Okay, I've analyzed the app.lua file. Here's a summary of its functionality:

This Lua script uses sdl3_ffi (SDL3 bindings for LuaJIT FFI) to create a window and render graphics. It demonstrates two rendering approaches: using the SDL Renderer API or direct surface blitting, controlled by the UseRenderer boolean variable.

Key Features:

1. SDL3 Initialization and Window Creation:

   • Initializes SDL video subsystem (SDL_Init(SDL_INIT_VIDEO)).
   • Creates a 512x512 window titled "Hello Lena" (SDL_CreateWindow).
   • Makes the window resizable (SDL_SetWindowResizable).

2. Global SDL Function Access:

   • It modifies the global environment _G with a metatable. This allows calling SDL functions like SDL_Init directly, instead of SDL.Init, by automatically looking them up in the SDL table if they start with "SDL_".

3. Conditional Rendering Path (UseRenderer):

   • If UseRenderer is true (default):
     • An "software" SDL renderer is created (SDL_CreateRenderer).
     • Blend mode is set to SDL_BLENDMODE_BLEND.
     • Images are loaded as BMP surfaces (SDL_LoadBMP) and then converted to SDL textures (SDL_CreateTextureFromSurface).
     • Drawing uses renderer functions (SDL_RenderTexture, SDL_RenderFillRect).
   • If UseRenderer is false:
     • Images are loaded and used directly as SDL surfaces.
     • Drawing uses surface blitting functions (SDL_BlitSurfaceScaled, and a custom FillRect implementation using temporary surfaces for alpha blending).

4. Resource Loading and Management:

   • Loads "assets/lena.bmp" and "assets/alpha-blend.bmp".
   • The Image table dynamically points to either the Texture table or the Surface table based on UseRenderer.
   • RectangleFromXYWH is a helper function to create SDL_FRect (for renderer) or SDL_Rect (for surface) structures from a Lua table {x, y, w, h}.

5. Drawing Functions:

   • Render(): This function orchestrates the drawing of several images (Lena and a transparent BMP at different positions and scales) and a semi-transparent filled rectangle.
   • DrawImage(): Abstracts the image drawing logic for both renderer and surface paths.
   • FillRect(): Abstracts the rectangle filling logic. For the surface path, it manually creates a temporary surface, fills it with the specified color and alpha, and then blits it to the window surface to achieve alpha blending.

6. Main Loop:

   • Handles events: SDL_EVENT_QUIT (window close) and SDL_EVENT_KEY_DOWN (Escape or Q key to quit).
   • Clears the screen to a gray color.
   • Calls the Render() function to draw the scene.
   • Presents the rendered frame (SDL_RenderPresent for renderer) or updates the window surface (SDL_UpdateWindowSurface for surface blitting).

7. Cleanup:

   • Properly destroys all loaded surfaces.
   • If the renderer was used, it destroys all created textures and the renderer itself.
   • Destroys the window.
   • Quits SDL (SDL_Quit()).

In essence, the script is a demonstration of basic 2D rendering with SDL3 in Lua, showcasing how to handle images and primitive shapes with and without the SDL Renderer API, and managing resources correctly. The Render() function specifically draws multiple instances of "Lena" and a transparent BMP, along with a filled rectangle.