True Impostors - Eric Risser

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approach to this has been laid out in Relief Mapping of Non Height Field Surfaces [Policarpo et al. 2006] using a ray march followed by binary search as shown in cells F-H. This finds the first point of intersection into the volume. Due to the nature of GPU design, it is impractical from an efficiency standpoint to exit out of a loop early, therefore the maximum number of ray marches and texture reads must be performed no matter how early in the search the first collision is found. Rather than ignoring this ”free” data, a method is proposed to add rendering support for translucent material types through a localized approximation to ray tracing. Rather than performing a binary search along the viewing ray, the points along the W axis are used to define a linear equation for each height field as shown in cell I. In cell J these linear equations are then tested for intersection with the view ray in a similar fashion as shown in Parallax Occlusion Mapping [Tatarchuk. 2006]. The intersection which falls between the upper and lower bounding points is kept as the final intersection point. Since the material is translucent the normal is checked for this point and the view ray is refracted according to a dielectric value either held constant for the shader or stored in an extra color channel in one of the texture maps. Once the new view direction is discovered, the ray march continues and the process is repeated for any additional surfaces that are penetrated as shown in cells K-M. By summing the distances traveled through any volume, the total distance traveled through the model can be known and used to compute the translucency for that pixel. True Impostors also offers a simple yet effective animation scheme in which the animation frames are tiled on a single texture, offering quick look-ups using the pixel shader. Figure 4: Animated texture map with cooresponding models. Rather than using the entire texture to store a single model, the image is partitioned into discrete equally sized regions each storing a frame of the desired animation as shown in figure 4. To accommodate the new data representation, the UV coordinates stored in each vertex span the length of a single region as opposed to the entire texture map. Also, the UV coordinates of the texture map are no longer shifted by -.5, but shifted so that the middle of the target region lies on the origin of the coordinate frame. The animation can be looped through by passing a global time variable into the shader and using it to select the current target region. This makes True Impostors a powerful technique for rendering large herds of animals, schools of fish, and flocks of birds. 4 Results and Analysis True Impostors can produce nearly correct reproductions of geometry on a single billboard for any possible viewing direction. This is shown in Figure 5 where multiple dogs are displayed from arbitrary directions. True Impostors is the first per-pixel displacement mapping based impostor technique to achieve complete viewing freedom and thus is the first of such techniques to represent objects in true 3D. In addition to viewing independence, this paper tackled a new volume intersection/traversal method ideal for rendering translucent/refractive objects which can be applied to any multiple layered per-pixel displacement mapping technique. Figure 6 shows a glass sphere rendered using True Impostors with correct double refraction. A trace-through of the view vector is also shown to confirm that True Impostors is properly rendering the object. Figure 6: refraction through a glass sphere using True Impostors Given the lack of a true root finding method during refractive rendering, True Impostors suffers from very little error; however, there is a case where minor artifacts will occur. When entering or leaving a volume such that one of the linear steps falls on a pixel in the texture map which does not define the volume being encountered, there is a chance that the line-segment intersection test can return a point that also does not lie on a pixel defining the target volume. The normal of the surface is needed to refract the view ray, in general this method rarely finds the exact point of intersection, but since the surface and therefore normal is generally continuous, the small level of error is unnoticeable by the human eye. However, at such rare points where a normal not lying on our surface is returned, the resulting image will show blatant artifacts. To soften these artifacts, the normal textures are mipmapped so each sampled point is actually the average of several neighboring points. For the most part this alleviates the rendering error; this does however leave a crease where two surfaces meet to form a volume. Because multiple points on the normal map are averaged, there is no smooth transition between layers defining a volume as shown in figure 7. This crease can be remedied through a simple art hack. When generating the normal maps for each surface, blend the color values concentrated around the edge of a volume, this should force the normals to conform at the edge and produce a smooth curve. In addition to refraction, True Impostors can also reflect points on a surface achieving complex local interactions between mirrored
Figure 5: The same impostor is shown from multiple arbitrary viewing directions. True Impostors is the first technique which has successfully been able to render view directions which significantly deviate from the profile, as shown in the third and last image. Figure 7: A visible crease is shown along the intersection of two surfaces. Due to averaging normal values. surfaces. As a proof of concept a sphere is rendered using reflection in figure 8. Figure 8: Reflective impostor. The method performs similarly to other per-pixel displacement mapping techniques; however, there are concerns unique to True Impostors. Performance is fill-rate dependent. Since billboards can occlude neighbors it is crucial to perform depth sorting on the CPU to avoid overdraw. Since True Impostors is fill-rate dependent, level of detail is intrinsic, and this is a great asset. With True Impostors it is possible to represent more geometry on screen at once then could be achieved using standard polygonal means, an example is shown on the first page where a model of Jupiter is rendered using a quarter of a million asteroids to comprise its ring. True Impostors was implemented in C++ using DirectX 9. All benchmarks were taken using a GeForce 6800Go and two GeForce 7800’s run in SLI. Although the vertex processor is crucial to True Impostors, the resulting operations do not put a heavy workload on the vertex processing unit and do not result in noticeable drops in performance. The performance of this technique is primarily determined by the fragment processing unit, the number of pixels on the screen, the number of search steps taken for each pixel, and which rendering technique is used. When performing ray casting the performance mirrored that of Relief Mapping of Non-Height-Field Surface Details [Policarpo et al. 2006] due to the similar ray casting technique used in both methods. The Jupiter model consisting of a quarter of a million impostors with a linear search size of 10 steps and a binary search size of 8 steps rendered in a 1024x768 window at 10 frames per second on the 6800Go and 35 - 40 frames per second on the 7800SLI. The ray tracing technique was rendered in a 800x600 window using 20 search steps and achieved on average 7-8 frames per second on the 6800Go and 25- 30 frames per second on the 7800SLI. No comparisons are made between the performances of the two techniques due to the fundamentally different rendering solutions they both offer. The only generalized performance statement made about the two techniques is that they both have been shown to achieve real time frame-rates on modern graphics hardware. 5 Discussion True Impostors offer a quick, efficient method for rendering large numbers of animated opaque, reflective or refractive objects on the GPU. It generates impostors with very little rendering error and offers inherent per-pixel level of detail. These results are achieved by building upon the concepts laid out in Relief Mapping of Non- Height-Field Surface Details [Policarpo et al. 2006] and Parallax Occlusion Mapping [Tatarchuk. 2006]. By representing volume data as multiple height fields stored in traditional texture maps, the vector processing nature of modern GPUs is exploited and a high frame rate is achieved along with a low memory requirement. By abandoning the restrictions inherent in keeping per-pixel displacement mapping a subsurface detail technique, a new method for rendering staggering amounts of faux-geometry has been achieved, not by blurring the line between rasterization and ray tracing, but through a hybrid approach, taking advantage of the best each has to offer. This method is ideal for video games as it improves an already widely used technique.
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True Impostors - Eric Risser

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