Re: Converting a 24-bit bitmap (with 256 unique colors) to an 8-bit bitmap without changing the colors???

"John H." <>
Thu, 4 Mar 2010 12:54:09 -0800 (PST)
On Mar 4, 1:09 pm, "Peter Olcott" <> wrote:

Do you think that it could copy from 32-bit to 8-bit

I think so. Below is an example.
With some thinking, you might be able to figure out a way to work
directly with the pixel bits (via CImage::GetBits) that can do things

#include "stdafx.h"
#include <atlimage.h>
#include <set>

int main()
    // Load the source image:
    CImage img24;

    // Gather all the colors in the image:
    std::set<COLORREF> colors;
    for(int i=0; i<img24.GetHeight(); ++i)
        for(int j=0; j<img24.GetWidth(); ++j)

    // Make a color table holding the colors:
    RGBQUAD table[256];
    std::set<COLORREF>::const_iterator color = colors.begin();
    for(int index=0; index<256; ++index)
        table[index].rgbBlue = GetBValue(*color);
        table[index].rgbGreen = GetGValue(*color);
        table[index].rgbRed = GetRValue(*color);
        table[index].rgbReserved = 0;

    // Make the destination image:
    CImage img8;
    img8.Create(img24.GetWidth(), img24.GetHeight(), 8);
    img8.SetColorTable(0, 256, table);

    // Copy the pixels:
    HDC const hdc = img8.GetDC();
    img24.BitBlt(hdc, 0, 0);


    return 0;

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