Transcript Basic Shadow Methods Jim X. Chen George Mason University 11/7/2015 CS 752 Interactive Graphics Software.

Basic Shadow Methods
Jim X. Chen
George Mason University
11/7/2015
CS 752 Interactive Graphics Software
Light source
Light
Creator
Receiver
Shadow
Observation: Shadows are places light does not reach.
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Definitions
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Light sources
Shadow creator(occluder)s and receivers
Light source
Creator and
receiver
Creator
Receiver
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Shadows
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Shadow Volume
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Shadow planes
Volume formed from
shadow planes
Open and infinite
Inside in shadow outside in light
Must be clipped and
capped
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P o in t lig h t s o u rc e
o b je c t
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lig h t v e rte x
S h a d o w V o lu m e
Shadow Volume
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A point is shadowed iff it is in at least one shadow
volume
Light Cap
Side
Shadow Volume
Dark Cap (at infinity)
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Ray Tracing Shadows
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Ray tracing casts shadow feelers to a point light
source.
Many light sources are illuminated over a finite
area.
The shadows between these are substantially
different.
Area light sources cast soft shadows
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Penumbra
Umbra
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Soft Shadows
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Soft Shadows
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Umbra – No part of the light source is
visible.
Penumbra – Part of the light source is
occluded and part is visible (to a varying
degree).
Which part? How much? What is the Light
Intensity reaching the surface?
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Anti-Aliasing
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Supersampling
Jittering – Stochastic Method
6
10
2
13
3
14 12
8
15
0
7
11
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9
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1
eye
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Supersampling
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1 sample per pixel
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Supersampling
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16 samples per pixel
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Supersampling
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256 samples per pixel
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Monte Carlo Integration
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For each hit point, use a bundle of rays and
take the average - Expensive
Monte Carlo Approach
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Using a randomly chosen ray at each hit point
Average the value from each ray
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Result: Hard Shadow - Cube
Without antialiasing
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With antialiasing
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Result: Soft Shadow - Ball
Hard shadow
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Soft shadow
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More Shadow Algorithms
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Fake Shadow
Vertex Projection
Shadow Z-Buffer
Shadow Volume
…
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Fake Shadow
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No exact calculation
Approximation of shadow
position and shape
Estimated by center or
anchor of object
Pro: simple, fast
Contra: flat ground, only
ground shading, not
exact, rotate limitations
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Vertex Projection
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Object projected to ground
Exact mathematical
calculation
Pro: still simple, exact, no
rotate limitations
Contra: flat ground, only
ground shading
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Projection Shadows
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Project creator geometry onto receiver plane [Blinn]
Projection matrix M
p=Mv
l=(lx,ly,lz)
y
v
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y=0
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p
Projection Shadows
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Render projected polygons to an image (renderto-texture)
Apply image as a texture onto the receivers
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Advantage:
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Compute texture coords on the fly
Use projective texturing
Texture can be projected onto multiple shadow
receivers
Do not need to regenerate texture if static scene
Limitation: objects can either cast or receive a
shadow, not both
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Shadow Z-buffer
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Shadow Z-buffer
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Based on hidden surfaces
Light’s point of view rendering
into Z-Buffer
Camera’s point of view
rendering, lookup in Z-Buffer
Transformation between
camera and light view
Z-value compare - shadowed
or lit
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Shadow Z-buffer
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Render the scene twice
First from the light source
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Second from the camera
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Need z-buffer only
Each time we scan a pixel P (xv,yv,zv) transform
P to light space (xs,yz,zs) and test zs against
stored value in z-buffer from light
If less than or equal to this value it is lit
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Shadow Z-buffer
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“Less than or equal” test is imprecise
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it is only accurate in the image space of the
light
self-shadowing: small fudge factor
Imagine a shadow throw over complex objects
or long distances
point-sampling: area-sampling
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Shadow Volume Techniques
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Shadow Volumes
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Just like a polygon - you are inside a volume if
you need to cross a surface to exist it
General idea of shadow volumes is count the
number of shadow planes you cross
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+1 for front facing
-1 for back facing
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If total is >0 you are in shadow
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Special case if the eye itself is in shadow
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Shadow Volumes
Two stages:
1) Preprocessing
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Find all planes of the shadow volume and their
plane equations
2) At run-time
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Determine shadow plane count per pixel
Use a scan-line method OR stencil test
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Using Stencil Test
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Three steps:
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silhouette generation
drawing of shadow volume(s)
rendering the shadow
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Shadow Volume Stencil Test
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A stencil buffer is screen sized buffer (18bit) that stores a flag about a rendering
operation
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E.G. stencil[x,y] is negated if zbuffer[x,y] is
less than current z value (i.e. stencil is set if
and only if z buffer test passes)
Many uses in graphics
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Shadow Volume Stencil Test
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Render the scene into the RGB and z-buffer
Turn z-buffer writing off, then render all
shadow polygons with the stencil buffer
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Increment stencil count for front-facing
Decrement for back facing
Re-render scene with lighting OFF and only
render pixels where stencil is non-zero
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Shadow Volumes cont.
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Step 1: Silhouette generation
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boundaries between adjacent front-facing and
back-facing polygons  silhouette
adding light vertex to silhouette vertexes 
shadow planes
shadow planes together with object  shadow
volume
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Shadow Volumes cont.
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Step 2: Drawing of Shadow volume
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rendered in stencil buffer
P o in t lig h t s o u rc e
O b je c ts
C a m e ra
1
0
1
2
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Shadow Volumes cont.
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Step 3: Rendering the Shadow
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stencil buffer holds shadow
render a polygon using stencil buffer bits
shadow for one light created
Repeat for next light from step 1
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Shadow Volumes cont.
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Pro:
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greatly improved realism
hardware support of stencil buffer
Contra:
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high requirement on fill-rate
sharp shadow
additional scene management
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Shadow Volume BSP Trees
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Instead of calculating shadows in image
calculate in object space
Break up objects into shadowed and unshadowed objects
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Saves time shading pixels
More polygons (potentially many more)
Precision problems
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Starting the SVBSP Tree
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Continuing the SVBSP Tree
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Finishing the SVBSP Tree
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Can continue until ALL polygons are in the SVBSP
tree
Usually put shadow casting polygons in the tree
first, and then filter remaining polygons down the
tree to see if they are lit or un-lit
A polygon that ends up at an in-node is stored
there as shadowed but doesn’t force the tree to
be expanded
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Summary
Projected Polygons
Good: Simple, quick, and all hardware can do it.
Bad: Can only cast shadows to a plane.
Project to Create a Texture
Good: Semitransparent, can reuse texture created, and
can project shadow onto any surfaces.
Bad: Objects either cast or receive shadows, not both.
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Summary, continued
Shadow Maps
Good: Anything to anything, constant cost regardless of
complexity, map can sometimes be reused.
Bad: Only feasible on some hardware, frustum limited.
Shadow Volumes
Good: Anything can shadow anything, including selfshadowing, and the shadows are crisp.
Bad: shadow polygons must be generated and rendered
(lots of polygons & fill), CPU intensive.
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