From: "Earl F. Glynn"
Subject: Re: 8 bit BMP to 8 bit JPEG
Date: 21 Jan 2000 00:00:00 GMT
Newsgroups: borland.public.delphi.graphics

"Bob Villiers" <100522.66@COMPUSERVE.COM> wrote in message
news:VA.00000163.01476379@cisppp...
> Could someone please show me how to convert a 8 bit bitmap to a 8 bit
> JPeg. I can convert to 24 bit ok but not 8bit.

The online documentation for TJPEGImage says:
"Use PixelFormat to set the pixel format of the JPEG image to 8-bit for
video drivers that cannot display 24-bit.  The native format of a JPEG image
is 24-bit.  PixelFormat is used for decompression, that is, for reading
files.  PixelFormat will also apply to a bitmap if the image is copied to
it."

I took a look at the JPEG.PAS source and the PixelFormat defaults to the
video mode.
There is once section of code that sets
jc.d.quantize_color := TRUE;
jc.d.desired_number_of_colors = 236;
if PixelFormat is jf8bit or jc.d.out-color_space = JCS_GRAYSCALE, which
would
make sense for 256-color display mode (Windows reserves 20 of the colors).

In TJPEGImage.GetMap, the FBitmap.PixelFormat is set to pf8bit when the
JPEG's Pixelformat is jf8bit (or out_color_space is JCS_Grayscale).

So in several quick tests I could not detect any change in a JPEG converted
from a BMP.
An 8 bit BMP creates the same JPEG regardless of the JPEG's PixelFormat
setting.  Likewise for a 24 bit BMP in creating a JPEG.

BUT, I could detect some difference while reading a JPEG and creating a BMP.
Consider the following:

procedure TForm1.Button2Click(Sender: TObject);
  VAR
    Bitmap:  TBitmap;
    JPEG:  TJPEGImage;
begin
  JPEG := TJPEGImage.Create;
  TRY
    JPEG.PixelFormat := jf8bit;
    JPEG.LoadFromFile('Fireworks.JPG');

    Bitmap := TBitmap.Create;
    TRY
      Bitmap.Assign(JPEG);
      Bitmap.SaveToFile('Fireworks.BMP');
      Image1.Picture.Graphic := Bitmap
    FINALLY
      Bitmap.Free;
    END;

    FINALLY
    JPEG.Free
  END
end;

The code above will result in a pf8bit Fireworks.BMP because
the JPEG.PixelFormat = jf8bit.  This code will work in either
high color or true color since palettes are handled well.

BUT, if you set JPEG.PixelFormat := jf24bit, while in 256 color
mode, the above code will create a 24-bits/pixel bitmap, which
will not have the correct palette, and will not display quite right.

So, I don't think the JPEG.PixelFormat buys you very much.

--
efg

Earl F. Glynn
Overland Park, KS  USA

efg's Computer Lab:  http://www.efg2.com/Lab