Re: Converting a GDI+ 24-bit RGB Bitmap to an Indexed Format Bitmap

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I would suggest computing a new Palette using one of the many color
reduction algorithms available.
MSDN has the code for the octree quantization algorithm.

Once you have a new palette, create a new bitmap with the same width and
height of your resized image.
Use the original indexed pixel format for this new image.

Since your resized image is 24bpp, you need to translate the 3 colors in
each pixel into an index of your new color palette.
If you create a logical palette via GDI, you may use the function
GetNearestPaletteIndex to get the new index for the pixel.
You use Lockbits, to cycle through each 24bpp pixel and take the index
returned by GetNearestPaletteIndex and set the
corresponding pixel in your indexed image to this index.

..
"Alain M. Dube" <adube@xxxxxxxxxxxxxxxxxxx> wrote in message
news:%23jEldIYyHHA.5024@xxxxxxxxxxxxxxxxxxxxxxx
Hello;

Can someone shed some light on how I can (using GDI+ and/or GDI) convert a
Bitmap whose pixel format is PixelFormat24bppRGB to a bitmap with any
variation of the PixelFormatIndexed pixel format? Note that the source
bitmap is "indexed" and converted to 24-bit RGB, then resized. No color
changes are made. Once completed, I'd like to return the bitmap to it's
original pixel format.

In theory, the source bitmap's palette should be reusable. I thought that
it would be a simple matter of iterating through the 24-bit RGB bitmap
data,
one pixel at a time, attempting to locate the pixel's color in the source
bitmap's palette. The colors values to not seem to match at all. The
color
palette values I see are actually in the source bitmap file. if I save
the
24-bit RGB bitmap to disk, the colors that I see as bitmap pixel data
(while
stepping througnb the code) are present in the saved bitmap file. The
color
values in the palette and in the 24-bit RGB data seem to be incompatible.

Thanks in advance;

Alain.





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