/* * This example creates an SDL window and renderer, and draws a * rotating texture to it, reads back the rendered pixels, converts them to * black and white, and then draws the converted image to a corner of the * screen. * * This isn't necessarily an efficient thing to do--in real life one might * want to do this sort of thing with a render target--but it's just a visual * example of how to use SDL_RenderReadPixels(). * * 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 #include /* We will use this renderer to draw into this window every frame. */ static SDL_Window *window = NULL; static SDL_Renderer *renderer = NULL; static SDL_Texture *texture = NULL; static SDL_Texture *converted_texture = NULL; static int texture_width = 0; static int texture_height = 0; #define WINDOW_WIDTH 640 #define WINDOW_HEIGHT 480 /* This function runs once at startup. */ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[]) { SDL_Surface *surface = NULL; char *bmp_path = NULL; if (!SDL_Init(SDL_INIT_VIDEO)) { SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't initialize SDL!", SDL_GetError(), NULL); return SDL_APP_FAILURE; } if (!SDL_CreateWindowAndRenderer("examples/renderer/read-pixels", WINDOW_WIDTH, WINDOW_HEIGHT, 0, &window, &renderer)) { SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create window/renderer!", SDL_GetError(), NULL); return SDL_APP_FAILURE; } /* Textures are pixel data that we upload to the video hardware for fast drawing. Lots of 2D engines refer to these as "sprites." We'll do a static texture (upload once, draw many times) with data from a bitmap file. */ /* SDL_Surface is pixel data the CPU can access. SDL_Texture is pixel data the GPU can access. Load a .bmp into a surface, move it to a texture from there. */ SDL_asprintf(&bmp_path, "%ssample.bmp", SDL_GetBasePath()); /* allocate a string of the full file path */ surface = SDL_LoadBMP(bmp_path); if (!surface) { SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't load bitmap!", SDL_GetError(), NULL); return SDL_APP_FAILURE; } SDL_free(bmp_path); /* done with this, the file is loaded. */ texture_width = surface->w; texture_height = surface->h; texture = SDL_CreateTextureFromSurface(renderer, surface); if (!texture) { SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create static texture!", SDL_GetError(), NULL); return SDL_APP_FAILURE; } SDL_DestroySurface(surface); /* done with this, the texture has a copy of the pixels now. */ converted_texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_STREAMING, WINDOW_WIDTH, WINDOW_HEIGHT); if (!texture) { SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create conversion texture!", SDL_GetError(), NULL); return SDL_APP_FAILURE; } return SDL_APP_CONTINUE; /* carry on with the program! */ } /* This function runs when a new event (mouse input, keypresses, etc) occurs. */ SDL_AppResult SDL_AppEvent(void *appstate, 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. */ SDL_AppResult SDL_AppIterate(void *appstate) { const Uint64 now = SDL_GetTicks(); SDL_Surface *surface; SDL_FPoint center; SDL_FRect dst_rect; /* we'll have a texture rotate around over 2 seconds (2000 milliseconds). 360 degrees in a circle! */ const float rotation = (((float) ((int) (now % 2000))) / 2000.0f) * 360.0f; /* as you can see from this, rendering draws over whatever was drawn before it. */ SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255); /* black, full alpha */ SDL_RenderClear(renderer); /* start with a blank canvas. */ /* Center this one, and draw it with some rotation so it spins! */ dst_rect.x = ((float) (WINDOW_WIDTH - texture_width)) / 2.0f; dst_rect.y = ((float) (WINDOW_HEIGHT - texture_height)) / 2.0f; dst_rect.w = (float) texture_width; dst_rect.h = (float) texture_height; /* rotate it around the center of the texture; you can rotate it from a different point, too! */ center.x = texture_width / 2.0f; center.y = texture_height / 2.0f; SDL_RenderTextureRotated(renderer, texture, NULL, &dst_rect, rotation, ¢er, SDL_FLIP_NONE); /* this whole thing is _super_ expensive. Seriously, don't do this in real life. */ surface = SDL_RenderReadPixels(renderer, NULL); if (surface && (surface->format != SDL_PIXELFORMAT_RGBA8888) && (surface->format != SDL_PIXELFORMAT_BGRA8888)) { SDL_Surface *converted = SDL_ConvertSurface(surface, SDL_PIXELFORMAT_RGBA8888); SDL_DestroySurface(surface); surface = converted; } if (surface) { /* Turn each pixel into either black or white. This is a lousy technique but it works here. In real life, something like Floyd-Steinberg dithering might work better: https://en.wikipedia.org/wiki/Floyd%E2%80%93Steinberg_dithering*/ int x, y; for (y = 0; y < surface->h; y++) { Uint32 *pixels = (Uint32 *) (((Uint8 *) surface->pixels) + (y * surface->pitch)); for (x = 0; x < surface->w; x++) { Uint8 *p = (Uint8 *) (&pixels[x]); const Uint32 average = (((Uint32) p[1]) + ((Uint32) p[2]) + ((Uint32) p[3])) / 3; if (average == 0) { p[0] = 0; // turn off alpha for pure black pixels. } p[1] = p[2] = p[3] = (average > 50) ? 0xFF : 0x00; } } /* upload the processed pixels back into a texture. */ SDL_UpdateTexture(converted_texture, NULL, surface->pixels, surface->pitch); SDL_DestroySurface(surface); /* draw the texture to the top-left of the screen. */ dst_rect.x = dst_rect.y = 0.0f; dst_rect.w = ((float) WINDOW_WIDTH) / 2.0f; dst_rect.h = ((float) WINDOW_WIDTH) / 2.0f; SDL_RenderTexture(renderer, converted_texture, NULL, &dst_rect); } SDL_RenderPresent(renderer); /* put it all on the screen! */ return SDL_APP_CONTINUE; /* carry on with the program! */ } /* This function runs once at shutdown. */ void SDL_AppQuit(void *appstate) { SDL_DestroyTexture(converted_texture); SDL_DestroyTexture(texture); /* SDL will clean up the window/renderer for us. */ }