Question

I am building an application that has “virtual windows”. The output is TImage object.

1) The application loads window skin files into TPNGObject‘s:

2) Then application has to create a new blank TPNGObject, and resize the skin files to needed sizes and draw them on that blank image. Should look something like this:

3) And the final output on TImage:

The problem is that I do know how to create a completely blank off screen image. Of course I could simply render the skin files on to TImage each time, but it’s easier and better to resize skin files and create the window once, instead.
I’m using the PNG Library by Gustavo Daud, version 1.564 (31st July, 2006).

Answer 1

I apologize to people that I messed their heads up.

It turns out CreateBlank works as wanted. The problem was that I was drawing PNG on PNG canvas (PNG.Canvas.Draw). Canvas doesn’t really support transparency. To draw a translucent PNG on another PNG you will need a procedure/function that merges those both layers together. With some googling I ended up with this procedure:

procedure MergePNGLayer(Layer1, Layer2: TPNGObject; Const aLeft, aTop: Integer);
var
  x, y: Integer;
  SL1,  SL2,  SLBlended: pRGBLine;
  aSL1, aSL2, aSLBlended: PByteArray;
  blendCoeff: single;
  blendedPNG, Lay2buff: TPNGObject;
begin
  blendedPNG := TPNGObject.Create;
  blendedPNG.Assign(Layer1);
  Lay2buff:=TPNGObject.Create;
  Lay2buff.Assign(Layer2);
  SetPNGCanvasSize(Layer2, Layer1.Width, Layer1.Height, aLeft, aTop);
  for y := 0 to Layer1.Height - 1 do
  begin
    SL1 := Layer1.Scanline[y];
    SL2 := Layer2.Scanline[y];
    aSL1 := Layer1.AlphaScanline[y];
    aSL2 := Layer2.AlphaScanline[y];
    SLBlended := blendedPNG.Scanline[y];
    aSLBlended := blendedPNG.AlphaScanline[y];
    for x := 0 to Layer1.Width - 1 do
    begin
      blendCoeff:=aSL1[x] * 100/255/100;
      aSLBlended[x] := round(aSL2[x] + (aSL1[x]-aSL2[x]) * blendCoeff);
      SLBlended[x].rgbtRed   := round(SL2[x].rgbtRed + (SL1[x].rgbtRed-SL2[x].rgbtRed) * blendCoeff);
      SLBlended[x].rgbtGreen := round(SL2[x].rgbtGreen + (SL1[x].rgbtGreen-SL2[x].rgbtGreen) * blendCoeff);
      SLBlended[x].rgbtBlue  := round(SL2[x].rgbtBlue + (SL1[x].rgbtBlue-SL2[x].rgbtBlue) * blendCoeff);
    end;
  end;
  Layer1.Assign(blendedPNG);
  Layer2.Assign(Lay2buff);
  blendedPNG.Free;
  Lay2buff.Free;
end;

Usage:

var
  PNG1, PNG2: TPNGObject;
begin
  PNG1 := TPNGObject.CreateBlank(COLOR_RGBALPHA, 16, 500, 500);
  PNG2 := TPNGObject.Create;
  PNG2.LoadFromFile('...*.png');
  MergePNGLayer(PNG1, PNG2, 0, 0);
  // PNG1 is the output

And again, I am really sorry to users that wanted to help, but couldn’t due to not understanding me.