CSL 859: Advanced Computer Graphics

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CSL 859 Advanced Computer Graphics

• Dept of Computer Sc. Engg.
• IIT Delhi

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Shadow Volumes
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Shadow Volumes
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Stencil Buffer

• Like color/z buffer one entry per pixel
• Traditionally a few bits of Z-buffer
• Stencil value also masks whether to render
• Render to Stencil
• Stencil operation
• void StencilFunc (enum func, int ref, uint mask
  ) NEVER, ALWAYS, LESS, LEQUAL, EQUAL,
  GEQUAL, GREATER, or NOTEQUALvoid
  StencilOp (enum sfail, enum zfail, enum zpass)
• KEEP, ZERO, REPLACE, INCR, DECR, INVERT
• glEnable(GL_STENCIL_TEST)

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Stencil Shadow Volume Overview

• Render the scene with shadow color
• For each light source
• Using the depth from pass 1, construct a mask in
  the stencil buffer enabling pixels not in shadow
• Shadow volume constructed by computing
  silhouettes of occluders
• And capping the volumes
• Render the scene again, lit this time.
• Stencil buffer masks out shadowed areas

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Stencil Generation

• Disable depth and color writes
• Set the stencil operation to increment on depth
  pass (counting shadows in front of the object)
• Enable back-face culling (ie front drawing)
• Draw the shadow volumes (ie front faces)
• Set the stencil operation to decrement on depth
  pass
• Enable front-face culling
• Draw the shadow volumes (ie back faces)

What if Eye is in shadow?
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Stencil Shadow Volumes
a
b
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Reverse Stencil

• Disable depth and color writes
• Enable front-face culling
• Set the stencil operation to increment on depth
  fail (counting shadows behind the object).
• Render the shadow volumes (ie back faces)
• Enable back-face culling
• Set the stencil operation to decrement on depth
  fail.
• Render the shadow volumes (ie front faces)

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Shadow Map Aliasing
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Perspective Shadow Maps

• Stamminger Grettakis 02

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Deep Shadow Map

• Deep shadow maps
• Instead of storing a depth at each pixel
• Store a series of depths w/ reduction of light
  transmitted
• Important for complex self-shadowing models
• Hair, clouds

Lokovic Veach 2000
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Deep Shadow Maps
Without self-shadowing
With self-shadowing
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Shadows from Area Light

• Accumulation buffer
• Higher resolution than final image
• Sample area light
• Render from multiple light-points
• Average

Heckbert Herf 97 Overlapping shadows have
undesired artifacts
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Fake Soft Shadows

• Extend shadow map Chan Durand 03
• Use extra primitives (smoothies) to soften shadows

lights view (blockers only)
lights view (blockers smoothies)
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Fake Soft Shadows

• Shadows not geometrically correct
• Shadows appear qualitatively like soft shadows

Hard shadows
Fake soft shadows
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Shadow Map Creation

• Render blockers into depth map

observers view
lights view
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Silhouette Detection

• Find blockers silhouette edges in object space

object-space silhouettes
observers view
lights view
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Construct Smoothies

• Blocker only

silhouette vertex
silhouette edges
blocker exterior
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Construct Smoothies

• Blocker smoothies

t
silhouette vertex
silhouette edges
t
smoothie edge
smoothie corner
blocker exterior
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Construct Smoothies

• Smoothie edges rectangles in screen space with a
  fixed width
• Smoothie corners connect adjacent smoothie edges

t
t
geometry
shading
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Render Smoothies

• Store depth and alpha values into smoothie buffer

Smoothie Buffer (alpha)
Smoothie Buffer (depth)
lights viewpoint
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Compute Shadows

• Compute intensity using depth comparisons

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Hard Shadow

• Image sample behind blocker (intensity 0)

light source
smoothie
blocker
receiver
completely in shadow
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Soft Shadows

• Image sample behind smoothie (intensity ?)

light source
smoothie
blocker
receiver
partially in shadow
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No Shadow

• Image sample illuminated (intensity 1)

light source
smoothie
blocker
receiver
illuminated