Path: utzoo!attcan!utgpu!watmath!watcgl!bmacintyre
From: bmacintyre@watcgl.waterloo.edu (Blair MacIntyre)
Newsgroups: comp.sys.amiga.tech
Subject: Double Buffering
Message-ID: <11833@watcgl.waterloo.edu>
Date: 9 Oct 89 15:31:09 GMT
Reply-To: bmacintyre@watcgl.waterloo.edu (Blair MacIntyre)
Distribution: na
Organization: University of Waterloo
Lines: 41

I'm trying to create a double buffered display.  I had it working for the
longest time using the following method:
	2 BitMap structs - one in Screen->Viewport.RasInfo->BitMap
			   the other in Screen->RastPort.BitMap
	swap them to double buffer
	call MakeScreen( Screen )
	     ReThinkDisplay();

A while ago someone told me to use ScrollVPort( &screen->Viewport ) to achieve
the double buffered affect, so I looked up the function and low and behold,
it says that by "leaving the dx,dy offsets 0 and changing the BitMap pointer
a double buffering effect can be achieved" ( or something to that effect ).

The problem is this:
	I substitute the ScrollVPort call for the other two calls and the
	following happens:  the double buffering works ( a Good thing! ) but
	all the lines on the display cease to be drawn correctly.  What I
	mean is this.   A line that used to be rastorized like this

				********
			********
		********
	********

	now appears like this
				********
		********
			********
	********

	This happens to all the lines on the display ( I am not using text
	so I don't know if it would happen to it as well )

Any ideas what may be causing this?  I'm at a loss.

Blair
-- 
= Blair MacIntyre, bmacintyre@watcgl.{waterloo.edu, UWaterloo.ca}          // =
=   now appearing at the Computer Graphics Lab, U of Waterloo!           \X/  =
= Ignore my opinions ... they are the product of an imagination preoccupied   =
= with sex, or so they tell me ... *sigh*                                     =