[cairo] using cairo_stroke_preserve with transparent color

[ds]

Thanks a lot! I realized with tmp surface. I am not to happy with it,
but the performance is not as bad as I expected.

Detlef

Am Dienstag, den 25.05.2010, 07:56 +0300 schrieb M Joonas Pihlaja: 
> On Mon, 24 May 2010, ds wrote:
> 
> > Is there any standard way to draw a transparent path with a line width
> > and make inbetween displaying?
> 
> I think you're asking "how can I stroke and fill a path in a way that 
> the stroked part of the path doesn't bleed into the filled part", 
> right?
> 
> The basic approach is to use a temporary surface where you first 
> stroke your border and then fill in a way that _overwrites_ the filled 
> parts rather than merges the colours.  Then you composite the 
> temporary surface to your target surface.  Here's an example:
> 
> cairo_push_group(cr);  /* creates the temporary surface. */
> {
> 	<add your path using move_to, line_to, etc.>
> 
> 	<set the stroke colour, possibly with alpha>
> 	cairo_set_operator(cr, CAIRO_OPERATOR_OVER);
> 	cairo_stroke_preserve(cr);
> 
> 	<set the fill colour, possibly with alpha>
> 	cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
> 	cairo_fill(cr);
> }
> cairo_pop_group_to_source(cr);
> cairo_paint(cr); /* composites the temporary surface to the target */
> 
> You can avoid the temporary surfaces if you can use 
> CAIRO_OPERATOR_SOURCE for all your path rendering (although internally 
> cairo will probably use temporary surfaces, so this is a coin toss.)
> 
> You _may_ get faster results if the stroke and fill colours have
> the same alpha, like this:
> 
> cairo_push_group(cr);
> {
> 	cairo_set_operator(cr, CAIRO_OPERATOR_OVER);
> 
> 	<add your path using move_to, line_to, etc.>
> 
> 	<set the opaque stroke colour, no alpha>
> 	cairo_stroke_preserve(cr);
> 
> 	<set the opaque fill colour, no alpha>
> 	cairo_fill(cr);
> }
> cairo_pop_group_to_source(cr);
> cairo_paint_with_alpha(cr, <the alpha>);
> 
> A third method which sometimes works is to use cairo_clip() to clip 
> out the inside of the path which shouldn't interact with the stroke 
> before you stroke.
> 
> Hope that was actually what you were looking for. :)
> 
> Cheers,
> 
> Jooans
>