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ASnogarD

bitmap font performance issue (SDL and C++ )

4 posts in this topic

Hi

I am developing a small framework that I can use when starting a new game project, the framework provides the basics I need each project ... like initialising , putting text on the screen, loading images, feedback from the code.
It was all going fine until I added the FPS meter, then I notice the FPS take a dive when the console was displaying messages... a HUGE dive, from max of 500 FPS ( no message from console) to 293 with only 23 messages being displayed.

To display text on the screen I use a basic bitmap font system, taken from Lazy Foo's tutorial and mixed with sdltutorials code.
In brief it :

Loads the bitmap font image onto a SDL_Surface, converting the image using SDL_DisplayFormat .
Stores the clipping data for each character in an static array of SDL_Rect objects
Uses ASCII values of each character to reference the relevant SDL_Rect object to provide clipping data to the draw function
Draw function is the typical blit from source ( the surface holding the font image) to destination ( main display).

video is set using SDL_HWSURFACE | SDL_DOUBLEBUF flags.

I realise it is a lot of looping and work to put up an image for each character in a string of text, but losing 200+ FPS for loosely spaced text on a screen normal ?

Question: Is a huge loss of updates per second to be expected when dsplaying a lot of text on the screen using a bitmap font system ?
Is there anyway to avoid this loss in SDL / C++ ? Edited by ASnogarD
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A drop in FPS from 500 to 293 is only an extra 1.4 milliseconds per frame, but I think that the main problem is that you are redrawing the text every frame. What happens to your FPS if you only draw the message once?
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I do clear the screen each loop using a function which does :

SDL_Rect clearBox = { 0, 0, SCREEN_WIDTH, SCREEN_HEIGHT };
SDL_FillRect(SDL_GetVideoSurface(), &clearBox , 0);

... you have given me some partial ideas though ( conditional screen clearing, partial screen clearing , small window for new messages and a large window to view all messages while the game loop gets paused when viewing the large console window )

So the drop is to be expected if I am redrawing the entire screen each update then ?

Thank you for the reply.
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For all my text in SDL I use SDL_ttf and ttf fonts (fontspace.com is a great website for these). Load the font (TTF_OpenFont), then load the text to a SDL_Surface (I use TTF_RenderText_Blended for quality, there are others that are quicker with lower quality), then blit the surface to screen. All of this is done in my [url="http://sednihp.wordpress.com/2012/06/02/111/"]ImageCache class[/url], which is probably similar to what you're trying to achieve.

This doesn't give me any noticeable slowdown at all and I redraw the entire screen every frame (same as you), but then all of my games are capped at 60fps. Edited by sednihp
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I also used SDL_ttf. One nice thing with that is that if you write the code well you only need to re-create the texture if the text changes, which most text doesn't do very often, so it's extremely fast. As fast as drawing one textured quad per text line.

Hmmm, yes, I did this together with openGL... I've never used SDL without openGL, though. :/ Maybe what I'm saying is irrelevant.
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