Ray Tracing for the Movie ‘Cars’
Per H. Christensen Julian Fong David M. Laur Dana Batali
Pixar Animation Studios

编辑整理和翻译 : Happy_高兴

This paper describes how we extended Pixar’s RenderMan renderer with ray tracing abilities. In order to ray trace highly complex scenes we use multi-resolution geometry and texture caches, and use ray differentials to determine the appropriate resolution. With this method we are able to efficiently ray trace scenes with much more geometry and texture data than there is main memory. Movie quality rendering of scenes of such complexity had only previously been possible with pure scanline rendering algorithms. Adding ray tracing to the renderer enables many additional effects such as accurate reflections, detailed shadows, and ambient occlusion.
这篇论文描述了怎样使用光线追踪的功能来拓展Pixar的RenderMan 渲染。为了使得复杂的场景的光线追踪效果更好,我们使用了多种分辨率的模型和材质缓存,还使用了光线微分来决定一个适合的分辨率。使用这种方法我们可以很高效的完成对有多个模型的场景和材质数据的光线追踪。对如此复杂的场景的电影级别的渲染,在之前只有使用纯扫描线渲染算法的可能性。将光线追踪添加到渲染器中可以得到很多精确的反射,更细腻的阴影和环境OCC。
The ray tracing functionality has been used in many recent movies, including Pixar’s latest movie ‘Cars’. This paper also describes some of the practical ray tracing issues from the production of ‘Cars’.
Pixar’s RenderMan renderer (PRMan) is a robust production renderer that is used for many CG movies and special effects [1]. PRMan uses the REYES scanline rendering algorithm [4]. About five years ago, at the request of our external customers, we started a project to add on-demand ray tracing to PRMan.
At roughly the same time, John Lasseter and his team started working on ‘Cars’, a movie that would turn out to be an ideal testing ground and showcase for the ray tracing functionality. There were two main rendering challenges in making the movie. First, ‘Cars’ has scenes that are much more complex than past Pixar movies; for
example, wide desert landscapes with many sagebrush and thorn covered cacti, and a racing oval with 75,000 cars as spectators. Second, ray tracing effects such as correct reflections, shadows, and ambient occlusion were needed to get the desired artistic look. Ray tracing these very complex scenes in manageable time was quite a challenge.
同年,John Lasseter和他的团队也开始加入“汽车总动员”的电影制作中,这部电影或许是测试光线追踪功能的理想胚胎。这部电影中有两个主要的渲染挑战。首先,“汽车总动员”有比以往Pixar电影中更复杂的场景;例如,广袤的沙漠,有山艾树、仙人掌在其中,还有追踪观众席中75000个汽车模型。其次,光线追踪影响诸如正确的反射、阴影和环境OCC等等这些必要的因素来得到一个满意的视觉效果。在安排的时间里完成光线追踪复杂的场景确实是一个挑战。
The REYES algorithm is very efficient at handling complex scenes. For ray tracing, we use ray differentials [6, 14] to select the optimal tessellation level of surfaces and the proper MIP map level for textures. A multiresolution geometry cache keeps recently tessellated geometry ready for fast access. Similarly, a multiresolution texture cache keeps recently accessed texture tiles ready for fast access. This combination of ray differentials and caching makes ray tracing of very complex scenes feasible.
This paper first gives a more detailed motivation for the use of ray tracing in ‘Cars’, and lists the harsh rendering requirements in the movie industry. It then gives an overview of how the REYES algorithm deals with complex scenes and goes on to explain our work on efficient ray tracing of equally complex scenes. An explanation of our hybrid rendering approach, combining REYES with ray tracing, follows. Finally we measure the efficiency of our method on a test scene, and present a few production details from the use of ray tracing for ‘Cars’.

Please refer to Christensen et al. [3] for an overview of previous work such as the Toro [10] and Kilauea [7] renderers. Recent related work includes an interactive out-of-core renderer by Wald et al. [15], the Razor project by Stoll et al. [13], and level of detail representations for efficient ray tracing of simplified geometry [2, 18]. The focus of those projects is more on interactive or real-time rendering than on movie-quality images.
