SDL: Why does this not work?

Meta Adam · 2005-01-21T10:29:29

Hello, I'm creating a simple tile engine in SDL, and right now, I just want to create a map, put a sprite on it and be able to move it. I got this code, and it compiles fine, but it just shows the map and not the sprite. Can someone help me figure this out? #include <iostream> #include <SDL/SDL.h> using namespace std; //yes, I know this is evil. SDL_Surface *screen; SDL_Surface *grass; SDL_Surface *water; SDL_Surface *wall; SDL_Surface *tree; SDL_Surface *mySprite; int sxpos = 0; int sypos = 0; int map[10][10] = {{4, 4, 4, 4, 4, 4, 4, 4, 4, 4}, {4, 1, 1, 1, 1, 1, 1, 1, 1, 4}, {4, 1, 2, 2, 1, 1, 2, 2, 1, 4}, {4, 1, 3, 3, 3, 3, 3, 3, 1, 4}, {4, 1, 2, 2, 1, 1, 2, 2, 1, 4}, {4, 1, 1, 1, 1, 1, 1, 1, 1, 4}, {4, 1, 3, 3, 3, 3, 3, 3, 1, 4}, {4, 1, 1, 1, 4, 4, 1, 1, 1, 4}, {4, 1, 1, 1, 1, 1, 1, 1, 1, 4}, {4, 4, 4, 4, 4, 4, 4, 4, 4, 4}}; int xpos = 0; int ypos = 0; int InitImages() { grass = SDL_LoadBMP("grass.bmp"); water = SDL_LoadBMP("water.bmp"); wall = SDL_LoadBMP("wall.bmp"); tree = SDL_LoadBMP("tree.bmp"); mySprite = SDL_LoadBMP("mysprite.bmp"); return 0; } int SDL_BlitSurface(SDL_Surface *src, SDL_Rect *srcrect, SDL_Surface *dst, SDL_Rect *destrect); void drawIMG(SDL_Surface *img, int x, int y) { SDL_Rect dest; dest.x = x; dest.y = y; SDL_BlitSurface(img, NULL, screen, &dest); } int drawMap() { xpos = 0; ypos = 0; for(int y=0; y<=9; ++y) { xpos = 0; for(int x=0; x<=9; ++x) { if(map[y][x] == 1) { drawIMG(grass, xpos, ypos); SDL_Flip(screen); } if(map[y][x] == 2) { drawIMG(water, xpos, ypos); SDL_Flip(screen); } if(map[y][x] == 3) { drawIMG(wall, xpos, ypos); SDL_Flip(screen); } if(map[y][x] == 4) { drawIMG(tree, xpos, ypos); SDL_Flip(screen); } xpos += 32; if(x == 9) { ypos += 32; } } } return 1; } void sprite() { drawIMG(mySprite, sxpos, sypos); } void drawScene() { drawMap(); sprite(); SDL_Flip(screen); } int main(int argc, char *argv[]) { Uint8* keys; if(SDL_Init(SDL_INIT_VIDEO) < 0) { cout<<"Unable to init SDL."<<endl; SDL_GetError(); } atexit(SDL_Quit); screen = SDL_SetVideoMode(640, 480, 32, SDL_HWSURFACE | SDL_DOUBLEBUF); if(screen == NULL) { cout<<"Unable to set video mode."<<endl; SDL_GetError(); } InitImages(); drawMap(); int done=0; while(done == 0) { SDL_Event event; while ( SDL_PollEvent(&event) ) { if ( event.type == SDL_QUIT ) { done = 1; } if ( event.type == SDL_KEYDOWN ) { if ( event.key.keysym.sym == SDLK_ESCAPE ) { done = 1; } } } keys = SDL_GetKeyState(NULL); if(keys[SDLK_UP]) { for(int sxu=0; sxu<=32; sxu++) { for(int syu=0; syu<=32; syu++) { sypos -= 1; } } } if(keys[SDLK_DOWN]) { for(int sxd=0; sxd<=32; sxd++) { for(int syd=0; syd<=32; syd++) { sypos +=1; } } } if(keys[SDLK_LEFT]) { for(int sxl=0; sxl<=32; sxl++) { for(int syl=0; syl<=32; syl++) { sxpos -= 1; } } } if(keys[SDLK_RIGHT]) { for(int sxr=0; sxr<=32; sxr++) { for(int syr=0; syr<=32; syr++) { sxpos += 1; } } } drawScene(); } return 0; } Thanks.

