Shadows

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Shadows

• Computer Graphics

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Shadows
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Shadows

• Extended light sources produce penumbras
• In real-time, we only use point light sources
• Extended light sources can be simulated using
  multiple point light sources
• Due to inter-objectreflection, even pointlight
  sources can formpenumbras

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Shadows

• Most real-time shadow algorithms produce hard
  shadows rather than soft shadows
• Hard shadows contain no penumbra
• Soft shadows contain a penumbra
• Additionally, most real-time shadow algorithms
  only produce geometric shadows
• Only the shape of the shadow is computed, but not
  its internal intensity

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Shadow Algorithms

• There are many different real-time shadow
  algorithms (no clear winner yet)
• Projected shadows
• Shadow Z-buffer
• Shadow volumes

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Projected Shadows

• Project the vertices of the occluder onto the
  ground/wall plane based on the location of the
  light source
• Convex hull of projected vertices form the shadow
  polygon
• The shadow polygon can then be rendered in
  shadow color

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Projected Shadows

• Main drawback of projected shadows is that they
  only project properly onto flat surfaces

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Shadow Z-Buffer

• Shadow computation is integrated into Z-buffer
  computation
• Additional buffer (called a Shadow Z-buffer) is
  needed
• Forming the shadow is a 2 step process
• Computation of depth information
• Render scene with modified Z-buffer algorithm

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Shadow Z-Buffer

• To compute the depth information
• The scene is rendered from the point of view of
  the light
• No intensities (colors) are stored in the
  framebuffer
• Depth information is stored in the Shadow
  Z-buffer
• Thus, the Shadow Z-buffer contains pixel-based
  information on the depth of objects from the
  light
• In particular, it contains the distance from the
  light source to the surface nearest the light
  source

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Shadow Z-Buffer

• Next render the scene the normal was from the
  point of view of the camera, using a modified
  z-buffer test
• If point is visible (i.e. passes normal z-buffer
  test), a coordinate transform is used to map the
  point into the coordinate system of the light
• The (x, y) parts of the transformed point are
  used to lookup the value in the Shadow Z-buffer
• If the transformed z value is greater than the
  value stored in the Shadow Z-buffer then the
  point is in shadow and is rendered using a
  shadow intensity

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Shadow Z-Buffer
Fails depth test
Passes depth/shadow tests
Passes depth/Fails shadow

• Note that one needs a separate Shadow z-buffer
  for each light source in the scene

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Shadow Volumes

• A volume is swept out in space by the shadow of
  the occluder

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Shadow Volumes

• Of course, the shadow volume is not actually
  rendered, but is used to determine the shadowed
  areas on the receiver

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Shadow Volume

• To compute the shadow volume one must first find
  the silhouette edge of the occluder
• The silhouette edge is made up of edges of the
  object that separate faces that are lit from
  faces that are facing away from the light source
• Rays are cast from the light source through the
  silhouette edges to form the shadow volume

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Shadow Volume

• Any polygons that lie inside the shadow volume
  are in shadow

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Shadow Volume Implementation

• The first step was to find the silhouette edge
• Preprocess the object so that for each polygon
  you have a list of all connecting faces
• Perform the back-face culling test on the faces
  in the object from the point of view of the light
• Include any edge between a culled face and a
  non-culled face as a silhouette edge

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Shadow Volume Implementation

• Next form the shadow volume from the silhouette
  edge
• Form long quads with the edge vertices as 2
  points and the other 2 points determined by the
  rays from the light through the edge vertices

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Shadow Volume Implementation

• Piercing is a potential issue to think about when
  creating the shadow volume
• Does the occluder cast shadows through walls
• The Shadow Volume is then to create a shadow
  using a multi-pass rendering of the scene using
  the stencil buffer

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Stencil Buffer

• A Stencil Buffer is another buffer like the color
  (frame) buffer and the Z-buffer
• On most hardware this is an 8 bit buffer
• The Stencil Test is a test that occurs
  immediately before the Z-buffer test
• If the Stencil Test fails, the fragment is
  discarded rather than passed on to the Z-buffer
• Fragment wont show up in the color buffer either

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Stencil Buffer

• The Stencil Test is a test of the value currently
  stored in the Stencil Buffer
• The Stencil Function controls the type of Stencil
  Test to perform

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Stencil Buffer

• There are 3 possible outcomes of the combined
  Stencil and Z-buffer tests
• Stencil Test fails
• Stencil Test passes, Depth Test fails
• Stencil Test passes, Depth Test passes
• The Stencil Operation lets you specify what
  happens to the Stencil Buffer values in each of
  these cases

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Stencil Buffer

• The possible Stencil Operations are
• GL_KEEP ? old value kept
• GL_ZERO ? value replaced by zero
• GL_REPLACE ? value replaced by given reference
  value (allows setting to a specific value)
• GL_INCR ? value incremented by 1
• GL_DECR ? value decremented by 1
• GL_INVERT ? value bitwise inverted