请参阅Christensen et al 对之前的渲染器Toro 和 Kilauea进行全面了解。最近相关的工作包括一个交互式的 out-of-core 渲染器,和使用光线追踪对简单模型的细腻呈现。这些工程的关注点更多的集中在了互动或者实时渲染上而不是电影级图片。


There are several reasons why the directors chose to use ray tracing for ‘Cars’: realistic reflections, sharp shadows, and ambient occlusion.
Real cars are usually shiny, and the reflections are an important visual cue to the shape and material of the car. With scanline algorithms such as REYES, reflections are usually computed using environment maps. However, this approach breaks down when the reflected points are close to the reflecting points, or if the reflected points are also reflectors. Figure 1 shows two early pre-production test images of Luigi, a yellow Fiat 500 “Topolino”. The images compare environment mapping with ray traced reflections. While the environment map reflection in figure 1(left) shows a good approximation of the distant environment, it does not capture interreflections such as the reflections of the eyes in the hood seen in figure 1(right).
Shadows give strong cues about the lighting and about the placement of objects relative to each other. Scanline algorithms traditionally compute shadows using shadow maps [11], and there are still many cases where a shadow map is the most efficient way of generating high-quality shadows. However, some of the scenes in ‘Cars’ use expansive sets but also have a lot of tiny, detailed geometry. This can lead to resolution problems in shadow maps. Furthermore, many scenes contain thousands of light sources, so keeping track of the shadow map files can
become an asset management problem. Figure 2 shows a frame from the final movie with Lightning McQueen leading a race. This is an example of a large scene requiring very fine shadow detail. The scene contains nearly 1000 light sources. Using ray traced shadows eliminates the resolution and asset management problems.
Another use of ray tracing is for ambient occlusion. Ambient occlusion [9, 19] is a measure of how much light reaches a point from a uniformly lit hemisphere. Ambient occlusion is widely used in movie production since it gives a good indication of creases on surfaces and spatial proximity of objects, and is a cheap (but crude) approximation of global illumination. Figure 3 shows ambient occlusion on three cars in Radiator Springs. Ambient occlusion is usually computed by shooting many rays to sample the coverage of the hemisphere above each point.
光线追踪的另一个使用方面是环境OCC。环境OCC是一个测量有多少盏灯从由统一的被灯光照射的半球到达一个点的。光线OCC被广泛使用在电影产品中,因为它在处理表面接缝和一个物体的空间关系方面表现非常好,而且在得到近似全局光方面非常便宜(但是非常粗糙)。图3展示了环境OCC在三辆车间的渲染。环境OCC通常是通过计算 发射多条射线来采样覆盖在每个点上的半球来得到的。
3 电影渲染的要求
The rendering requirements in the movie industry are extremely
• Scene geometry is far too large to fit in memory in tessellated form.
• Many surfaces are displacement-mapped.
• There may be thousands of textures (too many to fit all in memory at full resolution) to control reflection
parameters and displacements.
• There can be thousands of light sources.
• All illumination and surface reflection characteristics are controlled by fully programmable, complex
In addition, images are typically rendered at high resolution with motion blur and depth of field. Furthermore, no spatial or temporal aliasing is acceptable: no staircase effects, “crawlies”, popping, etc.
4 复杂场景使用REYES渲染
The REYES algorithm has many desirable properties such as coherent shader execution, coherent access to geometry and texture data, simple differential calculations, efficient displacement, fast motion blur and depth-of-field, and the ability to render very complex scenes.
The REYES algorithm divides each surface into smaller patches, and each patch is tessellated into a regular grid of tiny quadrilaterals (aka. micropolygons). The small patches are easy to place into one (or a few) image tiles. A patch can be thrown away if it is entirely behind other opaque patches.
