Fit To Screen Implementation in SDL2 and C

fixing_fit.c

#include <stdio.h>
#include <stdlib.h>
#include <SDL2/SDL.h>
#include <stdint.h>
#include <stddef.h>

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

#define STB_IMAGE_RESIZE_IMPLEMENTATION
#include "stb_image_resize.h"

void stb_fix_colors(uint32_t *img, int width, int height) {
    for (int i = 0; i < width; i++) {
        for (int j = 0; j < height; j++) {
            uint8_t *rgba = (uint8_t *)&img[i + j * width];
            uint8_t new[4];
            new[3] = 0xff;
            new[2] = rgba[0];
            new[1] = rgba[1];
            new[0] = rgba[2];
            uint32_t new_color = *(uint32_t*)&new;
            img[i + j * width] = new_color;
        }
    }
}

double fit_to_screen_scale_factor(double width, double height, double target_width, double target_height) {
    double factor_x = target_width / width;
    double factor_y = target_height / height;

    double ret = factor_x > factor_y ? factor_y : factor_x;
    return ret;
}

int main(void) {
    SDL_Init(SDL_INIT_VIDEO);

    int width, height, channels;

    double new_x, new_y;

    uint32_t *img = (uint32_t *)stbi_load("2560x1440.png", &width, &height, &channels, 4);

    double factor = fit_to_screen_scale_factor(width, height, 1280, 800);

    new_x = width * factor;
    new_y = height * factor;

    uint32_t *new_img = malloc((int)new_x * (int)new_y * 4);

    if (!img || !new_img) return -1;

    stb_fix_colors(img, width, height);
    stbir_resize_uint8((void*)img, width, height, 0, (void*)new_img, (int)new_x, (int)new_y, 0, 4);

    SDL_Window * window = SDL_CreateWindow("Fixing Fit",
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        1280, 800,
        0);

    while (1) {
        SDL_Event event = {0};
        SDL_Surface *window_surface = SDL_GetWindowSurface(window);
        uint32_t *fb = window_surface->pixels;

        uint32_t res_x, res_y;
        res_x = window_surface->w;
        res_y = window_surface->h;

        {
            uint32_t offset_x = (res_x / 2) - ((int)new_x / 2);
            uint32_t offset_y = (res_y / 2) - ((int)new_y / 2);

            memset(fb, 0, res_x * res_y * 4);

            for (int j = 0; j < new_y; j++) {
                 memcpy(&fb[offset_x + (j + offset_y) * res_x], &new_img[j * (int)new_x], (int)new_x * 4);
            }
        }

        SDL_UpdateWindowSurface(window);

        while (SDL_PollEvent(&event)) {
            if (event.type == SDL_QUIT) exit(0);
        }
    }
}