SDL/examples/renderer/clear/renderer-clear.c

/*
 * This example code creates an SDL window and renderer, and then clears the
 * window to a different color every frame, so you'll effectively get a window
 * that's smoothly fading between colors.
 *
 * This code is public domain. Feel free to use it for any purpose!
 */

#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>


/* We will use this renderer to draw into this window every frame. */
static SDL_Window *window = NULL;
static SDL_Renderer *renderer = NULL;

/* the current red color we're clearing to. */
static Uint8 red = 0;

/* When fading up, this is 1, when fading down, it's -1. */
static int fade_direction = 1;


/* This function runs once at startup. */
int SDL_AppInit(void **appstate, int argc, char *argv[])
{
    if (SDL_CreateWindowAndRenderer("examples/renderer/clear", 640, 480, 0, &window, &renderer) == -1) {
        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create window/renderer!", SDL_GetError(), NULL);
        return SDL_APP_FAILURE;
    }
    SDL_SetRenderVSync(renderer, 1);  /* try to show frames at the monitor refresh rate. */
    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
int SDL_AppEvent(void *appstate, const SDL_Event *event)
{
    if (event->type == SDL_EVENT_QUIT) {
        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
    }
    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once per frame, and is the heart of the program. */
int SDL_AppIterate(void *appstate)
{
    /* since we're always fading red, we leave green and blue at zero.
       alpha doesn't mean much here, so leave it at full (255, no transparency). */
    SDL_SetRenderDrawColor(renderer, red, 0, 0, 255);

    /* clear the window to the draw color. */
    SDL_RenderClear(renderer);

    /* put the newly-cleared rendering on the screen. */
    SDL_RenderPresent(renderer);

    /* update the color for the next frame we will draw. */
    if (fade_direction > 0) {
        if (red == 255) {
            fade_direction = -1;
        } else {
            red++;
        }
    } else if (fade_direction < 0) {
        if (red == 0) {
            fade_direction = 1;
        } else {
            red--;
        }
    }
    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once at shutdown. */
void SDL_AppQuit(void *appstate)
{
    /* SDL will clean up the window/renderer for us. */
}