Ray Tracing光线跟踪Contents 26 Real-Time 26.1 Fundamen

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详细说明：光线跟踪Contents 26 Real-Time Ray Tracing 26.1 Ray Tracing Fundamentals 26.2 Shaders for Ray Tracing 26.3 Top and Botton Level Acceleration Structures 26.4 Coherent 12 26.5 Denoising 27 26.6 Texture Filtering 34 26.7 Speculations References Bibliography ndex Acknowledgments Our sincere thanks the following for helping out with images, proofreading, expert knowledge, and morc: Kostas Anagnostou, Magnus Andersson, Colin Barrc-Briscbois Henrik Wall Jensen, Aaron Lefolin, Edward Liu, Ignacio Llamas, Rula Lober. boyd Meeji, Jacob Munkberg, Jacopo Pantaleoni, Steven Parker, Tomasz Stachowiak, and Chris Wyman Since version 1.0, a few people have kindly provided us corrections for this chapter namely pontus Andersson and Aaryaman Vasishta Chapter 26 Real-Time Ray Tracing I wanted change and excitement and to shoot off in all directions myself, like the colored arrows from a Fourth of Ja uly rocket. Sylvia plath Compared to rasterization-based techniques, which is the topic of large parts of this book, ray tracing is a method that is more directly inspired by the physics of light As such, it can generate substantially more realistic images. In the first edition of Real-Time Rendering. from 1999, we dreamed about reaching 12 frames per second for rendering an average frame of A Bug's Life(ABL) between 2007 and 2024. In some sense we were right. ABL used ray tracing for only a few shots where it was trul nccded, c g, reflection and refraction in a watcr droplet. Howcvcr, rcccnt advances in GPUs have made it possible to render game-like scenes with ray tracing in real time For example, the cover of this book shows a scene rendered at about 20 frames per second using global illumination with something that starts to resemble feature-film image quality. Ray tracing will revolutionize real-time rendering In its simplest form, visibility determination for both rasterization and ray tracing can be described with double for-loops. Rasterization is for(t in triangles) for(p in pixels) determine if p is inside t Ray tracing can, on the other hand, be described by for(p in pixels) (t in triangles) determine if ray through P hits T So in a sense, these are both simple algorithms. However, to make either of these fast, you need much more code and hardware than can fit on a business card 1 One important feature of a ray tracer using a spatial data structure, such as a bounding volume hierarchy(BVH), is that the running time for tracing a ray is O(log n), where m I The back of Paul Heckbert's business card froin the 1990s had code for a sinple, recursive ray 34 26. Real-Time Ray Tracing is the number of triangles in the scene. While this is an attractive feature of ray tracing it is clear that rasterization is also better than O(n), since GPUs have occlusion culling hardware and rendering engines use frustum culling, deferred shading, and Imany other techniques that avoid fully processing every primitive. So, it is a complex matter to cstimatc the running timc for rasterization in OO notation. In addition. the texture units and the triangle traversal units of a gpu are incredibly fast and have been optimized for rasterization over a span of decades The important difference is that ray tracing can shoot rays in any direction, not just from a single point, such as from the eye or a light source. As we will see in Section 26. 1, this Flexibility makes it possible to recursively render reflections and refractions[89, and to fully evaluate the rendering equation(Equation 11. 2). Doing so makos the images just look better. This propcrty of ray tracing simplifies content creation as well. since less artist intervention is needed 20. When using rasterizatiOn artists often need to adjust their creations to work well with the rendering techniques being used. However, with ray tracing, noise may become apparent in the images This can happen when area lights are sampled, when surfaces are glossy, when an environment map is integrated over, and when path tracing is used, for example That said, to make real-time ray tracing be the only rendering algorithm used for real-time applications, it is likely that several techniques, e. g, denoising, will be needed to make the images look good enough. Denoising attempts to remove the noise based on intelligent image averaging(Section 26.5). In the short-term, clever combinations of rasterization and ray tracing arc cxpectcd rasterization is not going away anly tilne sool. In the longer-lerIll, ray tracing scales well as processors becoine more powerful, i. e, the more compute and bandwidth that are provided the better images we can generate with ray tracing by increasing the number of samples per pixel and the recursive ray depth. For example, due to the difficult indirect lighting involved, the image in Figure 26. 1 was generated using 256 samples per pixel. Another image with high-quality path tracing is shown in Figure 26. 6, where the number of samples per pixel range from 1 to 65.536 Before diving into algorithins used in ray tracing, we refer you lo several relevant chapters and sections. Chapter 11 on global illumination provides the theory sur- rounding the rendering equation(Equation 11.2), as well as a basic explanation of ray and path tracing in Section 11. 2.2. Chapter 22 describes intersection methods, where ray against object tests are essential for ray tracing. Spatial data structures, which are used to speed up the visibility queries in ray tracing, are described in Section 19.1.1 and in Chapter 25, about collision detection 26.1 Ray Tracing Fundamentals Recall from equation 22. 1 that a ray is defined as q(t)=o+td 26.1. Ray Tracing Fundamentals Figure 26.l. a difficult scene with a large amount of indirect lighting rendered with 256 samples per pixel, with 15 as ray depth, and a million triangles. Still, when zooming it, it is possible to see noise inl this inlage. There are objects consisting of transparent plastic mlaterials, glass, and several glossy met surfaces as well, all of which are hard to render using rasterization.