Shadow

No, not the Hank Marvin kind!

Go to:

• http://developer.nvidia.com/object/doc_shadows.html

• Particularly, read the paper “shadow considerations” from 2004

Shadow solutions: 5 Categories

• Pre computed (texture based)• Volume based• Depth based• Hybrid• Raytrace

Pre computed shadows

• Lightmaps• Use textures to store lighting information• “Bake” textures using an offline process• Modulate the base texture with the lighting

texture to get the final result• Used in many games since Quake made the

technique popular

Pre computed shadows

• The “baking” process can be computationally heavy (using global illumination, radiosity)

• Have a look at “Turtle” http://www.illuminatelabs.com/

• They used an SGI machine for Quake• Textures take up memory (console issue)• Quality of lighting can be very nice because all

computation is done “offline”• Interesting factoid: Halo3 uses banks of XBOX360’s to

compute the lighting

Volume based shadows

• Uses a buffer called a “stencil buffer”• Often called stencil shadows• What we need to know, is whether a surface is being

shadowed by another surface• This involves a test to determine whether the

surface in question is occluded by the volume of the blocker surface

• We can do this by determining the volume of the space behind the blocker (with respect to the light source)

Volume based shadows

• First implementation seems to be “Severance blade of darkness”

• Actually, there was one called “into the shadows” by scavenger, but the company folded

• Carmack popularised them• Read Mark Kilgards slides on the zpass/zfail issues• There are new techniques working with them (see

nvidia site)

Depth based shadows• Use some kind of buffer to store a depth per light• This depth is the closest surface the light can “see”• Its basically a depth buffer (like the z buffer we use for surface

sorting)• If the distance from the light to the surface > than the closest depth

in the depth buffer, then the object is in shadow (i.e. Something is in front of it when looking towards the light)

• This all happens when rendering the object, basically it is a texture lookup

• BUT, its not without limitations (primarily sampling)• http://developer.nvidia.com/object/hwshadowmap_paper.html• http://developer.nvidia.com/object/cedec_shadowmap.html

Depth based shadows

• Good for complex geometry (because we are rendering with hardware, which is fast)

• Can be complex to handle the sampling issues properly in a scene

Hybrid shadows

• Take the better parts of precomputed, volume and image based shadows

• Use the appropriate method for the type of scene and shadowing requirements

• Can be complicated to get into an engine/shader

• Ideally, we’d prefer to have a single solution that just works!

Raytracing

• Physically correct• Handles complex light interactions with a scene, for

instance sub-surface scattering, reflection and refraction

• Computationally expensive• But seems we are getting more horsepower all the

time!• http://developer.nvidia.com/docs/IO/8230/GDC2003_

Gaffer2GameEngine.pdf• Consider that new hardware is going to offer more in

this area

Side issue: Deferred rendering

• See Hargreaves GDC presentation (2004)• http://http.developer.nvidia.com/GPUGems2/

gpugems2_chapter09.html• I’ve seen interviews with videocard vendors

that state that deferred shading is going to be used a lot more in the future.

• Crytek uses it for Cryengine• http://ati.amd.com/developer/siggraph06/We

nzel-Real-time_Atmospheric_Effects_in_Games.pdf

Different types of shadow map

• PSSM (Parallel split)• CSM (Cascaded)• Have a look at a few others too!