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Rendering. ICS 415. Rendering. It is the process of generating an image from a model, by means of computer programs. The model is a description of three dimensional objects in a strictly defined language or data structure.
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Rendering ICS 415
Rendering • It is the process of generating an image from a model, by means of computer programs. • The model is a description of three dimensional objects in a strictly defined language or data structure. • It would contain geometry, viewpoint, texture, lighting, and shading information.
Features of Rendering • A rendered image can be understood in terms of a number of visible features. • Rendering research and development has been largely motivated by finding ways to simulate these efficiently. • Some of these features are listed next.
Features of Rendering • shading — how the color and brightness of a surface varies with lighting. • texture-mapping — a method of applying detail to surfaces.
Features of Rendering • bump-mapping — a method of simulating small-scale bumpiness on surfaces. • fogging/participating medium — how light dims when passing through non-clear atmosphere or air.
Features of Rendering • shadows — the effect of obstructing (blocking) light. • soft shadows — varying darkness caused by partially obscured light sources.
Features of Rendering • reflection — mirror-like or highly glossy reflection. • Transparencyor opacity — sharp transmission of light through solid objects.
Features of Rendering • translucency — highly scattered transmission of light through solid objects. • refraction — bending of light associated with transparency.
Features of Rendering • diffraction — bending, spreading and interference of light passing by an object or aperture (hole or crack) that disrupts the ray.
Features of Rendering • indirectillumination — surfaces illuminated by light reflected off other surfaces, rather than. directly from a light source (also known as global illumination).
Features of Rendering • caustics (a form of indirect illumination) — reflection of light off a shiny object, or focusing of light through a transparent object, to produce bright highlights on another object.
Features of Rendering • depth of field — objects appear blurry or out of focus when too far in front of or behind the object in focus. • motion blur — objects appear blurry due to high-speed motion, or the motion of the camera.
Features of Rendering • photorealistic morphing —3D renderings to appear more life-like. • non-photorealisticrendering — rendering of scenes in an artistic style, intended to look like a painting or drawing.
Rendering Algorithms • Global illumination • Painter's algorithm • Radiosity • Ray tracing • Scanline algorithms like Reyes • Volume rendering • Unbiased rendering • Z-buffer algorithms
Global illumination • Global illumination is a general name for a group of algorithms used in 3D computer graphics that are meant to add more realistic lighting to 3D scenes. • Such algorithms take into account not only the light which comes directly from a light source (direct illumination), but also subsequent cases in which light rays from the same source are reflected by other surfaces in the scene (indirect illumination).
Global illumination • Theoretically reflections, refractions, and shadows are all examples of global illumination, because when simulating them, one object affects the rendering of another object (as opposed to an object being affected only by a direct light). • In practice, however, only the simulation of diffuse inter-reflection or caustics is called global illumination.
Global illumination • Images rendered using global illumination algorithms often appear more photorealistic than images rendered using only direct illumination algorithms. • However, such images are computationally more expensive and consequently much slower to generate.
Global illumination • Examples of algorithms used in global illumination: • Radiosity • Ray tracing • Beam tracing • Cone tracing, path tracing • Metropolis light transport • Ambient occlusion • Photon mapping • Image based lighting • some of these may be used together to yield results that are fast, but accurate.
Example-Global illumination • Rendering without Global Illumination. • Consider the next Figure. • Note that we are looking at a fully-enclosed scene through a one-way-transparency scheme (see the chrome sphere's reflection of the otherwise invisible white and green walls). • There is a lack of definition in areas that are outside the beam of direct light from the ceiling lamp. • For example, the geometry of the ceiling lamp's housing is obscured within a solid grey area produced by an ambient color. • Without the ambient color added into the rendering equation, this surface would be black.
Example-Global illumination • Rendering with global illumination • Note how light is reflected by surfaces. • Note how colors transfer (or "bleed") from one surface to another, an effect of diffuse inter-reflection. • Notice how colors from the red and green walls are diffusely reflected by other surfaces in the scene (one-way transparency is used to allow us to see "through" two walls from the outside while preserving their effect inside the scene). • Also notable is the caustic projected on the red wall as light passes through the glass sphere.
Demo-Global illumination • Vedio
Radiosity • Radiosity is a global illumination method. • Direct Illumination is a term that covers the principal lighting methods used by old school rendering engines such as 3D Studio and POV. • A scene consists of two types of entity: Objects and Lights. • Lights cast light onto Objects, unless there is another Object in the way, in which case a shadow is left behind. • Examples of Direct illumination techniques are: • Shadow Volumes, • Z-Buffer methods, • Ray Tracing. • Source (http://freespace.virgin.net/hugo.elias/radiosity/radiosity.htm)
Radiosity • Global illumination methods try to overcome some of the problems associated with Ray Tracing. • While a Ray Tracer tends to simulate light reflecting only once off each diffuse surface, global illumination renderers simulate very many reflections of light around a scene. • While each object in a Ray Traced scene must be lit by some light source for it to be visible, an object in a Globally Illuminated scene may be lit simply by it's surroundings.
