SDL Examples
This example creates an SDL window and renderer, and uses SDL_RenderTextureAffine to draw a 3D cube using only 2D rendering operations.
/* affine-textures.c ... */

/* affine-textures.c ... */

/*
* This example creates an SDL window and renderer, and then draws a cube
* using affine-transformed textures every frame.
*
* 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 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;

    SDL_SetAppMetadata("Example Renderer Affine Textures", "1.0", "com.example.renderer-affine-textures");

    if (!SDL_Init(SDL_INIT_VIDEO)) {
        SDL_Log("Couldn't initialize SDL: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    if (!SDL_CreateWindowAndRenderer("examples/renderer/affine-textures", WINDOW_WIDTH, WINDOW_HEIGHT, 0, &window, &renderer)) {
        SDL_Log("Couldn't create window/renderer: %s", SDL_GetError());
        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_Log("Couldn't load bitmap: %s", SDL_GetError());
        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_Log("Couldn't create static texture: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    SDL_DestroySurface(surface);  /* done with this, the texture has a copy of the pixels now. */

    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 float x0 = 0.5f * WINDOW_WIDTH;
    const float y0 = 0.5f * WINDOW_HEIGHT;
    const float px = SDL_min(WINDOW_WIDTH, WINDOW_HEIGHT) / SDL_sqrtf(3.0f);

    const Uint64 now = SDL_GetTicks();
    const float rad = (((float) ((int) (now % 2000))) / 2000.0f) * SDL_PI_F * 2;
    const float cos = SDL_cosf(rad);
    const float sin = SDL_sinf(rad);
    const float k[3] = { 3.0f / SDL_sqrtf(50.0f), 4.0f / SDL_sqrtf(50.0f), 5.0f / SDL_sqrtf(50.0f)};
    float mat[9] = { 
         cos      + (1.0f-cos)*k[0]*k[0], -sin*k[2] + (1.0f-cos)*k[0]*k[1],  sin*k[1] + (1.0f-cos)*k[0]*k[2], 
         sin*k[2] + (1.0f-cos)*k[0]*k[1],  cos      + (1.0f-cos)*k[1]*k[1], -sin*k[0] + (1.0f-cos)*k[1]*k[2], 
        -sin*k[1] + (1.0f-cos)*k[0]*k[2],  sin*k[0] + (1.0f-cos)*k[1]*k[2],  cos      + (1.0f-cos)*k[2]*k[2],
    };

    float corners[16];
    int i;

    for (i = 0; i < 8; i++) {
        const float x = (i & 1) ? -0.5f : 0.5f;
        const float y = (i & 2) ? -0.5f : 0.5f;
        const float z = (i & 4) ? -0.5f : 0.5f;
        corners[0 + 2*i] = mat[0]*x + mat[1]*y + mat[2]*z;
        corners[1 + 2*i] = mat[3]*x + mat[4]*y + mat[5]*z;
    }

    SDL_SetRenderDrawColor(renderer, 0x42, 0x87, 0xf5, SDL_ALPHA_OPAQUE);  // light blue background.
    SDL_RenderClear(renderer);

    for (i = 1; i < 7; i++) {
        const int dir = 3 & ((i & 4) ? ~i : i);
        const int odd = (i & 1) ^ ((i & 2) >> 1) ^ ((i & 4) >> 2);
        if (0 < (odd ? 1.0f : -1.0f) * mat[5 + dir]) continue;
        int origin_index = (1 << ((dir - 1) % 3));
        int right_index = (1 << ((dir + odd) % 3)) | origin_index;
        int down_index = (1 << ((dir + (odd^1)) % 3)) | origin_index;
        if (!odd) {
            origin_index ^= 7;
            right_index ^= 7;
            down_index ^= 7;
        }
        SDL_FPoint origin, right, down;
        origin.x = x0 + px*corners[0 + 2*origin_index];
        origin.y = y0 + px*corners[1 + 2*origin_index];
        right.x  = x0 + px*corners[0 + 2*right_index];
        right.y  = y0 + px*corners[1 + 2*right_index];
        down.x   = x0 + px*corners[0 + 2*down_index];
        down.y   = y0 + px*corners[1 + 2*down_index];
        SDL_RenderTextureAffine(renderer, texture, NULL, &origin, &right, &down);
    }

    SDL_RenderPresent(renderer);

    return SDL_APP_CONTINUE;
}

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