Amietia • Quake 2 Realtime GPU Pathtracing

May 2016

Quake 2 Realtime GPU Pathtracing

This is a from-scratch GPU-based pathtracer created specifically for Quake 2. It has several optimisations which are only viable due to the typical characteristics of Quake 2 such as support for parallogram-shaped lightsources, BSP ray traversal, and special handling of sky 'surfaces' (portals). It doesn't officially have a name, but you can call it Raylgun.

Supports all static and dynamic light types that were used in the original game, for which illumination was originally baked into static lightmaps.
All objects can cast shadows.
In terms of graphics API support, only OpenGL 3.3 is required.
Requires no offline pre-processing of input data, so any mod or level which works with Yamagi Quake 2 (which is a no-frills port) should work.
Still uses the rasterisation capabilities of the GPU anywhere that it makes sense.
All code modifications reside within the refresher (Quake 2's rendering subsystem) and the pathtracing codepath can be averted entirely at runtime without qualitative detriment to the original renderer's results.

Sam Lapere (of Ray Tracey fame) made a great post putting together a lot of my answers to his questions about this project, and also provided some useful tips on getting it running yourself. Sam is quite passionate about raytracing in videogames, be sure to check out his other posts and past projects.

Here are some average framerate measurements as measured by Quake 2 itself along with the graphics hardware that was used:

AMD FirePro W8100	49.3 fps
NVIDIA Quadro K6000	32.8 fps
GeForce GTX 960	31.1 fps
AMD FirePro W7100	29.3 fps
GeForce GTX 680	22.1 fps

Links

YouTube Video 1
YouTube Video 2
YouTube Video 3
GitHub Page

Console Commands

gl_pt_recompile_shaders	Forces the shader programs to be recompiled. This is usually necessary after altering some aspect of the pathtracer via the console because some parameters are inserted into the shader source code itself.
gl_pt_print_static_info	Prints some pathtracing-related information about the current map. This information only pertains to things which are processed at map loading time.

Console Variables

Name	Purpose	Requires vid_restart	Requires gl_pt_recompile_shaders
gl_pt_enable	Enables the pathtracing codepath. When this is set to zero, the original Quake 2 rendering is used and all other pathtracer settings are ignored.	No	No
gl_pt_stats_enable	Enables a continuous display of per-frame information. This is useful for finding out how well the pathtracer is performing.	No	No
gl_pt_bounces	The number of secondary bounces of light. A value of 0 means all illumination is direct. Note: The sky is not sampled by secondary bounces.	No	Yes
gl_pt_shadow_samples	The number of shadow rays to cast towards an individual lightsource. A higher number of shadow samples means less noise in shadows. Zero shadow samples means no shadows at all.	No	Yes
gl_pt_light_samples	The number of light samples to make. A higher number of light samples means less noise overall, but the shader must re-compute importance sampling weights for each light sample (since storing them in registers is prohibitively slow). Zero light samples means no light from explicitly-sampled lightsources.	No	Yes
gl_pt_sky_enable	Enables light emission from the sky. If this is disabled then the sky itself is still drawn, it just won't emit any light.	No	Yes
gl_pt_sky_samples	The number of BRDF-sampled rays to trace and test against skyportals.	No	Yes
gl_pt_ao_enable	Enables ambient occlusion	No	Yes
gl_pt_ao_radius	The ambient occlusion radius.	No	No
gl_pt_ao_color	The color of the ambient occlusion contribution. Note: This is a scalar value because Quake 2's console doesn't allow vector-valued variables.	No	No
gl_pt_ao_samples	The number of BRDF-sampled rays to trace for ambient occlusion.	No	Yes
gl_pt_translucent_surfaces_enable	Currently not used, but will be used to enable the application of pathtracing to translucent surfaces.	No	No
gl_pt_lightstyles_enable	Enables the use of lightstyles (animated colour and intensity of static entity lightsources).	No	No
gl_pt_dlights_enable	Enables the use of dlights (dynamic lightsources created at runtime by entities).	No	No
gl_pt_brushmodel_shadows_enable	Enables the casting of shadows by brushmodels (entities with rigid polygonal geometry defined by the BSP).	No	Yes
gl_pt_aliasmodel_shadows_enable	Enables the casting of shadows by aliasmodels (entities with deforming triangle mesh geometry defined by separate MD2 files).	No	Yes
gl_pt_bounce_factor	The scalar amount of absorption simulated at each surface-light interaction in secondary bounces. Has no effect if the number of bounces is zero.	No	No
gl_pt_diffuse_map_enable	Enables the modulation of light by the surface albedo maps (the original textures of Quake 2).	No	Yes
gl_pt_static_entity_lights_enable	Enables the creation of static entity lights when the map is loaded. These lights can be quite costly as for each one 4 triangular lightsources are created, arranged in a tetrahedron.	Yes	No
gl_pt_depth_prepass_enable	Enables a depth pre-pass which draws all entities into the depth buffer before pathtracing is performed. This eliminates overdraw (except for translucent surfaces) with minimal drawcall overhead so is recommended for improving performance.	No	No