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SDL Beginner Help!

Started by zippo88 Apr 3, 2009 at 12:34 PM 12 replies 3.6k views

Original Post

Apr 3, 2009 at 12:34 PM

Hi i've just started to program with SDL, and am having some trouble. I followed this guide: http://www.aaroncox.net/tutorials/arcade/Introduction.html I don't understand the part where you set the speed of your charachter / program. Anyone got any idea how i can work that into the code? Please explain in detail :). Here is the game: http://rapidshare.com/files/217032701/SDLSpel.rar.html and here is the code(please note that some parts are swedish): #include #include #include #include #include int main(int argc, char **argv) { bool gamerunning = true; bool keysHeld[323] = {false}; // everything will be initialized to false int level = 1; SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER ); SDL_Surface* screen = SDL_SetVideoMode(640, 480, 0, SDL_HWSURFACE | SDL_DOUBLEBUF); SDL_WM_SetCaption("SDLTest", 0); //Ladda BMP fil SDL_Surface* bakgrund = SDL_LoadBMP("Bakgrund.bmp"); SDL_Surface* gubbe = SDL_LoadBMP("Gubbe.bmp"); // Göm färgen Magneta för bild SDL_SetColorKey(gubbe, SDL_SRCCOLORKEY, SDL_MapRGB(gubbe->format, 255, 0, 255)); //BakgrundBildens koordinater som ska laddas SDL_Rect bakgrundDel; bakgrundDel.x = 0; bakgrundDel.y = 0; bakgrundDel.w = 640; bakgrundDel.h = 480; //BakgrundBildens koordinater i SDL rutan SDL_Rect bakgrundposition; bakgrundposition.x = 0; bakgrundposition.y = 0; bakgrundposition.w = bakgrundDel.w; bakgrundposition.h = bakgrundDel.h; //GubbBildens koordinater som ska laddas SDL_Rect gubbeDel; gubbeDel.x = 0; gubbeDel.y = 0; gubbeDel.w = 40; gubbeDel.h = 40; //GubbBildens koordinater i SDL rutan SDL_Rect gubbeposition; gubbeposition.x = screen->w / 2 - (gubbeDel.w / 2); gubbeposition.y = (screen->h / 2) - (gubbeDel.h / 2); gubbeposition.w = gubbeDel.w; gubbeposition.h = gubbeDel.h; SDL_Event event; while (gamerunning) { if (SDL_PollEvent(&event)) { if (event.type == SDL_QUIT) { gamerunning = false; } if (event.type == SDL_KEYDOWN) { keysHeld[event.key.keysym.sym] = true; } if (event.type == SDL_KEYUP) { keysHeld[event.key.keysym.sym] = false; } } if (keysHeld[SDLK_ESCAPE]) { gamerunning = false; } if ( keysHeld[SDLK_LEFT] ) { gubbeposition.x -= 1; gubbeDel.x = 40; } if ( keysHeld[SDLK_RIGHT] ) { gubbeposition.x += 1; gubbeDel.x = 40; } if ( keysHeld[SDLK_UP] ) { gubbeposition.y -= 1; gubbeDel.x = 0; } if (keysHeld[SDLK_DOWN]) { gubbeposition.y += 1; gubbeDel.x = 0; } if (level==1) { if (gubbeposition.x == screen->w) { gubbeposition.x = 1; bakgrundDel.x = 640; level += 1; } } if (level==2) { if (gubbeposition.x == screen->w) { gubbeposition.x = 0; bakgrundDel.x = 1280; level += 1; } else if (gubbeposition.x == 0) { gubbeposition.x = screen->w; bakgrundDel.x = 0; level -= 1; } } //Gör skärmen svart efter varje loop SDL_FillRect(screen, NULL, SDL_MapRGB(screen->format, 0, 0, 0)); //Hämta en del av en .BMP fil. FIL / Del av bild / Yta / bild position SDL_BlitSurface(bakgrund, &bakgrundDel, screen, &bakgrundposition); SDL_BlitSurface(gubbe, &gubbeDel, screen, &gubbeposition); SDL_Flip(screen); SDL_Delay(1); } SDL_FreeSurface(gubbe); SDL_FreeSurface(bakgrund); SDL_Quit(); return 0; }

Apr 5, 2009 at 3:20 PM

First of all, welcome to the GameDev forums =)
I'm going to answer your questions, but first let me advice you on how you should do from now on if you want help from people on GameDev ;)

1. Please post long code within code-blocks. This makes posts that contain code much easier to read. Please make posts easy to read if you want help. Many people will not answer posts if they are too difficult to read. See the forum FAQ for info on how to do that.

