Rendering

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Rendering

• ICS 415

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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.

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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.

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Features of Rendering

• shading how the color and brightness of a
  surface varies with lighting.
• texture-mapping a method of applying detail to
  surfaces.

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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.

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Features of Rendering

• shadows the effect of obstructing (blocking)
  light.
• soft shadows varying darkness caused by
  partially obscured light sources.

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Features of Rendering

• reflection mirror-like or highly glossy
  reflection.
• Transparency or opacity sharp transmission of
  light through solid objects.

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Features of Rendering

• translucency highly scattered transmission of
  light through solid objects.
• refraction bending of light associated with
  transparency.

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Features of Rendering

• diffraction bending, spreading and interference
  of light passing by an object or aperture (hole
  or crack) that disrupts the ray.

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Features of Rendering

• indirect illumination surfaces illuminated by
  light reflected off other surfaces, rather than.
  directly from a light source (also known as
  global illumination).

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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.

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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.

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Features of Rendering

• photorealistic morphing 3D renderings to appear
  more life-like.
• non-photorealistic rendering rendering of
  scenes in an artistic style, intended to look
  like a painting or drawing.

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Rendering Algorithms

• Global illumination
• Painter's algorithm
• Radiosity
• Ray tracing
• Scanline algorithms like Reyes
• Volume rendering
• Unbiased rendering
• Z-buffer algorithms

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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).

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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.

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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.

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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.

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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.

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Example-Global illumination
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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.

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Example-Global illumination
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Demo-Global illumination

• Vedio

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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/rad
  iosity/radiosity.htm)

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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.

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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.

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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.

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Radiosity
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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.

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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.

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Radiosity
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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.

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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.

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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.

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The process of performing Radiosity on a scene

• Now, lets choose one of the surfaces in the room,
  and consider the lighting on it.

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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.

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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?

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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.

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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.

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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.

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The process of performing Radiosity on a scene
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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.

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The process of performing Radiosity on a scene
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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.

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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.

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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.

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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.

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The process of performing Radiosity on a scene

• 4th 16th