Shading is done at the vertices of the grid. Shading an entire grid at a time is advantageous for data coherency and differential calculations as needed for e.g. texture filter sizes. The shading rate is decoupled from visibility calculations: there is typically only one shading point (grid vertex) per pixel on average, while the pixel sampling rate typically is 4×4 for static images and even higher for images with motion blur.
贴图被赋在每个网格的顶点上。在同一时间将贴图赋给整个网格是保持数据连续性的一个优势,微分计算也是必要的,比如材质过滤值等。贴图速度从可视化计算中被减弱:通常每像素上平均只有一个渲染点,像素采样率通常是4 * 4对于静态图片来说,如果加上运动模糊的话会更高。
The REYES algorithm is very good at handling complex scenes. First, it can completely ignore all objects outside the viewing frustum. Second, it renders only one small image tile (typically 16×16 or 32×32 pixels) at a time. This means that the computation only needs a small fraction of the scene geometry and textures at any
given time. This maximizes geometry coherency and minimizes the number of tessellated surfaces that need to be kept in memory at the same time. Furthermore, as soon as a surface has been rendered, its data can be removed from memory since they will no longer be needed. Surfaces are divided and tessellated according to their size
on screen, so large distant surfaces automatically get a coarse representation. Likewise, distant objects only need coarse textures, so only coarse levels in the texture MIP maps will be accessed for those objects. For all these reasons, REYES deals gracefully with very complex geometry and huge amounts of texture.
REYES算法很善于处理复杂场景。首先,它可以完全忽略所有物体外部的可视平截体。其次,它同时可以渲染16*16 或者32*32的图片像素。这意味着计算只需场景模型和材质中一个小的碎片。这个最大化的连续性模型和最小化的表面分段数需要同时被载入到内存中。此外,一旦表面被渲染了,它的数据就会从内存中被清除,因为我们已经不需要它了。物体表面根据在镜头中的大小被分割成小块。因此远处的表面会自动被处理成粗糙的展现。同样的,远处的模型只需要一个粗糙的材质,因此,只有粗糙级别的材质在材质库中才可被赋到这些远处的模型上。基于这些原因,REYES能够处理大型的复杂的场景。
5. 光线追踪处理复杂场景
Ray tracing also has several wonderful properties as a rendering algorithm: it is conceptually simple, its run-time only grows logarithmically with scene complexity, and it can easily be parallelized. But ray tracing has an important limitation: it is only efficient if the scene fits in memory. If the scene does not fit in memory, virtual memory thrashing slows the rendering down by orders of magnitude.
Ray tracing of complex scenes is inherently harder than REYES rendering of similar scenes. First, objects can’t be rejected just because they are outside the viewing frustum: they may cast shadows on visible objects or be reflected by them. Second, even if the image is rendered one tile at a time and the directly visible geometry is
ray traced very coherently, the reflection and shadow rays will access other geometry (and textures) in a less coherent manner. Even worse, the rays traced for diffuse interreflections and ambient occlusion are completely incoherent and may access any part of the scene at any time. Hence, we can’t delete an object even when the image tile it is directly visible in has been rendered — a ray from some other part of the scene may hit that object at any time during rendering.
For all the reasons listed above, ray tracing of very complex scenes may seem like a daunting task. However, the use of ray differentials and multi resolution geometry and texture caches makes it tractable.
5.1 Ray differentials
5.1 光线微积分
A ray differential describes the differences between a ray and its — real or imaginary — “neighbor” rays. Igehy’s ray differential method [6] traces single rays, but keeps track of the differentials as the rays are propagated and reflected. The differentials give an indication of the beam size that each ray represents, as illustrated in figure 4. The curvature at surface intersection points determines how the ray differentials and their associated beams change after specular reflection and refraction. For example, if a ray hits a highly curved, convex surface, the specularly reflected ray will have a large differential (representing highly diverging neighbor rays).
