Stretchdraw on TPNGImage

Question

When i call canvas.stretchdraw with a bitmap, the bitmap will be mirrored/flipped when left>right. This does not happen for a PNG. Is this a bug? What can I do to fix it?

To replicate, try this code:

procedure TForm1.TestStretchdraw;
var
  vBMP: TBitmap;
  vPNG: TPNGImage;
  X0,Y0,X1,Y1 : integer;
  R : TRect;

  procedure FlipRect;
  var
    T : integer;
  begin
    T := R.Left;
    R.Left := R.Right;
    R.Right := T;
  end;

begin
  vBMP := TBitmap.Create;
  vPNG := TPNGImage.Create;
  try
    vBMP.LoadFromFile('c:\temp\pic\pic.bmp');
    vPNG.LoadFromFile('c:\temp\pic\pic.png');

    X0 := 0;
    Y0 := 0;
    X1 := X0 + vBMP.Width;
    Y1 := Y0 + vBMP.Height;
    R := Rect(X0,Y0,X1,Y1);
    FlipRect;
    Canvas.StretchDraw(R,vBMP); //This image will be drawn mirrored

    X0 := vBMP.Width+10;
    Y0 := 0;
    X1 := X0 + vPNG.Width;
    Y1 := Y0 + vPNG.Height;
    R := Rect(X0,Y0,X1,Y1);
    FlipRect;
    Canvas.StretchDraw(R,vPNG); //This will not
  finally
    vPNG.Free;
    vBMP.Free;
  end;
end;

(but replace my testimages with some of your own)

Answer 1

Here I wrote a function that flips a png without any libraries like Gr32. Transparency remains in the flipped PNG.

procedure FlipPNG(aSource, aDest: TPngImage);
var
  X, Y: Integer;
  AlphaPtr: PByteArray;
  RGBLine: pRGBLine;
  PalleteLine: PByteArray;
  AlphaPtrDest: PByteArray;
  RGBLineDest: pRGBLine;
  PalleteLineDest: PByteArray;
begin
  aDest.Assign(aSource);

  if (aSource.Header.ColorType = COLOR_PALETTE) or
     (aSource.Header.ColorType = COLOR_GRAYSCALEALPHA) or
     (aSource.Header.ColorType = COLOR_GRAYSCALE) then
  begin
    for y := 0 to aSource.Height - 1 do
    begin
      AlphaPtr := aSource.AlphaScanline[y];
      PalleteLine := aSource.Scanline[y];
      AlphaPtrDest := aDest.AlphaScanline[y];
      PalleteLineDest := aDest.Scanline[y];
      for x := 0 to aSource.Width - 1 do
      begin
        PalleteLineDest^[aSource.Width - x -1] := PalleteLine^[x];
        if Assigned(AlphaPtr) then
          AlphaPtrDest^[aSource.Width - x -1] := AlphaPtr^[x];
      end;
    end;
  end else
  if (aSource.Header.ColorType = COLOR_RGBALPHA) or
     (aSource.Header.ColorType = COLOR_RGB) then
  begin
    for y := 0 to aSource.Height - 1 do
    begin
      AlphaPtr := aSource.AlphaScanline[y];
      RGBLine := aSource.Scanline[y];
      AlphaPtrDest := aDest.AlphaScanline[y];
      RGBLineDest := aDest.Scanline[y];
      for x := 0 to aSource.Width - 1 do
      begin
        RGBLineDest^[aSource.Width - x -1] := RGBLine^[x];
        if Assigned(AlphaPtr) then
          AlphaPtrDest^[aSource.Width - x -1] := AlphaPtr^[x];
      end;
    end;
  end;
end;

Answer 2

Yes, this is correct. StretchDraw calls the Draw method of the appropriate descendant of TGraphic . You can compare TBitmap.Draw with TPngImage.Draw yourself. TBitmap.Draw , naturally, simply calls the Windows API function StretchBlt . TPngImage.Draw , however, first calls AdjustRect :

procedure AdjustRect(var Rect: TRect);
var
  t: Integer;
begin
  if Rect.Right < Rect.Left then
  begin
    t := Rect.Right;
    Rect.Right := Rect.Left;
    Rect.Left := t;
  end;
  if Rect.Bottom < Rect.Top then
  begin
    t := Rect.Bottom;
    Rect.Bottom := Rect.Top;
    Rect.Top := t;
  end
end;

As you can see, it ruthlessly undoes your swapping of left and right .

Answer 3

I would not consider the case, where Left is larger than Right , a valid rectangle definition, so my answer would be: don't call it like that. You can ensure that the rectangle definition is valid before calling StretchDraw :

if (R.Left > R.Right) then
  Swap (R.Left, R.Right);
if (R.Top> R.Bottom) then
  Swap (R.Top, R.Bottom);