[cairo] Replicating Microsoft's AlphaBlend function with Cairo
sysrpl at gmail.com
Fri Aug 24 07:11:08 PDT 2012
How can I stretch draw an area of a gdk pixbuf to a cairo surface with a
user defined opacity?
I am trying to write a cross platform interface for working with bitmaps
and want to add alpha blending to my cairo stretch draw method. What I have
right now works just fine, but I am unable to think up a way of to mold
alpha blending into a series of cairo/gdk apis which work somewhat like
Microsoft's [AlphaBlend] function.
What I have so far is this:
procedure TGtkBitmap.Draw(const Source: TRect; Canvas: TCanvas;
const Dest: TRect; Alpha: Byte = $FF);
if FBuffer = nil then
if (WidthOf(Source) < 1) or (WidthOf(Dest) < 1) then
if (HeightOf(Source) < 1) or (HeightOf(Dest) < 1) then
D := TGtk2DeviceContext(Canvas.Handle).Drawable;
C := gdk_cairo_create(D);
gdk_cairo_set_source_pixbuf(C, FBuffer, 0, 0);
cairo_matrix_translate(@M, Source.Left, Source.Top);
cairo_matrix_scale(@M, WidthOf(Source) / WidthOf(Dest),
HeightOf(Source) / HeightOf(Dest));
cairo_matrix_translate(@M, -Dest.Left, -Dest.Top);
cairo_rectangle(C, Dest.Left, Dest.Top, WidthOf(Dest),
// what cairo functions can I combine here to vary
// the opacity of the pattern fill using Alpha argument?
Everything works fine, but I am unsure how to get alpha blending with a pix
buff pattern working. I can imagine one way which involves creating a
second cairo surface, drawing the entire pixbuf to it with a user defined
opacity, then creating pattern for the first surface using the new surface,
which all gets a little messy and probably is much slower than something
I'd be happy with.
Here is a [video recording] of what I have working so far. I'd like to
know from someone familiar with cairo, what can I insert into my routine
above to set the alpha level of the pixbuf source pattern?
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