Radiosity Lighting a simple scene with Direct Lighting: • A simple scene is modeled in 3D Studio. • We wanted the room to look as if it was lit by the sun shining in through the window. • So, we set up a spotlight to shine in. • When we rendered it, the entire room was pitch black, except for a couple of patches on the floor that the light reached.
Radiosity • Turning up the Ambient Light simply caused the room to appear a uniform grey, except for the uniformly red floor, and light patches. • Adding a point light source in the middle of the room brought out the details, but the scene doesn't have that bright glow that you expect from a sunlit room. • Lastly, we turned the background color to white, to give the appearance of a bright sky.
Radiosity Lighting a simple scene with Global Lighting: • We modeled the same scene radiosity renderer. • To provide the source of light, we rendered an image of the sky with Terragen(scenery generator), and placed it outside the window. • No other source of light was used. • With no further effort, the room looks realistically lit.
Radiosity Interesting points to note: • The entire room is lit and visible, even those surfaces facing away from the sun. • Soft shadows. • The subtle change in brightness across the wall to the left of the scene. • The grey walls, far from being grey, have a certain warmth to them. The ceiling could even be said to be ever so slightly pink.
The Workings of a Radiosity Renderer The basic premise of Radiosity. • Any light that hits a surface is reflected back into the scene. • That's any light. Not just light that's come directly from light sources. Any light. • That's how paint in the real world thinks, and that's how the radiosity renderer will work.
The Workings of a Radiosity Renderer • Anything that is visible is either emitting or reflecting light, i.e. it is a source of light. • Everything you can see around you is a light source. And so, when we are considering how much light is reaching any part of a scene, we must take care to add up light from all possible light sources. Basic Premises: • There is no difference between light sources and objects. • A surface in the scene is lit by all parts of the scene that are visible to it.
The process of performing Radiosity on a scene A Simple Scene • We begin with a simple scene: a room with three windows. • There are a couple of pillars and some alcoves, to provide interesting shadows. It will be lit by the scenery outside the windows, which we will assume is completely dark, except for a small, bright sun.
The process of performing Radiosity on a scene • Now, lets choose one of the surfaces in the room, and consider the lighting on it.
The process of performing Radiosity on a scene • As with many difficult problems in computer graphics, we'll divide it up into little patches (of paint), and try to see the world from their point of view. • From now on we'll refer to these patches of paint simply as patches.
The process of performing Radiosity on a scene • Take one of those patches. And imagine you are that patch. What does the world look like from that perspective?
The process of performing Radiosity on a scene View from a patch • Placing your eye very carefully on the patch, and looking outwards, you can see what it sees. • The room is very dark, because no light has entered yet. But we have drawn in the edges for your benefit. • By adding together all the light it sees, we can calculate the total amount of light from the scene reaching the patch. We'll refer to this as the total incident light from now on. • This patch can only see the room and the darkness outside. Adding up the incident light, we would see that no light is arriving here. This patch is darkly lit.
The process of performing Radiosity on a scene View from a lower patch • Pick a patch a little further down the pillar. This patch can see the bright sun outside the window. This time, adding up the incident light will show that a lot of light is arriving here (although the sun appears small, it is very bright). This patch is brightly lit.
The process of performing Radiosity on a scene Lighting on the Pillar • Having repeated this process for all the patches, and added up the incident light each time, we can look back at the pillar and see what the lighting is like. • The patches nearer the top of the pillar, which could not see the sun, are in shadow, and those that can are brightly lit. Those that could see the sun partly obscured by the edge of the window are only dimly lit. • And so Radiosity proceeds in much the same fashion. As you have seen, shadows naturally appear in parts of the scene that cannot see a source of light.
The process of performing Radiosity on a scene Entire Room Lit: 1st Pass • Repeating the process for every patch in the room, gives us this scene. Everything is completely dark, except for surfaces that have received light from the sun. • So, this doesn't look like a very well lit scene. Ignore the fact that the lighting looks blocky; we can fix that by using many more patches. What's important to notice is that the room is completely dark, except for those areas that can see the sun. At the moment it's no improvement over any other renderer. Well, it doesn't end here. Now that some parts of the room are brightly lit, they have become sources of light themselves, and could well cast light onto other parts of the scene.
The process of performing Radiosity on a scene View from the patch after 1st Pass • Patches that could not see the sun, and so received no light, can now see the light shining on other surfaces. • So in the next pass, this patch will come out slightly lighter than the completely black it is now.
The process of performing Radiosity on a scene Entire Room Lit: 2nd Pass • This time, when you calculate the incident light on each patch in the scene, many patches that were black before are now lit. The room is beginning to take on a more realistic appearance. • What's happened is that sun light has reflected once from the floor and walls, onto other surfaces.
The process of performing Radiosity on a scene Entire Room Lit: 3rd Pass • The third pass produces the effect of light having reflected twice in the scene. Everything looks pretty much the same, but is slightly brighter.
The process of performing Radiosity on a scene • The next pass only looks a little brighter than the last, and even the 16 th is not a lot different. There's not much point in doing any more passes after that. • The radiosity process slowly converges on a solution. Each pass is a little less different than the last, until eventually it becomes stable. Depending on the complexity of the scene, and the lightness of the surfaces, it may take a few, or a few thousand passes. It's really up to you when to stop it, and call it done.
