Converting 65536/256-color bitmap to 16 colors under Win16

Under Win16, I have a 65536-color or 256-color bitmap
(of size 32x32 because it's an icon's XOR mask) and I
want to convert it to a 16-color bitmap (I need to write
a 16-color XOR mask into a .ICO file). I am using Visual
C++ 1.52c under Windows 95/3.1.

What is the algorithm in C/C++ to do this?
sarrazipAsked:
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jnewlinCommented:
It depends on if you know what colors are available in the
16-bit image.  If you do, it's pretty easy.  If you don't
then you have to figure out how to choose the colors, which
can be pretty complex.

Assuming you know the color table for the 16-bit bitmap,
you can do something pretty simple, since the amount of
data that you are working with is small, you can do a linear
lookup on each pixel.  It's slow on large data, but should
be ok with a 32x32 bitmap.

All you do, is for each pixel in the original bitmap, get
a COLORREF that is the color of that pixel, then call
GetNearestColor(hdcOfDestBitMap, colorRef).
Then use the value that that returns as your the value for
the pixel that you are writing.  Of course you'll have to
have an HDC into the bitmap that you are writing.
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sarrazipAuthor Commented:
The original bitmap has 16-bit entries. I try to use
GetSystemPaletteEntries() to obtain a COLORREF
from a 16-bit entry, using the HDC returned by GetDC(),
but it fails.

Anyway, I have found a different method that involves
GetBitmapBits() and CreateBitmap() and no HDCs, assuming
that the goal is to produce a .ICO file.

Thanks.

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