRO_CONTENT_ALPHA / CAIRO_FORMAT_A8) surface,
respectively. This is done via the function copy_with_content() in the
attached program. The color-only part is multiplied with your color via
CAIRO_OPERATOR_MULTIPLY.
The multiplication result is then copied back to the target surface. By
doing this with cairo_mask(cr,
the_alpha_channel_of_the_original_surface), the alpha channel of the
original surface is recombined with the color components.

Since you also asked how to add tint with an ARGB-color: You can modify
the alpha-only surface before re-applying it. I think the following
should work (well, you have to extract the underlying cairo_surface_t
from the cairo_pattern_t in the code, but I bet you can figure out how
to do that):

 cairo_t *cr = cairo_create(alpha);
 cairo_set_source_rgba(cr, 0, 0, 0, alpha_factor);
 cairo_set_operator(cr, CAIRO_OPERATOR_IN);
 cairo_paint(cr);
 cairo_destroy(cr);

Cheers,
Uli

P.S.: Error handling left as an exercise for the reader. I am not sure
if ignoring the return value of cairo_pattern_get_surface() really is a
good idea...
-- 
This can be a, a little complicated. Listen, my advice is... ask
somebody else for advice, at least someone who's... got more experience
at...  giving advice.
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