Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!dartvax!eleazar.dartmouth.edu!earleh
From: earleh@eleazar.dartmouth.edu (Earle R. Horton)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Aligining bitmaps?
Message-ID: <15288@dartvax.Dartmouth.EDU>
Date: 28 Aug 89 22:20:37 GMT
References: <3791@ncsuvx.ncsu.edu>
Sender: news@dartvax.Dartmouth.EDU
Reply-To: earleh@eleazar.dartmouth.edu (Earle R. Horton)
Organization: Thayer School of Engineering
Lines: 41

In article <3791@ncsuvx.ncsu.edu> jnh@ecemwl.UUCP (Joseph N. Hall) writes:
>What is the most straightforward procedure for ensuring that a window's
>portBits begin on a word boundary?  What I would like to do is check after
>the user creates or resizes a graphics window and shift the window horizontally
>(if necessary) so that it will be aligned for later updates from an offscreen
>bitmap.

I don't know what you mean by straightforward, but ANDing the
horizontal coordinate with NOT-0x0F before calling MoveWindow() should
work fine.  Resize and create should not be a problem, but moving the
window is conceptually difficult for the average user since you will
15 times out of 16 place it on a different location than that to which
it has been dragged if you use the regular DragWindow() routine.  They
might not notice, but if they do they might perceive it as a bug.
Consider dragging the window by some other method that shows the
discrete end positions you will allow.

short x;
	x = whatever;
	x &= ~15;	/* Align x on word boundary not greater than x. */

>Also, how much speed difference is there between an aligned and unaligned
>one-to-one CopyBits?  I assume that performance is no better for an
>odd-byte-aligned bitmap than it is for a totally unaligned bitmap, right?

In four-bit mode, aligned BitMaps makes for noticeably faster
CopyBits().  Most of my experience is in using four-bit mode for
animation, but I think it is definitely worth the trouble to align
source and destination BitMaps when attempting any kind of animation
and the screen depth is greater than one.  Byte alignment seems to be
all that's necessary.  Where did you get your information that word
alignment is needed?

>I'm only messing with b/w bitmaps now, but I'd appreciate hearing the answer
>for both classic and color QuickDraw ...

     I don't notice any difference between aligned or non-aligned
BitMaps with a black and white screen.  On the other hand, these are
subjective results since I haven't done formal timing.

Earle R. Horton