Path: utzoo!utgpu!watserv1!watmath!att!rutgers!cs.utexas.edu!samsung!munnari.oz.au!uhccux!virtue!math1205
From: math1205@waikato.ac.nz
Newsgroups: comp.sys.atari.st
Subject: Double Ghost Effect
Message-ID: <646.266ba6c3@waikato.ac.nz>
Date: 5 Jun 90 00:34:11 GMT
Organization: University of Waikato, Hamilton, New Zealand
Lines: 28

I have been experimenting with sprite animation recently and came up
with an unexpected result which I can not explain.
The experiment uses the A_line routines and a 50Hz colour monitor to
animate a sprite moving back and forth across the screen. The program
was a simple loop which did the following:

a_drawsprite(x,y)  Vsync()  a_undrawsprite() update_pos(x)

The sprite used was a small 8x8 pixel ball and the x-increment used is
16 pixels. This worked nicely and gave a very smoothly moving ball at a 
refresh rate of 50Hz.

I seemed to recall from computer graphics literature that a refresh rate of
20 Hz should be sufficient to give the impression of animation. I therefore
modified the above loop to this:

a_drawsprite(x,y) Vsync() Vsync() a_undrawsprite() update_pos(x)

I then expected to see a ball travelling at half the speed and refreshed 
at 25Hz. What I actually saw was two very faint balls adjacent to each other 
travelling across the screen.
Even when the x-increment is reduced to 1 pixel the same effect is 
noticeable as a blurring of the image.

Can anyone explain the reason for this double ghosting effect?

Wayne Schou
MATH1205@waikato.ac.nz