Suykens andWillems [14] generalized ray differentials to glossy and diffuse reflections. For distribution ray tracing of diffuse reflection or ambient occlusion, the ray differential corresponds to a fraction of the hemisphere. The more rays are traced from the same point, the smaller the subtended hemisphere fraction becomes. If the hemisphere fraction is very small, a curvature-dependent differential (as for specular reflection) becomes dominant.
Suykens 和 Willems归纳了光线微积分在镜面反射和漫反射上的作用。对于漫反射和环境OCC的光线追踪分布,光线微积分回应从半球上的小部分上反弹回来的信息。越多的光线从一个相同的点被追踪,相对的半球部分会变得更小。如果半球部分变小了,一个依靠曲率的微积分会越有支配地位。
In Christensen et al. [3] we provided a comprehensive analysis of ray differentials vs. ray coherency. We observed that in all practical cases, coherent rays have narrow beams and incoherent rays have wide beams. This is an important and very fortunate relationship that enables ray tracing of very complex scenes. We exploit that relationship in the following sections by designing caches that utilize it.
在Christensen et al.中,我们对 光线微积分VS光线连续性 做了一个全面的分析。我们观察到,在所有的实际项目中,连续的光线是非常细的光柱,不连续的光线是宽的光柱。这是一个非常重要的并且非常幸运的关联,能够使得光线追踪更加复杂的场景。在之后的制作部分中我们通过设置缓存等拓展了这个相关性。
5.2 Multi resolution tessellation
5.2 多分辨率网格布线
REYES chooses tessellation rates for a surface patch depending on viewing distance, surface curvature, and optionally also view angle. In our implementation, the highest tessellation rate used for ray tracing of a patch is the same as the REYES tessellation rate for that patch. Subsets of the vertices are used for coarser tessellations, which ensures that the bounding boxes are consistent: a (tight) bounding box of the finest tessellation is also a bounding
box for the coarser tessellations. The coarsest tessellation is simply the four corners of the patch. One can think of the various levels of tessellation as a MIP map of tessellated geometry [17]. Figure 5 shows an example of five tessellations of a surface patch; here, the finest tessellation rate is 14×11.
In our first implementation [3], the tessellation rates used for ray tracing were 16×16, 8×8, . . . , 1. However, using the REYES tessellation rates and subsets thereof has two advantages: there are fewer quads to test for ray intersection (since we always rounded the REYES tessellations rates up for ray tracing), and there are fewer self-intersection problems if we use a hybrid rendering method (since the vertices of the two representations always coincide when using the current tessellation approach).
在我们的第一步执行中,光线追踪使用的网格密度是16*16,8*8 … 然而,使用REYES网格分布密度和它的子集可以有两个优势:有很少的四边形被用来做光线交互,如果我们使用混合渲染方案,将会有很小的自反射问题出现。
The example in figure 5 is a rectangular surface patch. We have a similar multi resolution tessellation method for triangular patches which arise from triangle meshes and Loop subdivision surfaces.
5.3 Multiresolution geometry cache
5.3 多分辨率物体缓存
We tessellate surface patches on demand and cache the tessellations. As shown above, we use up to five different levels of tessellation for each surface patch. However, we have chosen not to have five geometry caches; instead we use three caches and create the two intermediate tessellations by picking (roughly) a quarter of the vertices from the next finer tessellation level.
In our implementation, the coarse cache contains tessellations with 4 vertices (1 quad), the medium cache contains tessellations with at most 25 vertices, and the fine cache contains all larger tessellations (at most 289 vertices). The size of the geometry caches can be specified by the user. By default, the size is 20MB per thread allocated for each of the three caches. Since the size of the tessellations differ so much, the maximum capacity (number of slots) of the coarse cache is much higher than for the medium cache, and the medium cache has much higher capacity than the fine cache. We use a least-recently-used (LRU) cache replacement scheme.
For ray intersection tests, we choose the tessellation where the quads are approximately the same size as the ray beam cross section. We have observed that accesses to the fine and medium caches are usually very coherent. The accesses to the coarse cache are rather incoherent, but the capacity of that cache is large and its tessellations are fast to recompute.
