Moving Sprite Using Keyboard With SDL

In this tutorial we will explain how to handle events in SDL. Before starting this topic, the user is expected to have read the topic about rotate image using sdl and basic using keyboard to control the event. We feel this topic should we make in order for event handling logic becomes easy to understand.

Previously described, each event is handled by sdl through SDL_Event. Actually, SDL_Event is the general handling of each event. When only using the keyboard as a source of events, it can be replaced with SDL_KeyboardEvent SDL_Event. However, if this event requires a lot of source events, SDL_Event is the key. After a successful event is handled by SDL_Event, sdl collect event information that is being delayed. This task can be performed by SDL_PollEvent or SDL_PeepEvent. SDL_PollEvent a logical order. It will be worth 1 if there are events that are being delayed, and is 0 if no.

Events information collected includes events from the keyboard, mouse, joystick, and others. However, we focus on the event information from the keyboard. Event information from the keyboard includes two types, namely SDL_KEYDOWN (when pressed) and SDL_KEYUP (when not pressed). To find out if the button is pressed or released, then it will be done by SDL_keysym. SDL needs to know the symbol keys are used (pressed or released), in this case SDLKey will read the entire keyboard symbols used. See the figure below

Practice

Now, let we doing simple practice, how to control game from keyboard. This pratice show to you different way to make your game move smooth or move step by step.

Moving sprite when presses a key

This practice you will see that sprite (barrel) will rotated step by step each time while you pressing a keyboard. This happen because we don't give specification condition for making smooth movement. Please see the code below

#ifdef __cplusplus #include <cstdlib> #else #include <stdlib.h> #endif #ifdef __APPLE__ #include <SDL/SDL.h> #else #include <SDL.h> #endif #include <SDL_image.h> #include <string> #include <SDL_rotozoom.h> using namespace std; //give properties of our game windows application // wg = width of game windows // hg = height of game windows // bg = bit of game windows int wg = 320; int hg = 240; int bg = 32; //declare initial condition SDL_Surface* sprites = NULL; SDL_Surface* background = NULL; SDL_Surface* screen = NULL; SDL_Surface* cannon = NULL; SDL_Surface* rotate_cannon = NULL; //construct event in here SDL_Event event; //loading files SDL_Surface *load_image( std::string filename ) { //Get Free store for loadedImage SDL_Surface* loadedImage = NULL; //Get free store for optimizedImage SDL_Surface* optimizedImage = NULL; //Loading the image loadedImage = IMG_Load( filename.c_str() ); //Check status of loadedImage if( loadedImage != NULL ) { //Convert a surface to the display format optimizedImage = SDL_DisplayFormat( loadedImage ); //Frees the resources used by a previously created SDL_Surface SDL_FreeSurface( loadedImage ); //Then we go to mix the color if( optimizedImage != NULL ) { //Map the color key Uint32 mixlorkey = SDL_MapRGB( optimizedImage->format, 255, 255, 255 ); //Set it transparent SDL_SetColorKey( optimizedImage, SDL_SRCCOLORKEY, mixlorkey ); } } //Return the optimized image return optimizedImage; } //rotate image SDL_Surface* rotate(SDL_Surface* source, double angle, double zoom, int smooth) { //give initial for rotate