Example of image loading to pixels and display using SDL

image2pixelsSDL.cpp

/*
    Example of image loading to pixels and display using SDL
    
    The program is a minimal implementation of image loading 
    using SDL and display it on a window. It creates first a
    surface and retrieves the pixels from the last. Then it
    updates the texture using the pixels and area of the image.
    
*/


#include <stdlib.h>
#include <iostream>
#include <memory>

#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>

int main() {
    
    // variables
    SDL_Renderer *renderer = nullptr;
    SDL_Texture *texture = nullptr;
    SDL_Window *window = nullptr;
    SDL_Surface *surface = nullptr;
    SDL_Event event;
    
    // init sdl
    SDL_Init(SDL_INIT_EVERYTHING);
    
    IMG_Init(IMG_INIT_PNG);
    
    // create window
    SDL_CreateWindowAndRenderer( 500, 500, 0, &window, &renderer);
    
    // load custom image
    surface = IMG_Load("dino.png");
    
    // the format of the surface should be the same as the pixels and the textures
    // here the format is Uint32, so for simplicity, I hard coded this format hereafter.
    // but this is not a general case. Although there is an example code to check the format
    // automatically according to SDL wiki where we can check the read format and set the 
    // following format accordingly.
    // an example can be found here: https://discourse.libsdl.org/t/reading-a-bunch-of-pixelss-alpha-from-a-png-file-using-sdl-image-opengl-sdl-linux/14506/2
    std::cout << "DEBUG1: " << (int) surface->format->BytesPerPixel << std::endl;
    
    // get the pixels from surface
    // the pixels contain the whole image, we can choose part of it to display after
    Uint32* pixels = (Uint32*)(surface->pixels);
    Uint32* bgPixels[surface->w*surface->h];
    memset(bgPixels, 0, surface->w*surface->h*sizeof(Uint32));
    
    // for convience I create the texture from the surface, 
    // but I reset it by black color after
    texture = SDL_CreateTextureFromSurface(renderer, surface);
    // reset background color to bblack
    SDL_UpdateTexture(texture, NULL, bgPixels, 425*sizeof(Uint32));
    
    // the area to choose on the image to display
    SDL_Rect rect;
    rect.w = 80; // to be changed accordingly
    rect.h = 100;
    
    // update the texture (back as I set it before) area set by the rect 
    // by the corresponding pixels
    // Be careful that the pitch value represents the length of the row of the loaded image
    SDL_UpdateTexture(texture, &rect, pixels, 425*sizeof(Uint32));
    
    // if no pixels infos were involved in the maniplation, we can just load the image directly
    // to the texture without all above steps.
    //texture = IMG_LoadTexture(renderer, "dino.png");
    
    while (1) {
        SDL_RenderCopy(renderer, texture, NULL,NULL);
        SDL_RenderPresent(renderer);
        
        if (SDL_PollEvent(&event) && event.type == SDL_QUIT)
            break;
    }

    SDL_FreeSurface(surface);
    SDL_DestroyTexture(texture);
    IMG_Quit();
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}