Engines and Middleware Programming

Started by Meta Adam January 17, 2005 08:59 PM

14 comments, last by The Rug 20 years, 1 month ago

evillive2

779

January 18, 2005 04:57 PM

It isn't very well commented but a few people have found this to be helpful. Hopefully you can expand on the concepts a little but I think it has most of what you are looking for.

Evillive2

Inmate2993

222

January 19, 2005 03:14 PM

Alright, this guy is of the wrong impression of how the code flow. If you want to make a sprite move 1 pixel at a time, but across a full box, you need to program the system to automatically move the sprite after each iteration. For instance, look at this code:

while(1){  keys = SDL_GetKeyState(NULL);  if(keys[SDLK_UP])  {    if ( ( my_sprite.source_y == my_sprite.target_y )    &&   ( my_sprite.source_x == my_sprite.target_x ) )      my_sprite.target_y -= 32;  }   else if (keys[SDLK_DOWN])  {    if ( ( my_sprite.source_y == my_sprite.target_y )    &&   ( my_sprite.source_x == my_sprite.target_x ) )      my_sprite.target_y += 32;  }  if (my_sprite.source_y > my_sprite.target_y)  {    my_sprite.source_y -= 1;  }   else if (my_sprite.source_y < my_sprite.target_y)  {    my_sprite.source_y += 1;  }  blit( my_sprite );}

Its rudimentary, but you should see what I'm pointing at. You have to keep a target destination for the sprite, but draw where the sprite actually is from a different set of variables. Then, on each iteration of the loop, it'll move the sprite accordingly and not allow the player to interrupt the movement.

william bubel

vHaB

172

January 21, 2005 10:13 AM

The reason your movement is so fast is because you need to make it time-based.

Your frame rate is so fast that when you hold the key down like that, it registers several "keypresses" per frame. What you need to do is time how long a frame takes, decide how far you want it to move per frame, and increment the position accordingly.

This means that whether your game runs at 20fps or 200fps, the rate of movement is the same. Otherwise if for example it becomes a networked game, the player with the higher framerate has a distinct advantage.

code is something like this:

// these are for calculating FPS and movementfloat currentTime;float lastTime;float frameInterval;int SPEED;  // set this to how many pixels you want your sprite to move per SECOND.  Play with it until you find a value that seems right.// in your keypress event// if (<right movement key is down>)sprite.xpos += SPEED * frameInterval;// Draw frame here:void DrawFrame(void ) {    // calculate frame interval    currentTime = SDL_GetTicks() * 0.001f;    frameInterval = currentTime - lastTime;    lastTime = currentTime;    DrawTiles();    DrawSprite();}

Also, storing your sprite positions as floats makes all this a little easier. Convert to ints immediately before blitting.

andyZER0

128

January 21, 2005 10:18 AM

Why is

using namespace std;

evil?

Drew_Benton

1,865

January 21, 2005 10:21 AM

Quote:
Original post by andyZER0
Why is *** Source Snippet Removed *** evil?

I have no idea. I'd rather using the STL library nowadays then try to make everything better myself.

The Rug

628

January 21, 2005 10:29 AM

Quote:
Original post by andyZER0
Why is *** Source Snippet Removed *** evil?

Its (I think) generally considered bad practice to include the entire std namespace when you are only using a few bits of it. You wouldn't want to include the entire namespace when using just cout for instance, you'd just do:

using std::cout;

the rug - funpowered.com

SDL: Why does this not work?

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