From:	"David Mitchell" <dmitchell AT inet DOT net DOT nz>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: shifting left, and shifting right...
Date:	Tue, 20 Jul 1999 18:23:07 +1200
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Sahab Yazdani wrote in message <37937D02 DOT 5D28D561 AT geocities DOT com>...
>I'm having some trouble understanding bit-wise operations.  Lets take a
>transparency map as an example, it would be more efficeint to store the
>transparency map as a pixel per bit bitmap (ie black and white), but I
>have no clue how to store the individual pixels in the map.  Somebody
>told be that I have to use the shift left and right operators << & >>??
>Can anybody confirm this and also possibly send some code as to how to
>do it???

Lets say that you have a map which is 8x8 pixels (unrealistic I know, but
bear with me)

This means that you can store it in only 8 bytes as opposed to 64.

Right, to do this, you want to be able to set it, and read it.

To read it you would want to say:

    if (PixelMap[lineNum] & (1 << column))
    {
        // Then the bit is set
    }
    else
    {
        // It's not
    }

Where PixelMap is an array of unsigned char (in this case of size 8)

How it works is Lets say lineNum is 2, column is 3 (remember origin is 0, 0)
so your pixelMap looks like this (in Binary)

00101001
10110101
00101101        <- LineNum = 2
10100101
11011010
10111101
00101101
11011001

so the left side of the if statement is 00101101, and the right hand side is
1 << column
so it shifts the binary for 1 left column places.
When column is 3 this gives: 1 << 3 = 00001000

When you logical and this with 00101101
then you get:

00101101  and
00001000 =
------------
00001000

Since this is non-zero we get a true.  You will notice that the bit for the
left hand column is the right hand bit (LSB)

To set it is a little trickier because you don't want to clobber the other
values

if (!(pixelMap[lineNum] & (1<<column)))
{
    // It isn't set so set it
    pixelMap[lineNum] += (1 << column);
}

So with the above values but trying to set column 4
00101101 +
00010000 =
-----------
00111101

To unset it you just say the opposite

if (pixelMap[lineNum] & (1 << column))
{
    // It is set, unset it
    pixelMap[lineNum] -= (1 << column);
}

Hope this helps

David.

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