UCLAN Lecture

Advanced Computer GraphicsShadow Techniques

CO2409 Computer Graphics

Week 20

Lecture Contents

1.Basic Shadows

2.Pre-calculated Shadows

3.Shadow Mapping

Basic Shadows

•Shadows in a scene to help resolvethe relative positions of models

–Naive mistake to think that shadowshappen automatically with lighting

•A couple of basic techniques havelong been used:

–Draw a “blob” under a model

–Draw the model flattened (scaled to 0 inY) on the floor

•Can project straight down or away fromthe light

•Both methods require a flat floor towork correctly

Positions Ambiguous?

Shadows Resolve

Basic Shadows

•These methods are still frequently used, why?

•Basic methods are enough to resolve model positions

•Very cheap techniques – good when many models

–More advanced techniques are complex and slow

•Advanced techniques often drawattention to themselves:

–Sharp edges / too stark, problems incomplex cases

•In wide-open areas, shadows will tendto blur towards blobs in any case

–Viewer may not notice / appreciate betteraccuracy

Basic Shadows arestill widely used

Pre-calculated Shadows

•However, improved shadows can give a better sense ofspace and/or atmosphere

•Can pre-calculate light / shadows for the static models andlights in the scene

•Commonly used are staticshadow maps / lightmaps

–Light / shadow textures applied overthe main model textures

•The maps are pre-calculated

–Using high-quality techniques

•Sometimes called baking theshadows / lighting

Lightmass from Unreal 3

Tool to bake shadow / lightmaps

Static Shadows / Dynamic Models

•Baked shadows work well forstatic environments

–High quality, generated offline

•Dynamic models also need to beaffected by these shadows

•So store samples of the staticlighting at points where dynamicmodels will be

–Both on the floor and in the air

•Model uses local static lightingsamples to affect how it is lit

Dynamic Shadow Mapping

•We also need to cast shadows from dynamic models

•Dynamic Shadow Mapping is an extension of render-totexture techniques used for shadows

–Also called perspective shadow mapping (PSM)

–Or more commonly, just shadow mapping

•The scene is rendered to a texture (a shadow map), from

the point of view of the light

•Then the scene is renderednormally, but each pixel firsttested against shadow map

–The pixel is not lit if it is in shadowfrom the light

Shadow Mapping Method

•Create a render target texture for each light:

–Each “pixel” in the texture is a single floating-point value

•Instead of four R,G,B & A values

–This is a floating-point texture

•Render the scene from the light’spoint of view

–Treat the light like a camera

•For each pixel, render thedistance at that pixel only

–Result is a like a depth buffer for thescene from the light’s point of view

–Sometimes called a light map or(confusingly) a shadow map

Shadow Mapping Method

•After creating a textures for each light’s point of view, wenext render the scene normally

–Using an extra step to find shadowed areas

•In this main render step, we check the pixel’s visibilityfrom each light before applying lighting to it:

–Find the distance from the pixel we’re rendering to the light

–Also find where this pixel would appear in the light’s depth map

–Find the distance stored in the light’s depth map at that point

–If the distance stored in the map is less than the actual distancefrom pixel to light, then something is obscuring the pixel

•The pixel in shadow from this light

–This condition determines whether the pixel gets a diffuse /specular contribution from this light or not

–Repeat for all lights on this pixel

Shadow Mapping Diagram

Determining if the redpixel is in shadow fromthe white light

It is – the troll’s head isin the way. The red pixelis hidden in the light map

Shadow Mapping Detail 1

•To treat a light as a camera, we need a view & projectionmatrix for it

–So we need: position, orientation, FOV and near / far clip planes

•A spotlight is the simplest case:

–Already has a position and facing direction – can make view matrix

–Has a field of view (FOV) for projection matrix

•A point light shines in all directions

–We make six cameras pointing in eachof the world axis directions

–A FOV of 90° for each one

–This is an example of a cube map

Point Light = 6 Cameras

Shadow Mapping Detail 2

•A directional light poses two problems:

–No position as a camera. No FOV to use

•Solve first problem by “positioning” the directional lightvery far away – outside the scene

•Solve the second by using anorthogonal projection matrix

•Project vertices from 3D to 2Dalong parallel lines

–Not towards camera point like standardperspective projection

–Don’t need FOV

Shadow Mapping Issues

•Shadow mapping has two key problems:

–Texels of shadow map may be visible, affects shadow quality

–Polygons sometimes self-shadow

•Increase shadow map resolution for better quality

–But lower performance, increased memory

–Better to adapt resolution based on distance from viewer

•Good idea to blur or soften the shadows / map

–Many methods here, e.g. Variance Shadow Mapping

•Self-shadowing resolved by tweaking calculated andcompared depth

–Other solutions illustrated in lab