Date: Fri, 09 Aug 2002 15:07:38 -0400
From: Dean Verhoeven

I've been struggling for the last 2 days trying to figure out how
to display B/W images in pseudocolor using Delphi. I want to be
able to take an image with any range of pixel values, map those
values to palette indices in the range 0-255 in a bitmap, and
have that bitmap displayed according to a color map I define. I
want to be able to change the color map without having to change
the bitmap, and without provoking windows to change the bitmap.

You gave a good example of how to display an 8-bit image with a
user-defined palette on usenet. But it doesn't show how to change
the image's palette without redefining the image: just assigning
a new palette to the Bitmap.Palette doesn't work, one apparently
has to re-write the indices to the bitmap _after_ each change of
the palette to get the desired effect. This is not very
efficient.

The answer is the SetDIBColorTable() function, used like this:

var
   i           : integer;
   ColorTable  : array[byte] of TRGBQuad;
begin
   for i := 0 to 255 do
     with ColorTable[i] do
        begin
           rgbBlue     := i;
           rgbGreen    := i;
           rgbRed      := i;
           rgbReserved := 0;
        end;

    SetDIBColorTable(Bitmap.Canvas.Handle, 0, 256, ColorTable);
    Invalidate;  // cause Bitmap to be redrawn
end;

One doesn't even have to define a palette for the bitmap
(TLogPalette, CreatePalette and all that): just define the pixel
values of the bitmap, and then call a function like the one given
above to tell Windows what colors to display for what pixel
values (The Bitmap must be defined with Bitmap.HandleType  :=
bmDIB).

I got this answer from a usenet post (Fred Hovey, "Re: Changing
colors in 8-bit bitmaps - The Solution"), I thought you might be
interested. I also want to thank you for the information on your
web site, which is very helpful.

dean verheoven

--
Dean Verhoeven
ancona research inc.