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GRPHICS08 - Raytracing and Radiosity
The document discusses two primary non-real-time rendering techniques in 3D graphics: ray tracing and radiosity. Ray tracing involves tracing light paths to produce realistic images through high computational cost, while radiosity models light interactions as energy exchanges between patches to create diffuse lighting. Combining both methods in a two-pass algorithm yields optimal results, allowing for both specular and diffuse illumination in a scene.
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GRPHICS08 - Raytracing and Radiosity
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- 2. INTRODUCTION Thus farour discussion of 3D graphics has concentrated mainly of real-time rendering. In today’s lecture, we will discuss the two main techniques for non real-time rendering. Raytracing Radiosity
- 3. REFLECTION MODELS Localreflection model Imagine lights and objects are floating in dark space Considers only direct illumination Global reflection model Indirect light considered Light is reflected multiple times Much more computationally expensive. Outside the reach of most real-time applications. For now…
- 4. LOCAL VERSUS GLOBAL Direct illumination only Localmodel Light source Global model Direct and indirect illumination
- 5. GLOBAL ILLUMINATION Intheory, for this we: Trace rays from each light source to each point visible from that light source Trace reflections and refractions from this point to other surfaces Continue until: We reach the viewport, or We exit the scene entirely Approximations still used Can’t trace all rays of light from a light source.
- 6. RAY-TRACING Ray tracingis the process of tracing the path of light through pixels in an image. Capable of producing high degrees of realism. Still not quite photorealistic, but not far off. High computational cost Best used for scenes and animations that can be pre- rendered. Incorporates many aspects of physics Light reflection Light refraction Light scattering
- 7. RAY-TRACING Rays sendaway from the camera rather than to it Most rays of light emitted from a source would not hit the viewing plane. Comparable method called photon mapping exists. Many orders of magnitude more expensive to calculate. Provides advantages: Reflections and shadows realistically produced And disadvantages: Expensive to calculate each scene Still an approximation
- 8. RAY-TRACING Rays sent backwards fromthe camera. Much less computationally expensive than the alternative. http://www.codinghorror.com/blog/archives/001073.html
- 9. LIGHT INTERACTIONS Rememberour different kinds of light interactions. Perfect Specular Imperfect Specular Diffuse Must be able to combine light interactions in a ray-tracing model. Specular to Diffuse Diffuse to Specular And so on
- 10. TRANSMISSION OF LIGHT Indirect transmission of light occurs in several ways Reflection Transparent refractions Translucent diffusion Transparency should refract light It looks unrealistic otherwise. Translucency transmits light in multiple new directions.
- 11. RAY-TRACING For eachpixel, we calculate the ray from the centre of the viewport through that pixel. Find the intersection of that ray with the nearest object. Determine the pixel colour based o surface properties, orientation, and light intensities This may in itself have a component from reflected or refracted light rays If the surface is reflective or transparent, generate a new ray. Trace that onwards. Repeat for all pixels. If no intersection, pixel colour is background colour.
- 12. RAY-TRACING Secondary raysmay be used for diffuse reflection. Generate multiple new rays and trace them on. Depends on the kind of reflection needed. Each secondary ray is followed until it hits another object, light source, or background. Recursion is used to permit this relationship to extend to predefined limits.
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- 14. EFFICIENCY Expense ofcomputation based on number of calculations required. Consider a 1000x1000 resolution. On a scene with 1000 objects. With a single light source. Consider primary illumination first How about secondary rays? Further orders of illumination? How about a second light source? Or a third? Various optimisation schemes exist.
- 15. RADIOSITY Radiosity isbased on a model of radiative heat transfer. Assumes conservation of light energy in a closed environment. Energy emitted or reflected by every surface accounted for by: Absorption Reflection Refraction Fluorescence
- 16. RADIOSITY All surfacesare broken up into patches and elements. Patches can emit or send light They send light to other parts of the model. Elements can absorb light They receive light from other parts of the model. Number of patches relates to computational intensity. Each element is associated with a patch. Indeed, a patch may have many elements
- 17. RADIOSITY Radiosity worksthrough a system of progressive refinement Start with the patch which has most energy to ‘shoot’ Each of the elements receive the energy appropriately, and add to the energy of their own patches. Repeat with the patch which now has the most unspent energy. Repeat until all patches are empty, or some minimal bound is reached.
- 18. RADIOSITY Every polygon gets treatedas a patch (a light source) Every patch is associated with a number of elements, http://www.cs.princeton.edu/courses/archive/fall00/cs426/lectures/ra diosity/radiosity.pdf
- 19. PATCHES Patches maybe present in higher resolution than surfaces. One surface may be broken up into many patches. Subdivision algorithms used to further refine patch mapping over surfaces. Can be done blindly Or intelligently Focus at areas of high contrast in light levels.
- 20. RADIOSITY – AFIRST APPROACH Imagine a simple room Four walls A Ceiling A Floor No light source Radiosity makes each of the surfaces in the scene a light source. Some of these surfaces may have no energy to spend to begin with. Imagine the roof as a giant light source. Like in a supermarket Each surface stores: How brightly lit it is How much surplus energy it has to spend. We must calculate the interaction of energy to surface of all surfaces in the scene. Can be calculated in many ways. Most usual is by geometric distance. The resulting number is the form factor.
- 21. RADIOSITY Radiosity creates ascene of soft diffuse light. Represents the interaction of surfaces well, but cannot suitably deal with specular reflection. http://www.cs.dartmouth.edu/~spl/ Academic/ComputerGraphics/Fall2 004/outline.html
- 22. GLOBAL REFLECTION MODELS Ray Tracing Simulates perfect specular reflections Good for shiny objects reflecting in each other. Ray-traced images tend to have lots of shiny objects with perfect reflections. Is viewport dependant. Change the viewport and you need to rerender. Radiosity Simulates the interactions of diffuse light Good for matte surfaces Images rendered using radiosity tend to be softly lit rooms without shiny objects. Is viewport independent. Change the viewport and the radiosity image remains the same.
- 23. GLOBAL REFLECTION MODELS Best results obtained by combining the two into a two-pass method. Radiosity used to build a model of diffuse illumination across a scene. Raytracing used to build a viewport dependant illumination of the scene. Combination of two has representation of specular and diffuse radiation. Permits soft and hard lighting in a single scene.
- 24. TWO PASS -RAYTRACING
- 25. TWO PASS -RADIOSITY
- 26. TWO PASS -COMBINED
- 27. SUMMARY Non real-timerendering works through two primary methods. Ray-tracing Map the path of light from a viewport to objects. Radiosity Light interactions modelled as energy interactions between patches. Best results obtained by combining both approaches into a two-pass algorithm.