SDL/examples/renderer/17-read-pixels/read-pixels.c

/*
 * 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 <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;
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, &center, 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. */
}