(Model by Boyd Meeji, where o is the ray origin and d is the normalized ray direction, with t then being the distance along the ray. note that we use q here instead of r to distinguish it from the right vector r, used below. Ray tracing can be described by two functions called trace() and shade(). The core geometrical algorithm lies in trace, which is responsible for finding the closest intersection between the ray and the primitives in the scene and returning the color of the ray by calling shade(. For most cases we want to find an intersection with t >0. For constructive solid geometry, we often want negative distance intersections(those behind the ray as well To find the color of a pixel, we shoot rays through a pixel and compute the pixel color as some weighted average of their results. These rays are ca lled eye rays or camera rags. The camera setup is illustrated in Figure 26.2. Given an integer pixel oordinate, (a, y) with r going right in the image and y going down, a camera position C, and a coordinate frame, r, u, v(right, up, and view ), for the camera, and screen 26. Real-Time Ray Tracing (x,y) y Figure 26.2. A ray is defined by an origin o and a direction d. The ray tracing set up consist s of constructing and shooting one (or more) rays from the viewpoint through each pixel. The ray shown in this figure hits two triangles, but if the triangles are opaque, only the first hit is of interest. Note that the vectors r(right),u(up), and v(view )are used to construct a direction vector d(a, y) of a sample position(a, y) resolution of w xh, the eye ray q(t)=o+td is computed as s(x,y)=/2(x+0.5-1)r-f 2(3+0.5) (26.2 d(, y) where the normalized ray direction d is affected by f= tan(/2), with o being the cameras vertical field of view, and a=w/h is the aspect ratio. Note that the camera coordinate frame is left-handed, i.e., r points to the right, u is the up-vector, and v points away from the camera. toward the image plane, i.e., a similar setup to the one shown in Figure 4.5. Note that s is a temporary vector used in order to normalize d. The 0.5 added to the integer (a, y) position selects the center of each pixel, since (0.5, 0.5)is the floating-point center 33. If we want to shoot rays anywhere in a pixel, we would instead represent the pixel location using floating point values and the 0.5 offsets arc then not added In the naivest implementation, trace would loop over all the n primitives in the scene and intersect. the ray wit h each of them, keeping the closest intersection with t>0. Doing so yields O(n) performance, which is unacceptably slow except with a few primitives. To get to O(log n) per ray, we use a spatial acceleration data structure e.g., a bVh or a k-d tree. See Chapter 19. 1 for descriptions on how to intersection test a ray using a BVH Using trace() and shade( to describe a ray tracer is simple. Equation 26.2 is used to create an eye ray from the camera position through a location inside a pixel 26.1. Ray Tracing Fundamentals trace camera n rav trace pixel grid trace shade trace trace shade Figure 26.3. A camera ray is created through a pixel and a first call to trace( starts the ray tracing process in that pixel. This ray hits the ground planc with a normal n. Then shade() is called at this first hit point, since the goal of trace( is to find the ray's color. The power of ray tracing comes from the fact that shade () can ca ll trace( as a help when evaluating the BrDf at that point. Here his is done by shooting a shadow ray to the light source, which in this case is blocked by a triangle In addition. assuming the surface is specular, a reflection ray is also shot and this ray hits a circle At this second hit point, shade()is called again to evaluate the shading. Again, a shadow and a reflection ray are shot from this new hit point This ray is fed to trace(), whose task is to find the color or radiance( Chapter 8)that is returned along that ray. This is done by first finding the closest intersection along he ray and then computing the shading at that point using shade(). We illustrate this process in Figure 26. 3. The power of this concept is that shade(), which should evaluate radiance, can do that by making new calls to trace(). These new rays that are shot from shade() using trace() can, for example, be used to evaluate shadows recursive reflections and refractions, and diffuse ray evaluation. The term ray depth is used to indicate the numbcr of rays that have been shot recursively along a ray path The eve ray has a ray depth of 1, while the second trace() where the ray hits the circle in Figure 26.3 has ray depth 2 One use of these new rays is to determine if the current point being shaded is in shadow with respect to a light source. Doing so generates shadows. We can also take the eye ray and the normal, n, at the intersection to compute the reflection vector Shooting a ray in this direction generates a reflection on the surface, and can be donc rccursively. The samc proccss can be used to goncratc refractive rays. Pcrfcctly specular rellections and refractions along with sharp shadows is often referred to as Whitted ray tracing [89. See Sections 9.5 and 14.5. 2 for information on how to compute the reflection and refraction rays. Note that when an object has a different index of refraction than the medium in which the ray travels, the ray may be both reflected 26. Real-Time Ray Tracing n reflecti refraction Figure 26. 4. An incoming ray in the top left corner hits a surface whose index of refraction, n2, is larger than the index of refraction, n1, in which the ray travels, i.e., n2 >n1. Both a reflection ray d a refraction ra ch hit point(circles) and refracted. See Figure 26.4. This type of recursion is something that rasterization based methods struggle to solve by using various approximations to achieve only a subset of the effects that can be obtained with ray tracing. Ray casting, the idea of testing visibility between two points or in a direction, can be used for other graphical (and non-graphical) algorithms. For example, we could shoot a number of ambient occlusion rays from an intersection point to get an accurate estimate of that effect The functions trace(, shade(, and rayTraceImage(, wherc the latter is a lunction Chat creates eye rays through each pixel, are used in the pseudocode that follows. These short pieces of code shows the overall structure of a Whitted ray tracer, which can be used as a basis for many rendering variants, e.g., path tracing ray TraceImage() for (p in pixels) color of p= trace(eye ray through p trace(ray t find closest intersection return shade(pt shade(point) 10r=0 for(I in light sources) trace(shadow ray to l)i color t= evaluate brdf

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