c++: Convert 24bpp to 8 bpp or 1bpp image - c++

Question

I have to convert a 24bpp image to a 1bpp image or 8bpp image based on color table. The caller expects a unsigned char* in either case (which would be further processed or maybe for now debug output by sending the BITMAPINFOHEADER.biBitCount to its proper value, 8 or 1).
I have code to extract the color index into the palette (colorIndexArray is from color conversion or dithering algorithms)... I can get the info for an 8bpp bitmap...
But my problem is, I don't know how to put this info into a 1bpp bitmap
typedef struct {
unsigned int size;
unsigned char* pixels;
} ColorIndexArray;
unsigned char* convertImage(const ColorIndexArray& colorIndexArray, unsigned int paletteSize)
{
unsigned char* outputImage;
if (paleteSize > 2)
{
outputImage = (unsigned char*)LocalAlloc(LPTR, colorIndexArray.size);
for (int i=0; i<colorIndexArray.size; i++)
*(outputImage+i) = colorIndexArray.pixels[i];
// this works great
}
else // monochrome, caller has palette colors likely b/w (or purple/magenta or anything), must be 1bpp
{
outputImage = (unsigned char*)LocalAlloc(LPTR, colorIndexArray.size / 8);
// how can i place the unsigned char* info (which is already
// determined based on desired algorithm, representing index in
// color table) into the output image inside a single bit ?
// (obviously its value for a monochrome image would be 0 or 1 but
// it is saved as unsigned char* at the algorithm output)
// And how do I advance the pointer ?
// Will it be type safe ? Aligned to byte ? or do I have to fill
// with something at the end to make multiple of 8 bits ?
}
return outputImage;
}
Trying this after comment suggestion:
#include <GdiPlus.h>
....
else {
Gdiplus::Bitmap monoBitmap(w, h, PixelFormat1bppIndexed);
Gdiplus::BitmapData monoBitmapData;
Gdiplus::Rect rect(0, 0, w, h);
monoBitmap.LockBits(&rect, Gdiplus::ImageLockModeWrite, PixelFormat1bppIndexed, &monoBitmapData);
outputImage = (unsigned char*)monoBitmapData.Scan0;
for (unsigned int y = 0; y < h; y++)
{
for (unsigned int x = 0; x < w; x++)
{
if (colorIndexArray.pixels[x + y * w])
outputImage[y*monoBitmapData.Stride + x / 8] |= (unsigned char)(0x80 >> (x % 8));
}
}
monoBitmap.UnlockBits(&monoBitmapData);
}
return outputImage;
(Also need to allocate the memory for outputImage)

Answer 1

Based on the example suggested by Hans Passant (thank you also for pointing out how important the stride is), I wrote this little conversion
unsigned long stride = (((w + 31) & ~31) >> 3);
outputImage = (unsigned char*)LocalAlloc(LPTR, stride * h);
for (unsigned int y = 0; y < h; y++)
{
unsigned char* b = (unsigned char*)LocalAlloc(LPTR, stride);
for (unsigned int x = 0; x < w; x++)
if (colorIndexArray.pixels[x + y * w])
b[x / 8] |= (unsigned char)(0x80 >> (x % 8));
CopyMemory(outputImage + stride * y, b, stride);
}