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24 bit BMP on 256 color display

Posted on 1997-09-18
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Last Modified: 2010-04-03
When I use the standard image component to stick a 24 bit BMP file on a form the picture looks awful (like its only 16 colors). Is there a trick to make it look nicer without going to the hassle of converting it to a 256 color BMP first?

ta chris
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Question by:chrishart
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Expert Comment

by:JimBob091197
ID: 1345515
I once got around the problem like this:  (You convert the bitmap to 256 in Delphi, not the physical bitmap on disk.  The original bitmap on disk in my case was a 24 bit bitmap too.)

procedure TForm1.LoadBitmap;
var
  Bmp: TBitmap;
begin
  Bmp := TBitmap.Create;
  Bmp.LoadFromFile('C:\Temp\Test.bmp');
  Bmp.PixelFormat := pf8bit;
  Image1.Picture.Assign(Bmp);
  Bmp.Free;
end;

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Author Comment

by:chrishart
ID: 1345516
Thanks for the answer, it looked like it should do the trick.
I tried your code out but Sadly C++Builder does not seem to support
PixelFormat so it didn't work :-(
I'm sorry I didn't mention that I was using Builder, things are usually
the same.
It seems C++Builder's TBitmap is perhaps based on an old version of Delphi?

Thanks Chris
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LVL 5

Expert Comment

by:JimBob091197
ID: 1345517
Delphi's code for SetPixelFormat is below.  You should be able to convert it easily because it mostly deals with palettes and Wnidows API calls.

TPixelFormat = (pfDevice, pf1bit, pf4bit, pf8bit, pf15bit, pf16bit, pf24bit, pf32bit, pfCustom);

procedure TBitmap.SetPixelFormat(Value: TPixelFormat);
const
  BitCounts: array [pf1Bit..pf32Bit] of Byte = (1,4,8,16,16,24,32);
var
  DIB: TDIBSection;
  Pal: HPalette;
  DC: HDC;
begin
  if Value = GetPixelFormat then Exit;
  case Value of
    pfDevice:
      begin
        HandleType := bmDDB;
        Exit;
      end;
    pfCustom: InvalidGraphic(SInvalidPixelFormat);
  else
    FillChar(DIB, sizeof(DIB), 0);
    DIB.dsbm := FImage.FDIB.dsbm;
    with DIB, dsbm, dsbmih do
    begin
      bmBits := nil;
      biSize := sizeof(DIB.dsbmih);
      biWidth := bmWidth;
      biHeight := bmHeight;
      biPlanes := 1;
      biBitCount := BitCounts[Value];
      Pal := FImage.FPalette;
      case Value of
        pf4Bit: Pal := SystemPalette16;
        pf8Bit:
          begin
            DC := GDICheck(GetDC(0));
            Pal := CreateHalftonePalette(DC);
            ReleaseDC(0, DC);
          end;
        pf16Bit:
          begin
            biCompression := BI_BITFIELDS;
            dsBitFields[0] := $F800;
            dsBitFields[1] := $07E0;
            dsBitFields[2] := $001F;
          end;
      end;
      CopyImage(Handle, Pal, DIB);
      PaletteModified := Pal <> 0;
      Changed(Self);
    end;
  end;
end;

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Author Comment

by:chrishart
ID: 1345518
Thanks JimBob for your comment, that looks good to me, can you make it an answer so I can award you your points and close the topic!

Thanks very much Chris
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LVL 5

Accepted Solution

by:
JimBob091197 earned 300 total points
ID: 1345519
I hope it works ok for you...
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Author Comment

by:chrishart
ID: 1345520
Sorry about the delay and thanks again

Chris
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