Re: Converting 24bit image to 16bit

[EnderWiggenRules@aol.com]

In a message dated 10/5/00 5:58:25 PM Central Daylight Time, 
anime6@earthlink.net writes:

<< Someone help me please.  I need to write code to load a 24 bit, bitmap 
into a 16 bit buffer.  How do I do this.  Usually it doesn't work. >>

OK. If I'm remembering right, I think 24-bit images are stored with one byte 
for Red, Green, and Blue, and 16-bit is just the three attributes split over 
two bytes. So, if you load a 24-bit image in this order:
    RRRRRRRR GGGGGGGG BBBBBBBB
Each color attribute is 8-bit, meaning that it is in the range 0 to 255. 
16-bit (correct me if I'm wrong) is stored in this fashion:
    RRRRR GGGGGG BBBBB
The red and blue color attributes are in the range 0 to 31, and the green 
attribute is in the range 0 to 63, so what you need to do is convert the 0 to 
255 range to 0 to 31 or 63. This can be done easily by bit shifting. Here's 
pseudo-code:
    5BitRed=8BitRed>>3; // 8BitRed/8
    6BitGreen=8BitGreen>>2; // 8BitGreen/4
    5BitBlue=8BitBlue>>3;   // 8BitBlue/8
    Color=5BitBlue+(6BitGreen<<5)+(5BitRed<<11)
This is the conversion, though depending on the library/video card/driver you 
are using, the red and blue may be reversed in positioning.

-Ender Wiggin
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