<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">How can I stretch draw an area of a gdk pixbuf to a cairo surface with a user defined opacity?</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
<br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">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][1] function.</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
What I have so far is this:</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
    procedure TGtkBitmap.Draw(const Source: TRect; Canvas: TCanvas;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      const Dest: TRect; Alpha: Byte = $FF);</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">    var</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      D: PGdkDrawable;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      C: Pcairo_t;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      M: cairo_matrix_t;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">    begin</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      if FBuffer = nil then</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">        Exit;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      if (WidthOf(Source) < 1) or (WidthOf(Dest) < 1) then</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">        Exit;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      if (HeightOf(Source) < 1) or (HeightOf(Dest) < 1) then</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">        Exit;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      D := TGtk2DeviceContext(Canvas.Handle).Drawable;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      C := gdk_cairo_create(D);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      gdk_cairo_set_source_pixbuf(C, FBuffer, 0, 0);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      cairo_matrix_init_identity(@M);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      cairo_matrix_translate(@M, Source.Left, Source.Top);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      cairo_matrix_scale(@M, WidthOf(Source) / WidthOf(Dest),</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">        HeightOf(Source) / HeightOf(Dest));</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      cairo_matrix_translate(@M, -Dest.Left, -Dest.Top);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      cairo_pattern_set_matrix(cairo_get_source(C), @M);</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      cairo_rectangle(C, Dest.Left, Dest.Top, WidthOf(Dest), HeightOf(Dest));</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      // what cairo functions can I combine here to vary</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      // the opacity of the pattern fill using Alpha argument?</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">      cairo_fill(C);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
      cairo_destroy(C);</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">    end;</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
<br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">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.</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
Here is a [video recording][2] 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?</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
<br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">
  [1]: <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/dd183351%28v=vs.85%29.aspx" target="_blank" style="color:rgb(17,85,204)">http://msdn.microsoft.com/en-us/library/windows/desktop/dd183351%28v=vs.85%29.aspx</a></div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:13px;background-color:rgb(255,255,255)">  [2]: <a href="http://www.youtube.com/watch?v=iuorYlmbmvI" target="_blank" style="color:rgb(17,85,204)">http://www.youtube.com/watch?v=iuorYlmbmvI</a></div>