Extracting color from 32-bit pixel...

[Cheeze-It]

So... I started programming again. Been almost two years since I've
done anything. Went back to the project I was working on before I
quit and totally forgot where I was... so I started over.

Anywho, I came across some functions that extract the Red, Green,
and Blue bytes from a 32-bit pixel, but don't understand *why* I did
what I did. Here's the function.

unsigned char Pixel32_ExtractR(unsigned int pixel)
{
     return ((pixel >> 16) & 0xFF);
}

Thing is,I don't know why the & 0xFF is necessary. I know what & does,
but why did I do it here? Do I even need it? Wouldn't shifting the bits
alone be enough? Please explain why not, detailed.

Damnit, starting over blows. I don't remember *anything*. Hehe.

Thank ya,

ethic

[quzah]

Let's say you've got four bytes. Each byte in this case is 8 bits. You want the third one. You shift half way over, and now you have 2 bytes left. You only want one. If you do this:

return allthebytes >> 16;

You're returning 2 bytes. That's not what you want. So, you mask off the one byte that you want, and return that.

return ((allthebytes >> 16) & 0xFF);

Quzah.

[Magos]

You're returning 2 bytes.

Last I checked an unsigned char is only 1 byte...

[quzah]

So it is. I suppose I should've paid attention to the return type.


Quzah.

[jverkoey]

So in other words, in this example it's probably not really needed, but is almost more of something put in by habit.

I believe possibly endianness could affect this too, no? I don't know my endians well enough however to justifiably have a solid opinion on that.

[Dave_Sinkula]

I believe possibly endianness could affect this too, no? I don't know my endians well enough however to justifiably have a solid opinion on that.

Endianness is a effect of storage. Values don't have endianness. So if you are shifting a value, you need not worry about endianness.

If you would be examining one byte of a multibyte integer by its memory location, then you have introduced an endianness issue into the problem.

Other interesting descriptions.

[joed]

Why return an unsigned char instead of just "int"? Also, bitmap data can probably also be declared "int", since any math performed would usually be done on the extracted bytes. I use these macros instead of functions (for when the pixel format is fixed, and won't change):

#define makecol(b, g, r)	(b | (g << 8) | (r << 16))
#define makecola(b, g, r, a)	(b | (g << 8) | (r << 16) | (a << 24))

#define getr(c)		((c & 0x000000FF))
#define getg(c)		((c & 0x0000FF00) >> 8)
#define getb(c)		((c & 0x00FF0000) >> 16)
#define geta(c)		((c & 0xFF000000) >> 24)