Setting Canvas.Pixels data just results in black
lazarus / 39-Graphics

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zellfaze · 2013-05-08 19:55

I'm having a ton of trouble trying to work with Canvas.Pixels on a
TBitmap object.

I've been trying to get the following function working.  I understand
that this function actually does nothing, but if I can get it working,
then I can modify it to do what I need to do.  I am less worried about
speed, and more worried about readability, so I don't want to muck
with any of the ScanLine based stuff if at all possible.

Would anyone be willing to help me out?  When I run this function it
inevitably returns a TBitmap object that is just a black box.

Code: [Select]function copyWithCanvas(Bitmap: TBitmap): TBitmap;
var
  DestBitmap: TBitmap;
  x,y: Integer;
begin
  DestBitmap := TBitmap.create;
  DestBitmap.PixelFormat:= bitmap.PixelFormat;

  //Set the width and height of our destination to match the source
  DestBitmap.Width  := Bitmap.Width;
  DestBitmap.Height := Bitmap.Height;

  //Begin to copy
  x := 0;
  y := 0;
  //Copy each column
  while (x < DestBitmap.width) do
  begin
    //Copy each row
     while (y < DestBitmap.height) do
     begin
        //Copy the pixels themselves
        DestBitmap.canvas.Pixels[x, y] := Bitmap.canvas.Pixels[x, y];

        y := y+1;
     end;
     x := x+1;
  end;

  Result := DestBitmap;
end;
EDIT: Modified function name

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zellfaze · 2013-05-08 20:09

Code that calls the function is (snipped):
Code: [Select]  original := TBitmap.Create;
  try
    // Load from disk
    original.LoadFromFile(filename);

    //Mirror
    original := copyWithCanvas(original);

    // Write back to another disk file
    original.SaveToFile('output.bmp');
  finally
    original.Free;
  end;

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Blaazen · 2013-05-08 20:35

You reset y only once. You have to reset it (y:=0;) before each loop:
Code: [Select]//Begin to copy
  x := 0;
  //Copy each column
  while (x < DestBitmap.width) do
  begin
    //Copy each row
     y:=0;
     while (y < DestBitmap.height) do
     begin
        //Copy the pixels themselves
        DestBitmap.canvas.Pixels[x, y] := Bitmap.canvas.Pixels[x, y];
        y := y+1;
     end;
     x := x+1;
  end;

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zellfaze · 2013-05-09 12:32

*facepalm*

Thank you.  I can't believe that I didn't catch that.

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Blaazen · 2013-05-09 12:37

I think for..to loops can do the same job in 3 lines.

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zellfaze · 2013-05-09 14:24

They can.  Though once I modified the code to do what I actually
needed it to do the Begin and End Statements needed to stay.  The
for..to loop is still more efficient though.  I'll likely switch to
that.

This is what I actually was planning on making using that code:
https://github.com/zellfaze/lazarus-mirror-bitmap

There seemed to be a lack of instruction on how to mirror a bitmap
when Googling.  At some point I'll figure out how to do it with Scan
Line and add some documented functions to show how to do that to that
unit.

Thank you Blaazen for your help.  I still can't believe I didn't
notice that problem...