2. If you post short code, it doesn't matter if it's in code blocks.

3. If you post code: make sure it is commented. In ENGLISH. Not swedish ;)
You're lucky I happened to be swedish, so I could read them without getting upset. :P Even though I'm swedish I always write code and comments in english so that I can quickly ask anybody for help without having to translate everything first.

Okay, so let's get on to the solutions! Yay!

Your game loop (and most other game loops as well) updates like this:
* First do game logic and draw stuff to screen buffer (5 milliseconds)
* Then sit and wait for the next flicker of the screen (lots of milliseconds)
* Instantly swap the old screen image for the new one (0 milliseconds)
* Start loop all over again (0 milliseconds)

One pass through all that is called a frame. Or a tick.

As you can see, the whole process takes 5 + lots + 0 + 0 milliseconds in total. The wait-for-screen part is done within the SDL_Flip(screen); command. All this makes the game loop run with a constant framerate. So your game loop is normally executed some 100 times per second (this may vary depending on the system). You want it to always be like this.

In order to make your character move around faster, you DON'T want to make the game loop run faster. That speed is mostly constant. Instead you want the character to move further for every frame.

You do this by changing all lines that look like this:
gubbeposition.x -= 1;

into lines that look like this:
gubbeposition.x -= gubbeSpeed;

And to be able to do this we must of course have a variable called gubbeSpeed:
bool gamerunning = true;
bool keysHeld[323] = {false}; // everything will be initialized to false
int level = 1;
int gubbeSpeed = 2;

Ta-daaaaaaaa! I just made your "gubbe" move two pixels every frame instead of just one! Try playing around with that value, just for fun.

You will find that he does no longer wrap around the edges when he moves. That's because of this;
if (gubbeposition.x == screen->w)
{
...
}

Which now MIGHT not happen, because of the bigger jumps we just made him do. Instead you should have:
if (gubbeposition.x >= screen->w)

Good luck!

Apr 5, 2009 at 3:42 PM

Oh! I just read the tutorial you intended. You linked to the wrong page! I had to click around to find the thing about the speed and stuff. Now I better understand exactly what you asked for.

First do the modifications I mentioned in the above post. This gives you a variable speed. But it is FRAME RATE DEPENDENT. That means if you run the same code on some other computer that is twice as fast, your character will move twice as fast, because the game loop ticks twice as fast.

You do not want this. You want all players who download the game to have the same experiences, no matter how fast computers (frame rate) they got.

We accomplish this by multiplying the TimeSinceLastFrame with the speed! Notice that all your position variables must now be doubles.

SDL_Rect gubbeposition; // this is bad because SDL_rect structure only has integers in it.

You need to make your own class, so we can have all positions as doubles. This will be useful in the future. Like this:

class Cgubbe{double x;double y;double speed;string name;};Cgubbe gubbe;gubbe.x=100;gubbe.y=100;gubbe.speed=2.7;gubbe.name="Bosse Svensson";

And in the beginning of each frame:

long beginTime = SDL_GetTicks();long timeSinceLastFrame = 0;while(gamerunning){	timeSinceLastFrame = SDL_GetTicks() - beginTime;	beginTime = SDL_GetTicks();		// ... main code ...}

And FINALLY your
gubbeposition.x += gubbeSpeed;
commands need to change into
gubbe.x += gubbe.speed*TimeSinceLastFrame;

That's all. So if we got a fast computer, the TimeSinceLast number will be very small, and that will make the gubbe make small jumps between each frame. If we got a slow computer the TimeSincelast number will be large and the gubbe will make bigger jumps. Now we have FRAME RATE INDEPENDENT movement. Now he moves with the same speed on all computers.

You will have to figure out how to draw the image where you want it to go yourself. You'll have to round off the position to an integer somehow, and then somehow pass the right values to the SDL_BlitSurface() function. I recommend creating a helper function which makes things easier in the future. Something like:

void DrawImage(surface, atSurface, atLocationX, atLocationY);

Good luck!

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