Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!gatech!udel!princeton!andante!bwb
From: bwb@andante.UUCP (Bruce Ballard)
Newsgroups: comp.sys.mac.programmer
Subject: rowBytes for a pixmap
Message-ID: <21104@andante.UUCP>
Date: 20 Jul 89 15:29:54 GMT
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 21

I'm reading Knaster's Macintosh Programming Secrets for the
  first time, and I wonder whether the computation of rowBytes
  on pages 202 and 216 is known to be incorrect, or whether I am
  missing something.
For a bitmap (p. 202) he gives
      (((Right - Left - 1) div 16) + 1) * 2
  which is equivalent to the simpler formula
      ((Right - Left + 15) div 16) * 2
  but either is correct. For a pixmap (p. 216), however, he gives
      ((depth*(Right - Left - 1) div 16) + 1) * 2
  which he then uses to compute 208 as the rowBytes for an 8-bit-deep
  pixmap having 200 pixels across. But at this depth, we want one byte
  per pixel, right, so we should have a rowBytes of 200?
  It seems that a *correct* formula, which is easily derived from
  the alternate one I gave above, is
      (depth * (Right - Left + 15) div 16) * 2
  The difference is significant: for his 200-by-200 pixmap, 1600 of
  his 41600 bytes are wasted.
I love the book, but can someone clarify this point?
-Bruce Ballard
bwb@allegra.att.com