From: Patrick Martin <patrickmmartin@freenet.co.uk>
Subject: Re: fast Brightness/Contrast manipulation
Date: 18 Jul 2000 00:00:00 GMT
Message-ID: <39739929.3FA3ED84@freenet.co.uk>
References: <39749552.DC087F38@numog.eng.mcmaster.ca>
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For Images of 8bit colour depth and below, you can directly manipulate a
DIB, palette - that is as instant as it gets.
You can easily drag the colour/lightness in real time, for example.
It would be a simple matter to convert higher colur depths for the
purposes of a preview, and then do the real calculation on a pixel basis
for the real result.


Proviso - there is apparently a Palette Handle leaked when
TBitmap::PixelFormat = pf8bit is called
So, you must use an Assign to a  pre-prepared 8 bit TBitmap


E.g here is some example code

 PLogPalette pal = NULL;
  HPALETTE hpal;
  int i;

  try
  {
    pal = (PLogPalette) malloc( sizeof(TLogPalette) +
sizeof(TPaletteEntry) * 256);
    pal->palVersion = 0x300;
    pal->palNumEntries = 256;


    for (i = 0 ; i < 256 ; i++)
    {
      pal->palPalEntry[i].peRed = random(255);
      pal->palPalEntry[i].peGreen = random(255);
      pal->palPalEntry[i].peBlue = random(255);
    }
    hpal = CreatePalette(pal);
    if (hpal)
    {
      Bitmap->Palette = hpal;
    }
  }
  __finally
  {
    delete pal;
  }


Have a look at GetDIBColorTable:

example -


// saved palettes

RGBQUAD OldPalette[256];
RGBQUAD NewPalette[256];

// get the old palette

  GetDIBColorTable(Image1->Canvas->Handle, 0, 256, OldPalette);


  /* now munge the new palette */
   for (int i = 0 ; i < 256 ; i++)
   {
     NewPalette[i].rgbBlue = (short) (ScrollBar1->Position *
OldPalette[i].rgbBlue / 255);
     NewPalette[i].rgbGreen = (short) (ScrollBar1->Position *
OldPalette[i].rgbGreen / 255);
     NewPalette[i].rgbRed = (short) (ScrollBar1->Position *
OldPalette[i].rgbRed / 255);
   }

    SetDIBColorTable(Image1->Canvas->Handle, 0, 256, NewPalette);

    Image1->Invalidate();



}
//---------------------------------------------------------------------------



      /* cache old palette */
    GetDIBColorTable(Image1->Canvas->Handle, 0, 256, OldPalette);

You get the idea.

If you like this, you can owe me a beer.


Dean wrote:

> Hi,
>
> I am developing an image processing app and want to add a
> contrast/brightness tool similar to that seen in Scion Image which has
> contrast and brightness sliders which when moved change the appearance
> of a b&w image in a flicker free manner.  I use a short int buffer to
> hold the image data which can have values typically from -500 to 5000,
> and a lookup table to map the values down to 0-255 gray scale onto a
> Bitmap for bitblitting (?) onto a PaintBox canvas.  My guess is that
> there is some kind of color palette (?) that can be modified to change
> the appearance of the bitmap quickly and then modify the lookup table
> after the user is done but I don't know where to begin...
>
> Dean