image SDL_Surface* rotatefile = NULL; //give initial for optimizing rotate image SDL_Surface* optimizedrotate = NULL; //get rotate here rotatefile = rotozoomSurface(source, angle, zoom, smooth); //get optimizing optimizedrotate = SDL_DisplayFormat(rotatefile); //free surface SDL_FreeSurface( rotatefile ); return optimizedrotate; } //draw load files void drawfiles(SDL_Surface* imageSurface,SDL_Surface* screenSurface,int srcX, int srcY,int dstX, int dstY,int width, int height) { //defines a rectangular area of pixels for source SDL_Rect srcRect; srcRect.x = srcX; srcRect.y = srcY; srcRect.w = width; srcRect.h = height; //defines a rectangular area of pixels for destination SDL_Rect dstRect; dstRect.x = dstX; dstRect.y = dstY; dstRect.w = width; dstRect.h = height; SDL_BlitSurface(imageSurface, &srcRect, screenSurface, &dstRect); } //initial condition of all sdl bool init() { // initialize SDL video if ( SDL_Init( SDL_INIT_EVERYTHING ) < 0 ) { printf( "Unable to init SDL: %s\n", SDL_GetError() ); return false; } //Set up the screen screen = SDL_SetVideoMode( wg, hg, bg, SDL_ANYFORMAT ); //If there was an error in setting up the screen if ( !screen ) { printf("Unable to set 320x240 video: %s\n", SDL_GetError()); return false; } //If everything initialized fine return true; } //check load files bool load_files() { //Loading sprites sprites = load_image( "sprites.png" ); //If there was an error in loading sprites if (sprites == NULL) { printf("Unable to load sprites: %s\n", SDL_GetError()); return false; } //Loading cannon cannon = load_image( "cannon.png" ); //If there was an error in loading cannon if (cannon == NULL) { printf("Unable to load cannon: %s\n", SDL_GetError()); return false; } //If everything loaded fine return true; } //clean all memory void clean_up() { //Free the images SDL_FreeSurface( sprites ); SDL_FreeSurface( background ); SDL_FreeSurface( cannon ); SDL_FreeSurface( rotate_cannon ); //Quit SDL SDL_Quit(); } int main ( int argc, char** argv ) { //run init if( init() == false ) { return 1; } //Load the files with optimizing if( load_files() == false ) { return 1; } // load an image of background but with no optimizing background = IMG_Load("background.png"); if (background == NULL) { printf("Unable to load background: %s\n", SDL_GetError()); return 1; } double degree = 30; // program main loop bool done = false; while (!done) { // message processing loop while (SDL_PollEvent(&event)) { // check for messages switch (event.type) { // exit if the window is closed case SDL_QUIT: done = true; break; // check for keypresses case SDL_KEYDOWN: { // exit if ESCAPE is pressed if (event.key.keysym.sym == SDLK_ESCAPE) { done = true; break; } if (event.key.keysym.sym == SDLK_DOWN) { degree = degree - 1.0; } } } // end switch } // end of message processing // DRAWING STARTS HERE //rotating image rotate_cannon = rotate(cannon, degree, 1, 0); // clear screen SDL_FillRect(screen, 0, SDL_MapRGB(screen->format, 0, 0, 0)); // centre the image on screen //for background drawfiles( background, screen, 0, 0, 0, 0, 320, 240 ); //draw tank drawfiles( sprites, screen, 3,5,100,200,32,18); //draw bazooka drawfiles( sprites, screen, 176, 12,5,180,35,23); //draw drawfiles( rotate_cannon, screen, NULL, NULL, 50, 50, 40, 40); // DRAWING ENDS HERE // finally, update the screen :) SDL_Flip(screen); } // end main loop // free loaded bitmap clean_up(); // all is well ;) printf("Exited cleanly\n"); return 0; }

