09-transformations

09-transformations - NOTES (ray casting/antialiasing ) and...

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1 (ray casting/antialiasing ) and Transformations Lecture 9 CPSC 478/578 Spring 2005 NOTES Assignment # 1 ± due today 11:59pm ± TA office hrs in tonight zoo: 7- 9pm Quiz # 1 Feb. 23, 2005 ± Problems/short answer ± Covers material up to the end of lecture Feb. 15, 2005 ± Allowed 8.5x11 page of notes ± Will not ask for OpenGL code NOTES • Reading for this week: Text - Chapter 5 “Transformation Matrices” Text – Chapter 6 “Viewing” Text - Chapter 11 “A Full Graphics Pipeline” 3D Rendering • The color of each pixel on the view plane depends on the radiance emanating from visible surfaces View plane Eye position Simplest method is ray casting Rays through view plane How Many Rays? Aliasing • One of the most common artifacts in ray tracing is “jaggies.” Consider this example: • We would like a smoother transition: Recall: Aliasing (in general) • In general: – Artifacts due to under-sampling or poor reconstruction • Specifically, in graphics: – Spatial aliasing – Temporal aliasing Under-sampling
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2 Recall: Spatial Aliasing • Artifacts due to limited spatial resolution Recall: Spatial Aliasing • Artifacts due to limited spatial resolution “Jaggies” Recall: Image Processing Resampling • Image processing is a resampling problem We want to avoid aliasing! Recall: Ideal Low-Pass Filter • Frequency domain • Spatial domain x x x Sinc π sin ) ( = Recall: Gaussian Filter • Convolution with Gaussian filter Input Output Recall: Convolution and Filtering • Technical application –Pixel mask with weights
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3 Antialiasing in a Ray Tracer • We would like to compute the average intensity in the neighborhood of each pixel: • When casting one ray per pixel, we are likely to have aliasing artifacts • For a better result, we can cast more than one ray per pixel and average the result. This is known as super-sampling and averaging down . Antialiasing in a Ray Tracer Equal weights for all samples, Box filter, or weight closest to center (Gaussian) Antialiasing in a Ray Tracer • We may a want to to filter samples from neighboring pixels: Time -> Steady movement is rendered as a sudden jump Antialiasing in a Ray Tracer • We may a want to to filter samples from neighboring pixels: Time -> If (for example) use half of neighboring pixel, get smoother transition Antialiasing by Adaptive Super-Sampling • Casting many rays per pixel can be unnecessarily costly. e.g. if there are no rapid changes in intensity at the pixel, perhaps only a few samples are needed. • Solution: adaptive sampling • How do we decide to cast more rays in a particular area? Antialiasing by Jittering • Rays don’t have to be cast in regular pattern • Changing location randomly within the pixel avoids artificial features due to regular sampling
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4 Summary – Ray Casting • Intersection acceleration techniques are important – Bounding volume hierarchies – Spatial partitions • General concepts – Sort objects spatially – Make trivial rejections quick – Utilize coherence when possible
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09-transformations - NOTES (ray casting/antialiasing ) and...

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