5.4 Multiresolution texture cache
Textures are stored on disk as tiled MIP maps with 32×32 pixels in each tile. The size of the texture cache is chosen by the user; the default size is 10 MB per thread.
As with the geometry cache, the ray beam size is used to select the appropriate texture MIP map level for texture lookups. We choose the level where the texture pixels are approximately the same size as the ray beam cross-section. Incoherent texture lookups have wide ray beams, so coarse MIP map levels will be chosen. The
finer MIP map levels will only be accessed by rays with narrow ray beams; fortunately those rays are coherent so the resulting texture cache lookups will be coherent as well.
6. 其他的光线追踪操作遇到的情况
This section describes other efficiency and accuracy aspects of our implementation of ray tracing in PRMan.
6.1 Spatial acceleration data structure
Good spatial acceleration structures are essential for efficient ray tracing. We use a bounding volume hierarchy, the Kay-Kajiya tree [8]. This data structure is a good compromise between memory overhead, construction speed, and ray traversal speed. The data structure is built dynamically during rendering. While we are quite satisfied with the performance of the Kay-Kajiya tree, it is certainly worth considering other acceleration data structures in the future.
There is one more level of bounding volumes for the finest tessellations: bounding boxes for groups of (up to) 4×4 quads. These bounding boxes are stored in the fine geometry cache along with the tessellated points.
6.2 Displacement-mapped surfaces
Displacement shaders complicate the calculation of ray intersections. If the surface has a displacement shader, the shader is evaluated at the tessellation vertices to get the displaced tessellation. The bounding box of the displaced vertices is computed, and the Kay-Kajiya tree is updated with the new bounding box.
Although there exist techniques for direct ray tracing of displaced surfaces [12], we have found that applying displacement shaders to tessellated grids gives much faster rendering times — at least if the displaced tessellations are cached.
Each object that has displacement must have a pre-specified upper bound on the displacement; such bounds are important for the efficiency of both REYES rendering and ray tracing. Without a priori bounds, any surface might end up anywhere in the scene after displacement, and this makes image tiling or building an acceleration data structure futile.
每个物体需要置换的条件是必须有一个预先设定好的置换上限。这个上限对REYES渲染和光线追踪在高效性方面都是很重要的。 没有一个适合的上限,任何表面在置换后都有可能在场景的任何地方玩完,这将导致图片不能正确渲染。
The value of tight bounding boxes is so high that even if the first access to a displaced surface patch only needs a coarse tessellation, we compute a fine tessellation, run the displacement shader, compute the tight bounding box, update the Kay-Kajiya tree, and throw away those tessellation points that aren’t needed. This is a one-time
cost that is amortized by the reduction in the number of rays that later have to be intersection-tested against that surface patch. Tight bounding boxes allow us to ray trace displaced surfaces rather efficiently.
6.3 Motion blur
6.4 运动模糊
PRMan assumes that all motion is piecewise linear. There is a tessellation for the start of each motion segment plus a tessellation for the end of the last segment. These tessellations are computed on demand and stored in the geometry cache. For ray intersection tests, the tessellated vertex positions are interpolated according to the ray
time, creating a grid at the correct time for the ray.
The Kay-Kajiya node for a moving surface patch contains a bounding box for the start of each motion segment plus a bounding box for the end of the last segment. The bounding boxes are linearly interpolated to find the bounding box that corresponds to the ray time.
6.4 SIMD speedups
6.4 SIMD加速
We use SIMD instructions (SSE and AltiVec) to speed up the computation of ray intersections. When intersection-testing a Kay- Kajiya node bounding box, we test multiple slabs (x, y, and z planes) at once. The bounding boxes for groups of 4×4 quads of finely tessellated patches (stored in the fine geometry cache) are
intersection-tested four boxes at a time. And tessellated patches are intersection-tested 4 triangles (2 quads) at a time.
Since each ray is traced independently, our SIMD speedups do not rely on ray coherency. In contrast, Wald et al. [16] used SIMD instructions for tracing four rays at a time. This works fine if the rays are coherent, but fails to deliver a speedup if the rays are incoherent.
