Mail Archives: djgpp/1998/09/05/21:48:59
C.Rothwell <enquiries AT aditfree DOT com> wrote in article
<35C396B2 DOT C31C29EA AT aditfree DOT com>...
> Is there a fool proof way of getting somethng to run at the same speed
> on any PC without it slowing down the older machines?
>
>
You must use a time tick that do not depend on the CPU, that is the timer
(look at Allegro\demo\demo.c), to enstablish a time base
First install a timer handler at the required tick (install_int(handler,
time_base /* ms */)), then use the handler to count the number of tick
since the last refresh (the handler must be very fast). Then create a main
loop that check your tick, perform related operation (elapsed time = ticks
* time_base) and redraw the screen only if required.
On fast machine: Your game will be idle waiting for ticks. Do not care of
frame rate. Tipically the time base is higher than the maximum frame rate
(that is the veritical synch frequency - on my machine at 800x600x256 =>
61Hz => 61 frames/s)
On slow machine: More than one tick will occur between two complete game
loop. The player position will be updated at the right velocity (using
elapsed time), while the display will miss some frame.
Very slow machine: the machine is not able to handle all the interrupt
generated by the timer at the required tick ? Use time_base*2 or
time_base*N as required and calculate elapsed time as ticks*time_base*N. Of
course, the game "interaction" quality will be very low.
Note that the average elapsed time (or the idle time) give you an idea of
the machine speed (CPU+Video Car). You could automatically change the
graphic quality (by removing some features) to obtain good playable game
even of slow machine.
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