[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

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

>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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