[SDL] Re: extension for 2D with DRI

Mattias Engdegård f91-men at nada.kth.se
Sat Apr 15 12:27:04 PDT 2000


>I do not know for conversion between 50 and 60 Hz, but I know for a fact
>that conversion between 24 fps film movies and North American 60 Hz
>television is done by doubling frames. Yes, the "cheap and bad" method.

Cheap but not bad. The large common divisor makes the period short.
The usual 2:3-transfer alternates between 2 and 3 half-frames per film
frame (according to the well-informed TV/video standards FAQ,
http://www.bawue.de/~agnus/FAQ_Video.text). For an analogy in computer
graphics, think of a pattern dither: if you have a fine-grained, regular
pattern, it looks like a solid colour from afar, but large, irregular
patterns are more likely to show artifacts.

>The thing is, can you notice a punctual 1 fps difference when playing a
>game? *One* frame per second?

Yes. You are more likely to notice 1 extra frame out of 24, than 12 extra
frames, one inserted every 2 frames.

[Quake example]
>And when you look at
>the action, it is okay and looks pretty regular, doesn't it?

No doubt so, but I believe 3D games are more resistant to it. I will try
to construct a 2D demo where the effect can be seen, to convince myself
and any heretics :-)

>And if it uses PIO to do the
>transfer and that the transfer takes longer than the vertical retrace,
>then even if it *is* synchronized to the vertical retrace, you'll see
>artefacts.

Yes, possibly even worse since the tearing will occur at the same scan line
every time. This was not what happened on the Ultra, so its blitting isn't
synchronized.

>One of the reasons the new XFree86 4.0 servers are so fast is that they
>started using the video memory intelligently. Try the 500x500
>pixmap-to-window benchmark in x11perf on the old and new servers, you'll
>see what I mean.

Unfortunately the only Linux box I have access to right is not supported
by XF4.0, but I understand the improvement is considerable.




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