Remove BMP background and convert to trasparent PNG with Delphi

Question

I have a TBitmap, inside the TBitmap there is load a "map image" (image of a map). This map image have a white background and a lot of black line (no antialising only 2 colors white background and black line).

Now I must do this:

• Remove the white background from TBitmap (transparent background and black line);
• If possible and only if possible replace black line color with another color;
• Save the result as trasparent PNG image;

I don't have idea if these are possible. Suggestions?

NOTE I want avoid to use 3th part of class or VCL if possible. I can use FreeImage library if need because I just use it on my project. I use Delphi XE3.

Answer 1

Change pixelformat to pf1Bit. Create a palette with 2 entries, change the values of the TPaletteEntry to the desired color value (in the shown exaple to red). Create a TPNGImage, assign the bitmap and set the transparency for the PNG.

implementation
uses pngimage;
{$R *.dfm}

Type
  TMyPalette = Packed Record
     palVersion : Word;
     palNumEntries : Word;
     palPalEntry : Array [0..1] of TPaletteEntry;
   End;

Procedure ChangeBlackColor(bmp:TBitMap);
var
 pal:TMyPalette;
begin
   bmp.PixelFormat := pf1Bit;
   bmp.HandleType  := bmDIB;
    With pal Do
    Begin
      palVersion:=$0300;
      palNumEntries:=2;
      palPalEntry[0].peRed:= $FF;
      palPalEntry[0].peGreen:=$00;
      palPalEntry[0].peBlue:= $00;
      palPalEntry[0].peFlags:=PC_RESERVED;
      palPalEntry[1].peRed:= $FF;
      palPalEntry[1].peGreen:=$FF;
      palPalEntry[1].peBlue:= $FF;
      palPalEntry[1].peFlags:=PC_RESERVED;
    End;
   bmp.Palette := CreatePalette(pLogPalette(@pal)^)
end;



procedure TForm3.Button1Click(Sender: TObject);
var
 png:TPngimage;
 bmp:TBitmap;
begin
  // sample image
  Image1.Canvas.Rectangle(0,0,Image1.Width-1,Image1.Height-1);
  Image1.Canvas.Ellipse(1,1,Image1.Width,Image1.Height);



  bmp := Image1.Picture.Bitmap;

  ChangeBlackColor(bmp);

  png:=TPngimage.Create;
  try
     png.Assign(bmp);
     png.TransparentColor := clWhite;
     png.Transparent := true;
     Image2.Picture.Assign(png);
  finally
    png.Free;
  end;
end;