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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 |
Organization: | Inet Ltd, Christchurch, New Zealand |
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References: | <37937D02 DOT 5D28D561 AT geocities DOT com> |
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To: | djgpp AT delorie DOT com |
DJ-Gateway: | from newsgroup comp.os.msdos.djgpp |
Reply-To: | djgpp AT delorie DOT com |
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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