Free Direct

Free Direct
Website	https://freedirect.openeggbert.com
Programming language	C++
C++ standard	C++20
Reimplements	Subset of DirectX 3 (2D)
Built on	SDL 3
Licence	MIT

Free Direct is a C++ project that reimplements a narrow, game-driven subset of DirectX 3 (2D) using SDL 3 as an internal backend. The goal is not full DirectX compatibility, but a focused, minimal implementation sufficient to run Speedy Blupi (Epsitec) on modern platforms without Windows.

Architecture

DirectX 3 (subset)
      ↓
Free Direct
      ↓
SDL 3

Implemented components

Component	Status
DirectDraw	Implemented (narrow subset for Speedy Blupi)
DirectSound	Partially implemented (SDL 3 audio backend)
DirectPlay	Stubbed (dummy implementations)
Direct3D	Not implemented (not used by target game)

DirectDraw features: - Surface creation for primary and system-memory/offscreen surfaces - Blt / BltFast with clipping and source color key handling - Lock / Unlock for direct pixel access - Palette support (CreatePalette, SetEntries, GetEntries, SetPalette) - Primary surface presentation through SDL renderer - 8-bit rendering with palette conversion on present

DirectSound features: - SDL 3-backed audio playback for static PCM buffers - Supported formats: 8-bit unsigned and 16-bit signed LE, mono/stereo, 11025/22050/44100 Hz - Play(), Stop(), GetStatus(), SetVolume(), SetPan()

Presentation model

Blt / BltFast write to CPU pixel buffers and mark the primary surface dirty
Flip or Blt-to-primary triggers presentation: throttle check → dirty check → SDL_Texture upload → SDL_RenderPresent
VSync is enabled by default via SDL_SetRenderVSync
Target FPS configurable via FREE_DIRECT_TARGET_FPS=<n>

Debug options

Environment variable	Effect
`FREE_DIRECT_DEBUG_DDRAW=1`	Log DirectDraw calls
`FREE_DIRECT_DEBUG_DSOUND=1`	Log DirectSound calls
`FREE_DIRECT_DEBUG_COLORKEY=1`	Log color key diagnostics
`FREE_DIRECT_DEBUG_PERF=1`	Log performance counters (presents/s, blts/s)

Build

cmake -B build
cmake --build build
./build/FREE_DIRECT