Re: Drawing antialiased shapes with alpha transparency

[Daniel Serpell <daniel_serpell yahoo com>]

Hi!

El Wed, Jan 05, 2005 at 06:33:11PM +0100, Samuel Abels escribio:
> On Wed, 2005-01-05 at 11:40 -0300, Daniel Serpell wrote:
> > You could try cairo: www.cairographics.org
> 
> Now I am probably going completely off topic, but I can't think of a
> better place right now...
> 
[...]
> 
> Unfortunately libcairo is poorly documented. The examples look fairly
> simple, but are also incomplete (where is the pixmap created, what is
> cr, etc.); most of the documentation links on the cairo homepage are
> broken either.

See the examples at http://cvs.cairographics.org/cairo-demo/

> Do you know whether this supports rendering into an RGBA buffer, thus
> preserving the alpha? I haven't found any information on this.

I think is already supported.

> I like the general idea of cairo and though I would probably have to
> rewrite some parts of the canvas I would like to use it if it supports
> this.
> 
> Also, I don't know which of
> 
>   cairo_t*         cr;
>   cairo_surface_t* surface;
> 
> is the buffer. And is it a standard RGBA buffer that can be drawn using
> Gdk::Pixbuf::create_from_data()?

None of them :-)

You connect your own buffer to a cairo_t object using:

 cairo_set_target_image (cairo_t	*cr,
			char		*data,
			cairo_format_t	format,
			int		width,
			int		height,
			int		stride);

"format" is CAIRO_FORMAT_ARGB32 in your example, "data" is your own RGBA
buffer, "stride" is the size of each row in the buffer (in bytes).

For a very simple example, using only memory buffers, see:

  http://cvs.cairographics.org/cairo-demo/png/spiral.c?rev=1.2&view=auto

The buffer can be obtained from a Gdk::Pixbuf if you want.

        Daniel.