Making sprite move smooth

This practice will guide you how to make a sprite move smooth. Because in here, we give condition for making smooth movement. There are two key success for this. First, we create an array for event.key.keysym.sym, then we give condition true if pressed and false when released. Please see the code below

#ifdef __cplusplus #include <cstdlib> #else #include <stdlib.h> #endif #ifdef __APPLE__ #include <SDL/SDL.h> #else #include <SDL.h> #endif #include <SDL_image.h> #include <string> #include <SDL_rotozoom.h> using namespace std; //give properties of our game windows application // wg = width of game windows // hg = height of game windows // bg = bit of game windows int wg = 320; int hg = 240; int bg = 32; //declare initial condition SDL_Surface* sprites = NULL; SDL_Surface* background = NULL; SDL_Surface* screen = NULL; SDL_Surface* cannon = NULL; SDL_Surface* rotate_cannon = NULL; //construct event in here SDL_Event event; //loading files SDL_Surface *load_image( std::string filename ) { //Get Free store for loadedImage SDL_Surface* loadedImage = NULL; //Get free store for optimizedImage SDL_Surface* optimizedImage = NULL; //Loading the image loadedImage = IMG_Load( filename.c_str() ); //Check status of loadedImage if( loadedImage != NULL ) { //Convert a surface to the display format optimizedImage = SDL_DisplayFormat( loadedImage ); //Frees the resources used by a previously created SDL_Surface SDL_FreeSurface( loadedImage ); //Then we go to mix the color if( optimizedImage != NULL ) { //Map the color key Uint32 mixlorkey = SDL_MapRGB( optimizedImage->format, 255, 255, 255 ); //Set it transparent SDL_SetColorKey( optimizedImage, SDL_SRCCOLORKEY, mixlorkey ); } } //Return the optimized image return optimizedImage; } //rotate image SDL_Surface* rotate(SDL_Surface* source, double angle, double zoom, int smooth) { //give initial for rotate image SDL_Surface* rotatefile = NULL; //give initial for optimizing rotate image SDL_Surface* optimizedrotate = NULL; //get rotate here rotatefile = rotozoomSurface(source, angle, zoom, smooth); //get optimizing optimizedrotate = SDL_DisplayFormat(rotatefile); //free surface SDL_FreeSurface( rotatefile ); return optimizedrotate; } //draw load files void drawfiles(SDL_Surface* imageSurface,SDL_Surface* screenSurface,int srcX, int srcY,int dstX, int dstY,int width, int height) { //defines a rectangular area of pixels for source SDL_Rect srcRect; srcRect.x = srcX; srcRect.y = srcY; srcRect.w = width; srcRect.h = height; //defines a rectangular area of pixels for destination SDL_Rect dstRect; dstRect.x = dstX; dstRect.y = dstY; dstRect.w = width; dstRect.h = height; SDL_BlitSurface(imageSurface, &srcRect, screenSurface, &dstRect); } //initial condition of all sdl bool init() { // initialize SDL video if ( SDL_Init( SDL_INIT_EVERYTHING ) < 0 ) { printf( "Unable to init SDL: %s\n", SDL_GetError() ); return false; } //Set up the screen screen = SDL_SetVideoMode( wg, hg, bg, SDL_ANYFORMAT ); //If there was an error in setting up the screen if ( !screen ) { printf("Unable to set 320x240 video: %s\n", SDL_GetError()); return false; } //If everything initialized fine return true; } //check load files bool load_files() { //Loading sprites sprites = load_image( "sprites.png" ); //If there was an error in loading sprites if (sprites == NULL) { printf("Unable to load sprites: %s\n", SDL_GetError()); return false; } //Loading cannon cannon = load_image( "cannon.png" ); //If there was an error in loading cannon if (cannon == NULL) { printf("Unable to load cannon: %s\n", SDL_GetError()); return false; } //If everything loaded fine return true; } //clean all memory void clean_up() { //Free the images SDL_FreeSurface( sprites ); SDL_FreeSurface( background ); SDL_FreeSurface( cannon ); SDL_FreeSurface( rotate_cannon ); //Quit SDL SDL_Quit(); } int main ( int argc, char** argv ) { //run init if( init() == false ) { return 1; } //Load the files with optimizing if( load_files() == false ) { return 1; } // load an image of background but with no optimizing background = IMG_Load("background.png"); if (background == NULL) { printf("Unable to load background: %s\n", SDL_GetError()); return 1; } double degree = 30; // program main loop bool done = false; bool keyboardstate[323] = {false}; while (!done) { // message processing loop if (SDL_PollEvent(&event)) { if (event.type == SDL_QUIT) { done = true; } if (event.type == SDL_KEYDOWN) { keyboardstate[event.key.keysym.sym] = true; } if (event.type == SDL_KEYUP) { keyboardstate[event.key.keysym.sym] = false; } } if (keyboardstate[SDLK_DOWN]) { degree = degree - 1; } // end of message processing // DRAWING STARTS HERE //rotating image rotate_cannon = rotate(cannon, degree, 1, 0); // clear screen SDL_FillRect(screen, 0, SDL_MapRGB(screen->format, 0, 0, 0)); // centre the image on screen //for background drawfiles( background, screen, 0, 0, 0, 0, 320, 240 ); //draw tank drawfiles( sprites, screen, 3,5,100,200,32,18); //draw bazooka drawfiles( sprites, screen, 176, 12,5,180,35,23); //draw drawfiles( rotate_cannon, screen, NULL, NULL, 50, 50, 40, 40); // DRAWING ENDS HERE // finally, update the screen :) SDL_Flip(screen); } // end main loop // free loaded bitmap clean_up(); // all is well ;) printf("Exited cleanly\n"); return 0; }

for more code of this tutorial, you can get by press keyboard and by hold keyboard here.