From: Robert Rossmair <Robert.Rossmair@t-online.de>
Subject: Re: calculating the average color of a bitmap
Date: 02 Feb 2000 00:00:00 GMT
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Hi Martin,

Martin Schnieder schrieb:

>   bmp.pixelformat := pf24bit;

Note that converting a bitmap's pixel format is a time-consuming
process.  Chances are that this line will eat up more computing time
than the remainder of your routine if the bitmap is not a genuine
24-bit-bitmap.

I did some tests with a 889 x 932 bitmap.

execution times in msec (366 MHz Celeron):

RGB averaging code: 37

pixel format conversion

pf4bit:   67
pf8bit:   63
pf16bit:  94
pf24bit (no conv.):  0.01
pf32bit:  84

it is possible to speed up the averaging code a bit using assembler.
The following code needs about 23 msec (instead of 37) to scan the 889 x
932 bitmap mentioned above.

var
  X, Y, W: Integer;
  P: PRGBTriple;
  r, g, b, Pixels: Integer;
  averagecolor: TColor;
  Bmp: TBitmap;
begin

  //...

    r := 0;
    g := 0;
    b := 0;
    W := Width;
    for y := 0 to Height-1 do
    begin
      P := ScanLine[y];
      asm
        mov ecx, W
        and ecx, ecx
        jz @end
        push ebx
        push esi
        push edi

        mov edi, ecx

        mov ebx, b
        mov ecx, g
        mov edx, r

        mov esi, P

      @Next:
        movzx eax, BYTE PTR [esi]
        add ebx, eax
        movzx eax, BYTE PTR [esi]+1
        add ecx, eax
        movzx eax, BYTE PTR [esi]+2
        add edx, eax

        dec edi
        jz @finis

        movzx eax, BYTE PTR [esi]+3
        add ebx, eax
        movzx eax, BYTE PTR [esi]+4
        add ecx, eax
        movzx eax, BYTE PTR [esi]+5
        add edx, eax

        dec edi
        jz @finis

        movzx eax, BYTE PTR [esi]+6
        add ebx, eax
        movzx eax, BYTE PTR [esi]+7
        add ecx, eax
        movzx eax, BYTE PTR [esi]+8
        add edx, eax

        dec edi
        jz @finis

        movzx eax, BYTE PTR [esi]+9
        add ebx, eax
        movzx eax, BYTE PTR [esi]+10
        add ecx, eax
        movzx eax, BYTE PTR [esi]+11
        add edx, eax

        add esi, TYPE TRGBTriple * 4
        dec edi
        jnz @Next

      @finis:
        mov b, ebx
        mov g, ecx
        mov r, edx

        pop edi
        pop esi
        pop ebx
      @end:
      end;


Greetings, Robert
--
Robert Roßmair
http://home.t-online.de/home/Robert.Rossmair/
Programming environment: Delphi 5 Professional, WinNT 4.0 SP6