Working with Bitmaps and PNGs
lazarus / 39-Graphics

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Devin · 2006-06-07 23:14

Ok, so I am completely new at this, so any help you can give me is
much appreciated.

I have a bitmap displayed on the screen, 125 x 90 pixels.  The user
clicks a button, picks another 125 X 90 bitmap.  The program is
supposed to overlay the selected image onto the first, interpreting
black as transparent, and then replace the displayed image with the
resulting image.

I wrote some code to do this, where you basically loop through each
pixel of the overlay image, and if it's black you use the pixel from
the original image, else you use the pixel from the overlay to create
the new image.  However, it's not working correctly.

The program seems to interpret any solid colour as black, so a value
of rgb(255,0,0) would be interpreted as transparent, rather than red.

Here's the code I'm using
Code: [Select]
procedure TForm1.Button1Click(Sender: TObject);

var
   LoadImage: TBitmap;
   SwapImage: TBitmap;
   BackColour, OverColour, SwapColour: TColor;
   px, py: Integer;

begin
  OpenPictureDialog1.Execute;
  //this is where the magic happens.
  LoadImage:=TBitmap.Create;
  LoadImage.LoadFromFile(OpenPictureDialog1.FileName);
  LoadImage.Transparent:=False;
  SwapImage:=TBitmap.Create;
  SwapImage.Width:=90;
  SwapImage.Height:=135;
  SwapImage.Transparent:=False;
  //go pixel by pixel to create SwapImage
  for px:= 0 to 89 do begin
      for py:= 0 to 134 do begin
        //get overlay colour
        OverColour:=LoadImage.Canvas.Pixels[px,py]; //from overlay
image, overlay colour
        if (OverColour=$000000) then begin //if overlay pixel is black
          BackColour:=Image1.Canvas.Pixels[px,py];  //a fancy way of
doing absolutely nothing
          SwapColour:=BackColour;
        end else begin
          SwapColour:=OverColour; //if the overlay pixel is not black,
put that pixel in the swap value
        end;
        SwapImage.Canvas.Pixels[px,py]:=SwapColour; //put the swap
colour in the swap image
      end;
  end;
  //Swap Image becomes the displayed image
  Image1.Picture.Bitmap:=SwapImage;
  //kill the rest and let God sort 'em out
  LoadImage.Destroy;
  SwapImage.Destroy;
end;


I've never really worked much with graphics before, so I don't really
know what I'm doing.

As a second question: is there an easy way to convert & save the
resulting bitmap in PNG format?

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tony72 · 2006-06-08 11:31

I was playing with some Delphi code a while back, and found that
Canvas.Pixels did not work in Lazarus, I can't remember if it always
returned 0, but I think so. I don't think TCanvas is fully implemented
yet, basically. Nor did Bitmap.Scanlines[] work (my plan B for
accessing pixel values). This was in Linux, V9.12 or 13, but might
well be what you are experiencing.

You can work around it by using a TLazIntfImage instead, it's
Colors[x,y] property will return the pixel value.

Also in Delphi, you could use the Transparent & TransparentColor
properties of TImage to acheive what you're trying to do without
writing any code. No idea if that will work in Lazarus, worth checking
though.

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felipemdc · 2006-06-08 11:50

> I was playing with some Delphi code a while back, and found that
Canvas.Pixels did not work in Lazarus,

You mean it didn´t work =)

It was implemented very recently, I think it already works on 0.9.16

http://www.freepascal.org/mantis/view.php?id=1661

It works on Windows, Gtk 1 and Gtk 2

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Devin · 2006-06-09 01:08

Thank you both for the responses... I will try out some of those ideas
and see if I can get it working the way I want it to.

Any idea about converting to PNG?

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Devin · 2006-06-09 01:24

Quote from: "sekel"> I was playing with some Delphi code a while back,
and found that Canvas.Pixels did not work in Lazarus,

You mean it didn´t work =)

It was implemented very recently, I think it already works on 0.9.16

http://www.freepascal.org/mantis/view.php?id=1661

It works on Windows, Gtk 1 and Gtk 2

Okay, I updated to 0.9.16 and it works perfectly.  Thank you very much
for the help.  It's also reassuring to know that it wasn't just my
incompitence.

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CWBudde · 2006-06-09 08:57

How about http://pngdelphi.sourceforge.net/ ? They claim to be object
pascal, which might be also compatible with free pascal.

Christian