Re: Flipping Screens

[philip@labtam.labtam.oz.au (Philip Stephens)]

Mark Manning writes:

>What's the fastest you've been able to make animation go
>and still not get the flicker?  (ie: exceed the refresh
>rate)

  Technically, flicker-free animation can only be achieved if you draw your
pixels on the screen whilst the electron beam is busy scanning somewhere
else.  Typically you want to start drawing when the video-blanking period
begins (i.e. when the beam has reached the bottom and is zipping back up
the top).  Of course, you only have a limited amount of time before the
beam starts it's way down the screen again.
  But as a rough guide, if you can draw your stuff faster than 1/60 of a
second, you'll avoid most of the flicker.

>Secondly, how many things can you handle at one time one
>the screen before everything slows down too much?  Is there
>a table of number of items crossed against the size of the
>items?  After all, the larger the items are, the slower things
>will go - right?

  Right.  Except that the speed of animation is not just a function of the
object size, but also depends on how quickly you can draw pixels onto the
video screen.  As a very rough estimate, you can fill the entire super hires
screen in about 1/10th of a second.  The less you put on the screen, the
faster it's gonna get.  So for a typical game that doesn't try to do full
screen scrolling, you can easily get a flicker-free rate of animation.
And you CAN perform full screen scrolling flicker-free if you draw the
animation off-screen then copy it on-screen during the video-blanking
period, but the frame rate will only be that 1/10th of a second, at best.

>Thanks!  :)

  You're welcome.  Sorry I couldn't be more specific, but I don't have my
calculator and instruction timing sheet handy at the moment ;-)

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