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Stencil Buffer

• The Stencil buffer/test are only performed if
• The Stencil buffer is enabled
• The underlying hardware/drivers support it
• The stencil buffer is usually used as a mask
• Example creating a non-rectangular windshield
  for a first-person driving game
• Store 1s in the stencil buffer pixels where you
  wish the windshield to be and then set the
  stencil test to render if not equal to 0

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Shadow Volume Implementation

• Here is a version of the multi-pass algorithm for
  shadow volume rendering
• Render the scene as if it was entirely in shadow
  (ambient light only)
• Render front-facing shadow volume polygons to
  determine if any object polygons are behind them
• Render back-facing shadow volume polygons to
  determine if any objects from the second pass are
  in front of them
• Render the entire scene again, overwriting the
  results of the first pass with polygons that are
  not in shadow

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Shadow Volume Implementation

• A slightly modified version of the multi-pass
  algorithm
• Render the full scene normally (full lighting)
• Render front-facing shadow volume polygons to
  determine if any object polygons are behind them
• Render back-facing shadow volume polygons to
  determine if any objects from the second pass are
  in front of them
• Render the scene again, with only ambient light,
  overwriting the results of the first pass with
  polygons that are in shadow

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Shadow Volume Implementation

• So where does the stencil buffer come in?
• The stencil buffer will be used as a mask for our
  last rendering (either masking off all polygons
  that are in shadow or not in shadow depending
  on the version of the algorithm)
• The two shadow volume passes are used to create
  the mask in the stencil buffer

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Shadow Volume Implementation

• To calculate the stencil buffer values
• Disable the frame buffer and z-buffer writing,
  leaving depth testing enabled
• Initialize the stencil buffer to 0
• Set the stencil function to always pass
• This turns off masking ability of stencil buffer
• Set the stencil operation to increment if depth
  test passes
• Will increase the value in the stencil buffer if
  the newly rendered polygon has a depth closer
  than anything previously rendered

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Shadow Volume Implementation

• Enable back-face culling
• Only front-facing polygons will be rendered
• Render the shadow volume
• Given the previous setup
• No color values are changed in the frame buffer
• No z-values are changed in the z-buffer
• Only front-facing shadow polygons are processed
• The stencil buffer is incremented for each pixel
  where there is an object polygon behind the
  front-facing shadow polygon

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Shadow Volume Implementation

• Set the stencil operator to decrement if depth
  test passes
• Will decrease the value in the stencil buffer if
  the newly rendered polygon has a depth closer
  than anything previously rendered
• Enable front-face culling instead of back-face
• Only back-facing polygons will be rendered
• Render the shadow volume
• Given the previous setup
• No color values are changed in the frame buffer
• No z-values are changed in the z-buffer
• Only back-facing shadow polygons are processed
• The stencil buffer is decremented for each pixel
  where there is an object polygon behind the
  front-facing shadow polygon

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Shadow Volume Implementation

• The end result of this is a stencil buffer that
  has a positive value if the pixel at that
  location is in shadow
• Incremented because front-shadow in front of
  object polygon
• Not decremented because back-shadow is not in
  front of object polygon
• This stencil buffer can then be used as a mask by
  setting the stencil function to equality with
  zero or greater that zero depending on if you
  want to mask object polygons that are in shadow
  or not in shadow

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Planar Reflection

• Environment maps reflect a static texture mapped
  world onto a dynamic object
• Key is texture projection functions based on
  reflection vectors
• Planar reflections reflect a dynamic world onto a
  static plane
• Key is multipass rendering with a stenciling
  technique

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Planar Reflection
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Planar Reflection

• The basic concept is to draw the objects you want
  reflected twice
• First in their reflected position
• Then in their normal position
• In between these renderings the reflective planar
  surface needs to be drawn as a semi-transparent
  object whose color is blended with the color of
  the reflected object

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Planar Reflection
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Planar Reflection

• If the viewer can see under the reflective plane
  then the illusion is lost
• We can maintain the illusion even in these
  circumstances by using the stencil buffer

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Planar Reflection

• Note that what we want is for the reflected scene
  to only show up in the area of the reflective
  surface
• i.e. mask the rendering of the reflected scene
• The mask is, of course, the stencil buffer
• We setup the stencil buffer by rendering just the
  reflective surface

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Planar Reflection

• So the steps involved are
• Render the reflective surface to setup the
  stencil buffer as a mask
• Frame buffer should be turned off during this
  step
• Render the reflected scene
• Scaling can be used to perform the reflection
• The stencil buffer should be used as a mask
• Render the reflective surface again
• Semi-transparent and blended into the background
• Render the scene again (not reflected this time)