Re: [Algorithms] FPS Questions

[Stephen J B. <sj...@li...>]

On Tue, 1 Aug 2000, Keith Z.Leonard wrote:

> Actually, if we are going to be nit-picky the ~60hz of TV is interlaced, so
> you are seeing half-frames, with an actual "frame" rate of 29.97hz

Well, each scanline is painted ~30 times a second - but it draws all the odd
numbers lines in one 1/60th second period and all the even numbered scanlines
on the next 1/60th second.  Hence you see changes in the position of objects
60 times a second - so double imaging shouldn't happen as it would in a true
30Hz setup.

...and for some countries, it's 25Hz and 50Hz rather than 30 and 60
as it is here in USA.

What you get with TV is interlace flicker where one pixel high lines only
show up every alternate field - 30Hz.  Fortunately TV cameras and TV
graphics systems know about this and are very careful to deal with it.

There are also some very peculiar psycho-physical effects when an object
is moving vertically up or down the screen at more or less exactly one
scanline per field - this is called 'entrainment' - and I don't understand
it.  It makes some people very ill and other people mis-judge the speed
of things.

So what do console games do about interlace artifacts?  Are they actually
running at half the vertical resolution of broadcast TV - or do they have
fancy antialiasing filters to deal with it?

> Are movies also interlaced??

No - you can't interlace a film - it doesn't have raster structure.

> I thought that I read somewhere that movies had moved to a 48 hz refresh.

So we are reliably informed (I didn't know that before) - they display the
same entire image twice though - similar to PC's running at 30Hz.

> Also, none of this really matters to computer frame rate discussion,
> the end result is that people can see the difference between 30 and
> 60 fps, and the change is significant. 

For fast-moving images - yes.

Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
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