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Poor full screen performance


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#1 Danack   Members   -  Reputation: 122

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Posted 26 April 2000 - 02:46 AM

I''ve been writing some programs based on the tutorials and I''ve noticed that in full screen modes the frame rate is MUCH lower than in windowed modes. ie it drops from 40 fps to about 3. Does anyone reckon that this is because of my drivers (which I have updated recently), or just that I''m doing setting something up wrong? cheers dan Game production: Good, quick, cheap: Choose two.

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#2 Prosper/LOADED   Members   -  Reputation: 100

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Posted 26 April 2000 - 02:52 AM

Be more specific please. What your program actually does ? How do you call SetDisplayMode() ?

Edited by - Prosper/LOADED on 4/26/00 8:53:44 AM

#3 Morgan   Members   -  Reputation: 122

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Posted 26 April 2000 - 03:32 AM

Most likely it''s just because it''s running at a higher resolution. Do you start with a, lets say, 320x240 window and then when you go into full screen mode it is running at 800x600 or 1024x768? If so, it''s because it has to render more pixels to the screen than to the little window.

Morgan

#4 Danack   Members   -  Reputation: 122

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Posted 26 April 2000 - 11:07 PM

More specifics:

The programs drawing hardly anything....
theres only one big textured poly ( a rectangle ), ( which doesn''t go off the edge of the screen.
Theres also two filled polys and the rest is just some lines (around 200). There aren''t any complex calculations going on in the background.

I''m using the tutorial routines for setting up the screen, so what i''m doing is:

ChangeDisplaySettings(&dmScreenSettings,CDS_FULLSCREEN)

followed by CreateWindowEx(...)


It isn''t drawing more pixels as, the window size and the full screen resolution is the same. If i drag the window to be bigger, I do see the frame rate drop, but only by a reasonable amount, eg change it from 640,480 to almost 1600, 1200 and the frame rate drops from 40 to 20.

Dan

#5 Bad Monkey   Members   -  Reputation: 145

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Posted 27 April 2000 - 03:27 PM

Hi Danack,

Just a random thought that popped into my head... it could be that when you change the screen mode, the refresh rate of the monitor may be getting set back to some crap number like 60Hz (yuk). When you swap buffers, OpenGL implcitly waits for the next vertical retrace (unless you''ve disabled it with a driver utility), so it could be that your program is just barely missing each retrace, and has to wait for the next one before flipping, causing a lag between frames (poor framerate). Do you see what I''m getting at?

Actually, the more I think about it (if you''re running at 640x480 and drawing a simple scene with shite all polys), the less likely this seems... oh well

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#6 Danack   Members   -  Reputation: 122

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Posted 27 April 2000 - 11:49 PM

Bah! duff drivers.

Apparently my card doesn''t like full screen 24 bit colour.

If I set the colour depth to either 16 or 32 it works fine. If I set the colour depth to 24 bit then in 640, 480 mode, my program run at about 3 fps, if I set it to 800, 600 then the screens corrupted....hey ho, guess I''ll just have to use 32-bit colour then


#7 RandomJon   Members   -  Reputation: 122

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Posted 02 May 2000 - 12:59 PM

Fullscreen 24 bit colour isn''t available on any graphics card I know of
Besides in NeHe''s base code colour is set up to be 32 bit
so it will run really slow if you set up your screen as 16bit.
32bit colour is better anyway ''cos it has faster memory access and has an 8bit destination alpha in there for better blending stuff...

#8 Anonymous Poster_Anonymous Poster_*   Guests   -  Reputation:

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Posted 03 May 2000 - 11:55 AM

a pixel is only so big.

just because the resolution is static does not mean that it won''t be drawing more pixels when you maximize the screen.

it will draw magnitudes more because the objects are scaled to the window size. (in these tutorials anyway.)

#9 Danack   Members   -  Reputation: 122

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Posted 03 May 2000 - 10:52 PM

When I said fullscreen mode, I meant fullscreen, er, mode rather than a fullscreen windowed mode. So the resolution was the same for both the window and the fullscreen.

RandomJon:

My graphics card does claim to support 24bit modes...it shouldn''t change to that mode if it wasn''t supported.


Besides in NeHe''s base code colour is set up to be 32 bit
so it will run really slow if you set up your screen as 16bit.

32bit colour is better anyway ''cos it has faster memory access...


Hmm, can you justify either of these statements?



#10 Anonymous Poster_Anonymous Poster_*   Guests   -  Reputation:

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Posted 04 May 2000 - 04:22 AM

He cant, because those statements are full of shit.

#11 jaxson   Members   -  Reputation: 127

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Posted 04 May 2000 - 04:38 AM

These statements aren''t, as the Anonymous Poster so eloquently put it, "Full of shit." Instead, they aren''t quite specific enough. 32 bit is usually faster than 24 bit color because the pixels are aligned on 4 byte (ie 32bit) boundaries and processors and video cards tend to like that a LOT more than 24 bit which usually requires all kinds of extra addressing since 3 bytes doesn''t fit very nicely into 4 byte addressing.

As far as whether 32 bit is faster than 16 bit, it''s hard to say. I would think that it''s not likely. 16 bit is probably faster.

#12 RandomJon   Members   -  Reputation: 122

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Posted 04 May 2000 - 02:38 PM

cheers to jaxson for that
and no it''s not a full of shit atatement.
Your video card will not support 24 bit colour in fullscreen 3d mode. It will support it for windows but not in fulscreen.
and as for the faster access bit, what is windows? it would be a 32bit operating system. Most current hardware runs on a 32 bit memory address bus so sending anything less than that has no optimisations anymore...
The blocks of data have to be split up, so that gives you slower memory access..

In justifictaion of the second statement, if the prog is set to run using 32 bit color and you try and run using 16 bit it has to convert between the two all the time which causes the prog to slow down. Believe me on this I''ve found this out myself.
It also does the same thing the other way around and is slow for the same reason so set up your prog to use the current colour depth that you have your desktop set to 16 or 32 bit.




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