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From: John H. on 4 Mar 2010 15:54
On Mar 4, 1:09 pm, "Peter Olcott" <NoS...(a)OCR4Screen.com> wrote:
> Do you think that it could copy from 32-bit to 8-bit
> indexed?

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
faster.

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

int main()
{
// Load the source image:
CImage img24;
img24.Load("256colors.bmp");

// 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)
{
colors.insert(img24.GetPixel(j,i));
}
}

// 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;
++color;
}

// 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);
img8.ReleaseDC();

img8.Save("8bpp.bmp");

return 0;
}
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