[cairo] render in grayscale

[Per Hermansson]

Behdad Esfahbod wrote:
> On 09/22/2009 03:01 AM, Per Hermansson wrote:
>> Hi
>>
>> I'm using cairo to render a GTK+ widget (using gdk_cairo_create). The
>> rendering is partially done using an external library routine (which I
>> can't change).
>> Sometimes I want the widget to be rendered in grayscale.
>> Is it possible to modifiy cairo before or after the library has done the
>> rendering so that the output appears in gray scale?
>>
>> Per
>
> You can create an alpha-only temporary surface 
> (cairo_surface_create_similar with content=CAIRO_CONTENT_ALPHA), 
> render to it, and cairo_mask() that with a white rgb pattern on a 
> black background.
>
> behdad
>
Thanks for the help and quick reply, I only get a black area when I try 
the code below:

cr = gdk_cairo_create(widget);
target = cairo_surface_create_similar(cairo_get_target(cr), 
CAIRO_CONTENT_ALPHA, width, height);
cr_tmp = cairo_create(target);

poppler_page_render(page, cr_tmp);

cairo_set_source_rgb(cr, 0, 0, 0);
cairo_paint(cr);
pattern = cairo_pattern_create_rgb(1, 1, 1);
cairo_set_source_surface(cr, target, 0, 0);
cairo_mask(cr, pattern);

Per