How to use ScanLine

Problem/Question/Abstract:

Can someone explain to me how to use the ScanLine function? I tried the example in the Help, only got a black box.

Answer:

type
  PRGBs = ^TRGBs;
  TRGBs = array[0..1000000] of TRGBTriple;

var
  r, g, b: Byte2DArray; {customer implemented}

procedure VirtualImageRGB.fromimage(t: TImage);
var
  x, y: Integer;
  P: PRGBs;
begin
  for y := 0 to t.picture.Bitmap.Height - 1 do
  begin
    P := PRGBs(t.picture.Bitmap.Scanline[y]);
    for x := 0 to t.picture.Bitmap.Width - 1 do
      with P^[x] do
      begin
        r[x, y] := rgbtRed;
        g[x, y] := rgbtGreen;
        b[x, y] := rgbtBlue;
      end;
  end;
end;

procedure VirtualImageRGB.toimage(t: TImage);
var
  x, y: Integer;
  P: PRGBs;
begin
  for y := 0 to t.picture.Bitmap.Height - 1 do
  begin
    P := PRGBs(t.picture.Bitmap.Scanline[y]);
    for x := 0 to t.picture.Bitmap.Width - 1 do
      with P^[x] do
      begin
        rgbtRed := r[x, y];
        rgbtGreen := g[x, y];
        rgbtBlue := b[x, y];
      end;
  end;
  t.canvas.draw(0, 0, t.picture.Bitmap);
end;