由于每个光线都是单独被追踪,我们的SIMD加速器没有依赖光线连续性。作为对比,Wald et al. 使用SIMD结构同时追踪4个光线。这项工作对于连续的光线是非常好的,但是对于不连续的光线传递加速却失败了。
6.5 Shading at ray hit points
6.5 给光线碰撞的点加材质
To compute the shading results at ray hit points, we could shade the vertices of ray tracing tessellations, store the colors in a cache, and interpolate the colors at the hit points. This would be a straighforward generalization of the REYES shading approach. But unfortunately the shading colors are usually view-dependent — highlights
move around depending on the viewing direction, for example. The shader may also compute different results depending on the ray level for non-realistic, artistic effects.
Instead we create 3 shading points for each ray hit (similar to Gritz and Hahn [5]). One shading point is the ray hit point, and the other two shading points are created using the ray differentials at the ray hit point. This way, the shader can get meaningful differentials for texture filtering, computation of new ray directions, etc. While
most shading functions are executed on all three shading points, some are only executed at the ray hit point—for example, rays are only traced from the ray hit point.
It is worth emphasizing that for production scenes, the dominant cost of ray tracing is typically not the computation of ray intersections, but the evaluation of displacement, surface, and light source shaders at the ray hit points. (This is also why ambient occlusion has gained popularity in movie production so quickly as an alternative
to more accurate global illumination solutions: even though it takes a lot of rays to compute ambient occlusion accurately, there are no shader evaluations at the ray hit points.)
6.6 Avoiding cracks
6.6 避免破面
Visible cracks can occur if the two grids sharing an edge have different tessellation rates. See figure 6 for an illustration. This is a potential problem both for REYES rendering and for ray tracing, and has to be dealt with explicitly.
如果两个网格面共同使用一个边的话,在不同布线密度下网格会产生明显的破面。图6. 这对于REYES渲染和光线追踪都是潜在的问题,必须明确的处理这个问题。
The easiest way to fix these cracks requires that all dicing rates are powers of 2. Then every other vertex on the fine tessellation side of the edge can be moved to lie along the straight line between its two neighbor points. This ensures that the vertices on both sides of the edge are consistent so there are no cracks. However, such power-of-two tessellation (aka. “binary dicing”) introduces too many shading points compared to more flexible tessellation rates, and it is therefore too expensive in practice when shaders are a bottleneck.
Instead, PRMan uses an alternative algorithm that glues all edges together [1, sec. 6.5.2], thus avoiding cracks. We call this algorithm “stitching”. Stitching moves the tessellation points so that the grids overlap, and introduces new quads if needed to fill remaining gaps. For REYES rendering, new quads that are introduced are never shaded, they only copy colors from their nearest neighbor.
We use a similar stitching algorithm for ray tracing. If any new quads are generated, they are stored in the geometry cache.
The tessellation rate of each surface patch is determined from the size of the patch bounding box relative to the ray beam size. Hence, the tessellation within a patch is kept consistent for each ray, and there are no cracks internally within a patch. (Such cracks are sometimes refered to as “tunnelling” [13].)
7 混合渲染:REYES 和光线追踪
PRMan uses a combination of the REYES algorithm and on demand ray tracing. REYES is used to render objects that are directly visible to the camera. Shading those objects can cause rays to be traced. With this hybrid approach there are no camera rays; all the first-level rays originate from REYES shading points.
With the methods described above, both the REYES and ray tracing algorithms can handle very complex scenes. So why not use ray tracing for primary rendering? It would unify our algorithm, eliminate large parts of the PRMan code base, and make software maintenance easier. However, so far the advantages of coherency, well defined differentials, graceful handling of displacement mapping, efficient motion blur and depth-of-field, decoupling of shading rate and pixel sample rate, etc. makes the REYES algorithm hard to beat for movie-quality rendering.
