Basic Using Keyboard To Control The Event

Hi everybody, we meet again in the topic about how to make a game with c++. Now, let’s continue our discussion. But first, if you don’t understand about c++ very well, then please follow the tutorial of c++ from us. We discuss it step by step. We all know that every files of the game always contains so many images. But in the previous post, we just discuss it only for one file of image. Not only that, all game must be able to controled by using keyboard, mouse, joystick or else. For that purpose, we want to continue our discussion about basic load many files and basic control for game.

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Select libraries

Yes, we must select libraries for our program. Of course, we need SDL.h and SDL_image.h as external libraries. Also we use the string as additional library, because we want to make a class for loading many files. With this way, we will be able to control algorithm of our program. In here, we use namespace just to make sure

#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> using namespace std;

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Global variables

All global variables needed by a program very depend on the creator. We using wg, wb, and bg as global variables not only for size of game windows application but also for several condition. And that condition you will know after this.

//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 = 16;

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Generate the surface and event

SDL_Surface's represent areas of "graphical" memory, memory that can be drawn to. And in here we using pointer because we want to get the actual data of graphics.  We give a free store (signed by NULL) for SDL_surface because every pointer that not pointing to anything is a good step if we give NULL first. On the previous post, SDL_Event located inside of “int main”. But, for now and next if you still do that your programs will not work. So that’s way we put SDL_Event outside of “int main”.

//declare initial condition SDL_Surface* sprites = NULL; SDL_Surface* background = NULL; SDL_Surface* screen = NULL; //construct event in here SDL_Event event;

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Function for loading images

On our explanation above we must be able to loading many files by using one command. And for that purpose we must make a function for loading images. Remember, we will try to loading image with the non bmp extensions. So using IMG_Load from SDL_image is a good idea. But first please give NULL for SDL_Surface for images. Images need a free place before come to your game. SDL_DisplayFormat is using to takes a surface and copies it to a new surface of the pixel format and colors of the video framebuffer, suitable for fast blitzing onto the display surface. Without this, you will not be able to show the picture of game. Because in here, SDL_Surface is only takes a place for your images in computer memory. Keep in your main, after using SDL_Surface follow it by SDL_Display Format. SDL_FreeSurface is aimed to free the resources used by a previously created SDL_Surface. Basically, animation in a game is draw image several times. If you don’t clear previous image then old image will still on windows application during the games.

//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 ); } //Return the optimized image return optimizedImage; }

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Function for draw image

Imagine if you have so many files to draw and then you must write it one by one. We don’t want to do that. And we need to make a new function for draw image once. See support files from us for this tutorial. If you don’t have it please go to Loading Image From Another Extention or download support files here. In sprites.png you will see so many photos. Every photo locates on the different pixels. Let say, you want to choose a tank on the upper left corner with the size width = 35 px and height = 25 px, you can use Paint application in operating windows. Then it photos define by SDL_Rect as a source.  After you choose that tank, you place on you windows everywhere. Of course depend on you. Let say, we define before that our game has width = 320 px and height = 240 px. And you want to place the source in coordinate x = 100 px and y = 100 px. This source on game windows application named as destination by SDL_Rect. The next step is blit it as fast as possible by using SDL_BlitSurface. Remember try to always follow SDL_Rect with SDL_BlitSurface.

//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); }

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Function for initial of SDL and function for check files

Basically, for initial is same with previous, the different is we using a Boolean. Also it happens for check files. If you are not giving specified address for your images. So you must, put the image inside of game project folder in your computer. If you don’t do it that your computer will give information that your program is not true.

//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 SDL_Surface* 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; } //If everything loaded fine return true; }

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Function for clean up your memory.

This also same with the previous post. The different here is we using SDL_Quit, and not using atexit. Because atexit is not always success for advance programs.

//clean all memory void clean_up() { //Free the images SDL_FreeSurface( sprites ); SDL_FreeSurface( background ); //Quit SDL SDL_Quit(); }

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Inside “int main”

The different between this script than before on our post is we are using classes.

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; } // 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; } } // end switch } // end of message processing // DRAWING STARTS HERE // 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 ); // 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; }

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Event

SDL_Event has two uses:
• Reading events
• Placing events
There are 14 event structure for SDL_Event: type, active, key, motion, button, jaxis, jball, jhat, jbutton, resize, expose, quit, user, and syswm. Reading events from the event queue is done with either SDL_PollEvent or SDL_PeepEvents. But it is still different, SDL_PollEvent aimed for poll pending events. But SDL_PeepEvents is for checks the event queue for messages and optionally returns them. So please select what you need. Because SDL_Event contains 14 structure data then when we want to handle those, we must using logic process. If there are no events then SDL_PollEvents will return 0. Type structure data is define as the type of event. There are 13 type of event: SDL_ACTIVEEVENT, SDL_KEYUP/DOWN, SDL_MOUSEMOTION, SDL_MOUSEBUTTONDOWN/UP, SDL_JOYAXISMOTION, SDL_JOYBALLMOTION, SDL_JOYBUTTONDOWN/UP, SDL_QUIT, SDL_SYSWMEVENT, SDL_VIDEORESIZE, SDL_VIDEOEXPOSE, SDL_USEREVENT. In this tutorial, we using SDL_QUIT (for quit a game) and SDL_KEYDOWN (for detect key button which pressed).

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Next step

Basically, the next step is same with previous post. Please run and save your project. If you success run your project then the image will same with the first image of this posting. We hope you understand. Thank you. Please download the source code for this tutorial.