How to paint on a Canvas with Transparency and Opacity?

Question

Overview

From the GR32 library, I am using TImgView32 to render a grid which will be my transparent background like so:

Answer 1

The Opacity feature of TCanvas.Draw() does not support what you are trying to do, at least not the way you are trying to use it. To accomplish the affect you want, you need to create a 32-bit TBitmap so you have a per-pixel alpha channel, then fill in the alpha for each pixel, setting the alpha to 0 for pixels you don't want, and setting the alpha to the desired opacity for the pixels you do want. Then, when calling TCanvas.Draw(), set its opacity to 255 to tell it to use just the per-pixel opacities.

procedure DrawOpacityBrush(ACanvas: TCanvas; X, Y: Integer; AColor: TColor; ASize: Integer; Opacity: Byte);
var
  Bmp: TBitmap;
  I, J: Integer;
  Pixels: PRGBQuad;
  ColorRgb: Integer;
  ColorR, ColorG, ColorB: Byte;
begin
  Bmp := TBitmap.Create;
  try
    Bmp.PixelFormat := pf32Bit; // needed for an alpha channel
    Bmp.SetSize(ASize, ASize);

    with Bmp.Canvas do
    begin
      Brush.Color := clFuchsia; // background color to mask out
      ColorRgb := ColorToRGB(Brush.Color);
      FillRect(Rect(0, 0, ASize, ASize));
      Pen.Color := AColor;
      Pen.Style := psSolid;
      Pen.Width := ASize;
      MoveTo(ASize div 2, ASize div 2);
      LineTo(ASize div 2, ASize div 2);
    end;

    ColorR := GetRValue(ColorRgb);
    ColorG := GetGValue(ColorRgb);
    ColorB := GetBValue(ColorRgb);

    for I := 0 to Bmp.Height-1 do
    begin
      Pixels := PRGBQuad(Bmp.ScanLine[I]);
      for J := 0 to Bmp.Width-1 do
      begin
        with Pixels^ do
        begin
          if (rgbRed = ColorR) and (rgbGreen = ColorG) and (rgbBlue = ColorB) then
            rgbReserved := 0
          else
            rgbReserved := Opacity; 
          // must pre-multiply the pixel with its alpha channel before drawing
          rgbRed := (rgbRed * rgbReserved) div $FF;
          rgbGreen := (rgbGreen * rgbReserved) div $FF;
          rgbBlue := (rgbBlue * rgbReserved) div $FF;
        end;
        Inc(Pixels);
      end;
    end;

    ACanvas.Draw(X, Y, Bmp, 255);
  finally
    Bmp.Free;
  end;
end;

procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;  
  Shift: TShiftState; X, Y: Integer);  
begin  
  DrawOpacityBrush(Image1.Picture.Bitmap.Canvas, X, Y, clRed, 50, 85);  
end;