Advanced Computer Graphics Shadow 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 resolve the relative positions of models –Naive mistake to think that shadows happen automatically with lighting A couple of basic techniques have long been used: –Draw a “blob” under a model –Draw the model flattened (scaled to 0 in Y) on the floor Can project straight down or away from the light Both methods require a flat floor to work 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 draw attention to themselves: –Sharp edges / too stark, problems in complex cases In wide-open areas, shadows will tend to blur towards blobs in any case –Viewer may not notice / appreciate better accuracy Basic Shadows are still widely used

Pre-calculated Shadows However, improved shadows can give a better sense of space and/or atmosphere Can pre-calculate light / shadows for the static models and lights in the scene Commonly used are static shadow maps / lightmaps –Light / shadow textures applied over the main model textures The maps are pre-calculated –Using high-quality techniques Sometimes called baking the shadows / lighting Lightmass from Unreal 3 Tool to bake shadow / lightmaps

Static Shadows / Dynamic Models Baked shadows work well for static environments –High quality, generated offline Dynamic models also need to be affected by these shadows So store samples of the static lighting at points where dynamic models will be –Both on the floor and in the air Model uses local static lighting samples 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-to texture 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 rendered normally, but each pixel first tested against shadow map –The pixel is not lit if it is in shadow from 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’s point of view –Treat the light like a camera For each pixel, render the distance at that pixel only –Result is a like a depth buffer for the scene 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, we next render the scene normally –Using an extra step to find shadowed areas In this main render step, we check the pixel’s visibility from 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 distance from 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 red pixel is in shadow from the white light It is – the troll’s head is in the way. The red pixel is hidden in the light map

Shadow Mapping Detail 1 To treat a light as a camera, we need a view & projection matrix 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 each of 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 light very far away – outside the scene Solve the second by using an orthogonal projection matrix Project vertices from 3D to 2D along parallel lines –Not towards camera point like standard perspective 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 and compared depth –Other solutions illustrated in lab