How to convert 24-bit DIB to 4-bit DIB ?

If I save a 24-bit DIB as .BMP-file it's about 1.8 MB large. I don't have so much colors in the DIB so it gives also enough good results to reduce to 4-bit colors. I tried it with MS-PAINT. MS-Paint allows you to convert to 4 bit and the result is very good and the file-size reduces to nearly 300 kB. Is there anywhere source-code in VC++ in doing this converting from own programs written in C++ ?

Thanks a lot
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Tommy HuiConnect With a Mentor EngineerCommented:
The idea is that you have 16 colors to which map everything else. The bad thing about this is that the 16 colors are already chosen for you. They exists in the standard VGA color palette. So basically, for every RGB you have, you need to figure out the distance between that RGB with the RGB of the standard colors to see which it is closest to. Then you change that RGB to the standard color.

atariAuthor Commented:
I've heard upon that, you have 20 ground colors in the standard-palette and the routines you find  for converting are calculating the "nearest colors". I've seen sample-code for that. But this is not the main problem. In the 24-bit-DIB the color information is inside the pixels and there is no color palette. If I want to reduce the DIB to 16 colors I need a color-palette, there I found somewhere in MSJ a sample-code "palette generator", but the main problem is to filter out the color-information out of the pixels, that include also the color-information if you have a 24-bit-DIB. Some sample-code would be very nice. There can nothing be found around the whole internet. I tested also Paint Shop Pro and this program also reduces the colors in a fine way. There must be some sample-code for it.

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