[SDL] How to make a game run at the same speed on any PC?

[Lilith Calbridge]

I can't claim to be an expert on this kind of programming so I can only
offer a guess.  I'm still kind of new to the process and have had to put
off getting into the game aspect of it in order to develop the graphics
for a presentation (using SDL) I'll be giving in January.

My feeling about the difference in speed is this....

Using the tick count is still very inaccurate.  Despite the speed of
the process, there's so much going on with processes that support the OS
that can't be interrupted that time resolution to 1 ms is really
impossible.  You can really expect something more on the order of 10 ms.
 The slower processor means that the OS is taking longer to deal with
its overhead so you may be coming in closer to the 10 ms resolution or
beyond.  Maybe not on every iteration of your loop but often enough to
cause some sort of timing difference.  

That's the downside of having to use the OS for support.  Then again,
who wants to write a program that you have to boot the system to play. 
This kind of thing was perhaps a bit easier to do on DOS based systems
because DOS could be cut from interfering with the program until it
finished unless you needed DOS to do some output for you.   But you'd
also have to deal with different processor speeds in a different manner
too.

Lilith

>>> On 7/23/2006 at 10:40 PM, in message <ea1fem$gga$1 at sea.gmane.org>,
leo28c at gmail.com wrote:
> Hmm...  Still not working :-(
> I'm testing the game with 2 comps, one of 2.8GHz and one with half of

> that (1.4GHz)...  The 1.4GHz PC takes like one second to load the 
> screen, and the 2.8GHz one is instant.  Then, if I press the Left
arrow 
> on both comps at the same time and leave them pressed, the 1.4GHz one

> takes just a bit longer to get to the end of the screen.  Why is
this? 
> Some misdesign?  I only have a guy walking around sand tiles with a
car 
> that he can get on by walking into it, no rotating or anything weird

> either...  How can I fix it?  Thanks!
> 
> PD:  It doesn't take
> 
> Lilith Calbridge wrote:
>> You don't have to use 500.  That was just a throwaway value.  Most
demo
>> code I've seen uses a value of 30, which ostensibly gives you a
frame
>> rate of about 30 frames a second, probably a little closer to 33. 
That
>> means the sampling rate of your events would be about fifteen times
>> faster than a half second interval.  I haven't tried this, and
probably
>> won't get around to it any time soon, but if you want to make your
>> sampling even faster than that you could probably create a shorter
delay
>> loop inside a larger loop to do your sampling and update what's
>> necessary before updating the screen.
>> 
>> In a project I've had to put off I was thinking of looking at the
>> keyboard state at the moment that the decision needs to be made.  I
have
>> an object that moves towards intersections and can only move
forward,
>> without speed change, until then .  So the only thing that matters
is
>> whether one of four possible keys is pressed at that specific time.

>> Maybe something like that is what you need.
>> 
>> Lilith
>> 
>>>>> On 7/23/2006 at 7:53 PM, in message
<ea15lh$uag$1 at sea.gmane.org>,
>> leo28c at gmail.com wrote:
>>> Hmm...  Now it all screwed up...  :-(
>>> I think my movement mechanism is wrong...  I'm doing it like, at 
>>> SDL_KEYUP event, the movement key for that direction goes to 0... 
>> But 
>>> at SDL_KEYDOWN, it the variable goes to 1, so the dude's moving...
>>> But since this function is called only every half a second, the
>> movement 
>>> is updated every half a second too!  So, it starts and stops
moving
>> half 
>>> a second after my key presses and releases... :-(
>>>
>>> Note that I add PLR_SPEED to the player's X/Y positions in the 
>>> update_screen function...  I tried putting it in the same function
as
>> 
>>> the event handling, but it didn't work either, it moved only every
>> half 
>>> a second...
>>>
>>> Do you know how to fix this?  Thanks very much! ;)
>>>
>>>
>>> Janne Junnila wrote:
>>> Peter Mackay wrote:
>>>> There's a great article on fixing your logic timestep here:
>>>> http://www.gaffer.org/articles/ 
>>>>
>>>> His other articles are also very good. Hope this helps,
>>>>
>>>> Peter
>>>>
>>>> "Janne Junnila" <janne.junnila at gmail.com> wrote in message 
>>>>
news:770a6dc40607230033x6df09271xbca8208b6872a5a6 at mail.gmail.com...
>>>>> Hi,
>>>>>
>>>>>> Anyways, I asked what update_interval was supposed to be... 
>> Thanks!
>>>>> Just a constant - it's the interval between game world updates
(in
>> ms I 
>>>>> think).
>>>>> Like update_interval = 500 would make it call the update
function
>> every 
>>>>> 1/2 sec.
>>>>>
>>>>> The point is to make the updating function to be called on a
fixed
>>>>> interval independent of the machine the program is run on. The
>> screen
>>>>> OTOH is updated as often as possible.
>>>>>
>>>>> - Janne Junnila 
>>>> Hi,
>>>>
>>>>> Anyways, I asked what update_interval was supposed to be... 
>> Thanks!
>>>> Just a constant - it's the interval between game world updates
(in
>> ms I 
>>> think).
>>>> Like update_interval = 500 would make it call the update function
>> every 1/2 
>>> sec.
>>>> The point is to make the updating function to be called on a
fixed
>>>> interval independent of the machine the program is run on. The
>> screen
>>>> OTOH is updated as often as possible.
>>>>
>>>> - Janne Junnila
>>>
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