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The past was an exciting year for computer graphics. Nvidia announced RTX graphics cards which brings real-time ray tracing to consumers. Following the announcement, we saw new releases of mainstream game series including Battlefield V and Shadow of the Tomb Raider, putting RTX powered graphics in their games. However, the recent new game releases branded with RTX graphics mostly use RTX for tracing reflections and shadows including soft shadow , many other possibilities with real-time ray tracing are still to be explored.
Of course, ray traced reflections and shadows can largely enhance the overall graphics quality, considering the importance of these two components and how bad their quality was even using very complex rasterization tricks.
But there are still tons of possible applications of real-time ray tracing that can lift the overall graphics quality to a new level. For example, we can achieve more faithful subsurface scattering in translucent objects like marbles and human skin.
Current technique use shadow maps to estimate the distanced traveled by light inside the object, which can fall short with concave objects and have artefacts around object edges. But ray tracing simply avoids all artefacts brought by rasterization tricks and everything will appear as they should be. This technique is known as instant radiosity, introduced by Alexander Keller in It resembles bidirectional path tracing and photon mapping in the sense that it also traces light paths and records hit positions along the paths.
These surface records are then used as point lights that represent discretized indirect lighting. Comparing to photon mapping, instant radiosity is cheap and effective to render smooth diffuse reflection, thanks to the low frequency spatial radiance distribution of point lights. A few hundreds of VPLs are enough to provide indirect lighting from a small light source with reasonable quality of course, there is singularity problem caused by the point light approximation, but that can be bypassed by setting a minimum distance between point lights and surface points , which often requires more than 1M photons when using photon mapping to eliminate the wavy artefact.
As a result, virtual point lights have been used in games for flashlights. An example is the rendering of indirect illumination in rooms from gun mounted flashlights in Gears of War 4. This technique gives real-time single bounce GI likely unshadowed. The following screenshots from the talk video a comparison between flashlight aiming at the red tapestry and the wall.
Clearly, the technique generates reasonable color bleeding as shown by the change of color on the ceiling. Looking carefully into the talk video, there are some temporal incoherence as the flashlight moves. However it is not obvious in a dark environment like this, especially when using a moving FPS game character. But the render quality can still be improved if the virtual point lights can be generated and evaluated at a lower cost. First, using more VPLs improves the temporal stability and reduces the bright blotches.
There are several options to get access to the ray tracing function in RTX cards. In all shaders, calling TraceRay executes the fixed function hardware scene traversal using an acceleration structure BVH. Because can a ray can hit any object, a shader table is used to store shading resources for each geometry object. Upon intersection, the entry for the current object can be retrieved from the shader table to determine which shaders and textures to use.
With these functions, we can virtually implement all possible ray tracing functions with DXR, except choosing and our own acceleration structure for example, k-d tree. Here I provide a brief code walk through of using DXR to implement the original brute force instant radiosity algorithm. Notice that it is not meant to be interactive as the original instant radiosity is an offline rendering algorithm that goes through all virtual point lights for each pixel, and shooting shadow rays to resolve the visibility.
In our experiment we will generate 1 million VPLs for at most two-bounce indirect diffuse reflection and render a x instant radiosity image. This means we need to trace My DXR program running on an RTX takes 25 minutes to render a instant radiosity image for Crytek Sponza k triangles , about mega rays per second which is quite impressive. Please notice that I a very unoptimzed way of tracing shadow rays trace shadow rays for one light for all pixels in each frame with which contains a fixed G-buffer overhead.
With proper optimization, the speed should at least reach 1 Giga rays per second. The VPL generation process is relatively fast, taking less than 2 ms. Here is the raygen shader I used to generate the VPLs. To sample light rays from a directional distant light sun light , I used a function like this to randomly sample a point from the top disk of the bounding cylinder of the scene bounding sphere aligned to the sun light direction a technique introduced in PBRT.
This point is then used as the origin of the light ray, which is then traced through the scene and make diffuse bounces when intersecting with surfaces. DXR requires us to specify the ray using a ray description RayDesc structure that contains the origin, tMin min parametric intersection distance along the ray , ray direction and tMax.
It is multiplied with the surface albedo and the cosine factor during each surface hit. The payload also stores the current recursion depth. This payload feeds the last argument of TraceRay, which can pass the information from ray generation to hit shader and between bounces.
An atomic counter is incremented and returned each time a new VPL is created to prevent writing to the same position. Following that, a diffuse reflection ray is sampled from the hemisphere centered at the surface normal to generate next bounce. Notice how DXR provides a wide range of fixed functions and variables that stores necessary ray tracing information we need for lighting computation.
Finally, we can gather the VPL contribution for all visible pixels to generate an irradiance buffer. Notice that texPosition and texNormal are surface world position and normal from the G buffer.
A shadow ray is traced to resolve the visibility between current pixel and VPL, and the max ray T is set to the light distance which means any hit is an occlusion.
This message can be stored in the ray payload as a boolean. After we iterate through all VPLs, we can modulate the irradiance buffer with the surface albedo to produce the final indirect illumination and add that to the direct illumination. And here is the final image. But none of them can reach real time performance yet. Dachsbacher, C. Reflective shadow maps. In Proceedings of the symposium on Interactive 3D graphics and games pp.
Malmros, J. Gears of War 4: custom high-end graphics features and performance techniques. Toggle navigation Daqi's Blog. Home Publications Music Photo About. Example Implementation Here I provide a brief code walk through of using DXR to implement the original brute force instant radiosity algorithm.
Sample defaultSampler, uv. SampleLevel defaultSampler, uv, 0. IsOccluded to translucent
Illumination in the Presence of Weak Singularities
Approximating illumination by point light sources, as done in many professional applications, allows for efficient algorithms, but suffers from the problem of the weak singularity: Besides avoiding numerical exceptions caused by the division by the squared distance between the point light source and the point to be illuminated, the estimator should be unbiased and of finite variance. We first illustrate that the common practice of clipping weak singularities to a reasonable value yields clearly visible bias. Then we present a new global illumination algorithm that is unbiased and as simple as a path tracer, but elegantly avoids the problem of the weak singularity. In order to demonstrate its performance, the algorithm has been integrated in an interactive global illumination system. Unable to display preview.
Using RTX to Accelerate Instant Radiosity