8 复杂场景的测试
Figure 7 shows a test example, a scene with 15 cars. The cars are explicitly copied, not instanced. Each car consists of 2155 NURBS patches, many of which have trim curves. The cars have ray-traced reflections (maximum reflection depth 4) and sharp shadows, while the ground is shaded with ray-traced ambient occlusion. This gives a mix of coherent and incoherent rays. The image resolution is 2048×1536 pixels.
During rendering the car surfaces are divided into 1.3 million surface patches, corresponding to 383 million vertices and 339 million quads (678 million triangles) at full tessellation. Storing all full tessellations would consume 4.6 GB. Instead, with multi resolution caching, the scene uses a total of 414 MB: Geometry cache sizes
are set to their default value (20 MB per cache per thread), a total of 120 MB. The Kay-Kajiya tree uses around 59 MB per thread. The top-level object descriptions use 126 MB plus 50 MB for trim curves.
在渲染过程中,汽车模型被分为一百三十万个片,相当于三亿八千三百万个点和三亿三千九百万个四边形。保存所有的网格信息需要4.6GB。相反,对于多分辨率缓存,场景使用414MB:物体缓存大小设定为初始值(每线程每个缓存20MB), 一共是120MB。Kay-Kajiya树每线程使用59MB。高精度物体在物体描述方面使用126MB加上50MB切线。
Rendering this image used 111 million diffuse rays, 37 million specular rays, and 26 million shadow rays. The rays cause 1.2 billion ray-triangle intersection tests. With multi resolution geometry caching, the render time is 106 minutes. The three geometry caches have a total of 675 million lookups and cache hit rates of 91.4%–95.2%. In contrast, if the ray differentials are ignored (the REYES tessellation rates are used for all ray intersection tests) and
no caching is done, the render time is 15 hours 45 minutes — almost 9 times slower.
More exhaustive tests and results can be found in Christensen et al. [3]. Although the render times reported there are quite obsolete by now, the time ratios and relative speedups are still representative.
更加详尽的测试和结果可以在Christensen et al. 中找到。尽管记录了渲染时间,但是现在很过时,时间比率和相对速度都还是很典型的。
9 “汽车总动员”光线追踪
Figure 8 shows “beauty shots” of two of the characters from ‘Cars’. These images demonstrate ray traced reflections, shadows, and ambient occlusion.
For convex surfaces like a car body, distant reflections do not need to be very accurate. In many shots, the maximum distance that a ray can hit geometry was set to 12 meters. If the ray didn’t hit anything within that distance, it would use a single held environment map instead.
The reflections in the movie were usually limited to a single level of reflection. There were only a few shots with two levels of reflection, they are close-ups of chrome parts that needed to reflect themselves multiple times. Figure 9 shows an example.
Figure 9: Chrome bumper with two levels of ray-traced reflection.
Figure 10 shows all the main characters in the ‘Cars’ movie. This is an example of a very complex scene with many shiny cars. The shiny cars reflect other cars, as shown in the three close-ups. The image also shows ray-traced shadows and ambient occlusion.
PRMan uses the REYES algorithm for rendering directly visible objects, and offers on-demand ray tracing for reflections, shadows, ambient occlusion, etc. It uses a multi resolution geometry cache and a multi resolution texture cache, and uses ray differentials to select the appropriate resolutions. Due to the observation that coherent rays have narrow beams while incoherent rays have wide beams, the method is efficient for ray tracing of complex scenes. The ray tracing functionality has been used for several movies, including Pixar’s ‘Cars’.
10 总结
We would like to thank our colleagues in Pixar’s RenderMan Products group for providing an inspiring and creative environment and for many helpful discussions. Loren Carpenter implemented most of the SIMD speedups and Brian Smits helped us optimize other aspects of the ray-tracing efficiency. Tony Apodaca headed the Cars
“Nitro” speed team. Erik Smitt clarified some of the movie production details.
All images from ‘Cars’ are copyright c Disney Enterprises, Inc. and Pixar Animation Studios.

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