How to create transparent png image from bitmap? - image

Question

My task is:
Create a TBitmap object.
Fill it with transparent color (alpha = 0).
Assign this bitmap to TPngImage.
Save PNG file with alpha transparency.
How can I do it in Delphi XE?
var
Png: TPngImage;
X, Y: Integer;
Bitmap: TBitmap;
begin
Bitmap := TBitmap.Create();
Bitmap.PixelFormat := pf32bit;
Png := TPngImage.Create();
try
Bitmap.SetSize(100, 100);
// How to clear background in transparent color correctly?
// I tried to use this, but the image in PNG file has solid white background:
for Y := 0 to Bitmap.Height - 1 do
for X := 0 to Bitmap.Width - 1 do
Bitmap.Canvas.Pixels[X, Y]:= $00FFFFFF;
// Now drawing something on a Bitmap.Canvas...
Bitmap.Canvas.Pen.Color := clRed;
Bitmap.Canvas.Rectangle(20, 20, 60, 60);
// Is this correct?
Png.Assign(Bitmap);
Png.SaveToFile('image.png');
finally
Png.Free();
Bitmap.Free();
end;
end;

Answer 1

More or less a copy of Dorin's answer.
It shows how to make a transparent png image and how to clear the background.
uses
PngImage;
...
var
bmp: TBitmap;
png: TPngImage;
begin
bmp := TBitmap.Create;
bmp.SetSize(200,200);
bmp.Canvas.Brush.Color := clBlack;
bmp.Canvas.Rectangle( 20, 20, 160, 160 );
bmp.Canvas.Brush.Style := bsClear;
bmp.Canvas.Rectangle(1, 1, 199, 199);
bmp.Canvas.Brush.Color := clWhite;
bmp.Canvas.Pen.Color := clRed;
bmp.Canvas.TextOut( 35, 20, 'Hello transparent world');
bmp.TransparentColor := clWhite;
bmp.Transparent := True;
png := TPngImage.Create;
png.Assign( bmp );
png.SaveToFile( 'C:\test.png' );
bmp.Free;
png.Free;
end;