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From: osborn@ux1 (James R Osborn)
Newsgroups: comp.sys.mac.programmer
Subject: Offscreen BitMaps
Summary: Tech Note 41 Error
Keywords: bitmap, offscreen
Message-ID: <3023@helios.ee.lbl.gov>
Date: 18 Jul 89 22:16:36 GMT
Sender: usenet@helios.ee.lbl.gov
Reply-To: osborn@ux1 (James R Osborn)
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 61


Hi folks,

I assume that what I have to say here is old hat to most mac
programmers.  However, I have used a piece of code given in
TN 41 for quite awhile without any problems and only recently
did I discover the problem.

The code I am refering to is that which calculates the necessary
rowbytes for your bitmap:

	{make it at least as big as we need}
	OffRowBytes:=((OffRight-OffLeft) div 8) +1;

	{make OffRowBytes even}
	If Odd(OffRowBytes) then
		OffRowBytes:= OffRowBytes -1;

This works great if you have a number ourRowBits which is:

	odd multiple of 8 bits <= ourRowBits <= even multiple of 8 bits

In this range, the row bits that are missed by the integer truncation
(from the division) are counted because of the extra byte needed to make
the row bytes even.

If you are unforunate enough to have a number ourRowBits which is:

	even multiple of 8 bits < ourRowBits < odd multiple of 8 bits

then, not enough bytes are allocated for the last few bits of your image.
I noticed this because things that were being drawn at the right edge of
my bitmap were "overflowing" to the beginning of the next row.


You could just add an extra rowbyte instead of subtracting, but this would
waste memory in certain cases.  Here is how I dealt with it (in C).  I make
no claims as to the efficiency of my code:

	int	numBytes, rowBits;

	rowBits = ourRect.right - ourRect.left;
	numBytes = rowBits / 8;

	if (odd(numBytes))		/* You must write your own odd() */
		numBytes += 1;
	else if (rowBits % 8)		/* % is mod for pascal people */
		numBytes += 2;

Now rowBytes should contain the proper (and even) number of bytes to contain
ourRect.

I hope this does some other people good.  It just goes to show you it's
always something.  Either it's incorrect tech notes or your mac is
smoking.  It's always something.

	|------------------------------|
	| James R. Osborn              |
	| Lawrence Berkeley Laboratory |
	| osborn@ux1.lbl.gov           |
	|------------------------------|