18.1Si31_2001

SI31Advanced Computer

GraphicsAGR

SI31Advanced Computer

GraphicsAGR

Lecture 18Image-based Rendering

Light MapsWhat We Did Not Cover

Learning More...

18.2Si31_2001

Model-based RenderingModel-based Rendering

Conventional approach is:– create 3D model of a virtual world– project each object to 2D and

render into frame buffer Scene complexity is a major

factor– real-time walkthroughs of complex

scenes needs powerful processing– affects major application areas such

as computer games and VR

18.3Si31_2001

Image-based RenderingImage-based Rendering

Goal:– make rendering time independent

of scene complexity Approach:

– make use of pre-calculated imagery– many variations - we look just at

two Question:

– where have we met pre-calculated imagery before?

18.4Si31_2001

Image Caching - ImpostorsImage Caching - Impostors

Basic idea:– cache image of an object rendered in one frame

for re-use in subsequent frames Technique

– project bounding box of object onto image plane to get rectangular extent for that view

– capture image and put in texture memory– for next view, render an ‘impostor’ which is a

quadrilateral in plane parallel to initial view plane, and texture map with the original image

– texture mapping uses current view so image is warped appropriately

18.5Si31_2001

Image CachingImage Caching

Validity of impostors:– once view direction changes

substantially, the impostor is no longer valid

– object then re-rendered Hierarchical image caching:

– use BSP trees to cluster objects in a hierarchy

– distant objects can be clustered and a single image used to render the cluster

18.6Si31_2001

Environment Mapping - Revision

Environment Mapping - Revision

Pre-computation:– from object at centre of scene we

rendered 6 views and stored resulting images as walls of a surrounding box

– caches light arriving at object from different directions

Rendering time– specular reflection calculation then

bounced a viewing ray onto point on interior of box and used its colour as the specular colour of the object

18.7Si31_2001

Light FieldsLight Fields

Concept:– for every point, cache the light or

radiance emanating from that point in each direction

– rendering involves looking up (very large) table

– five dimensions: (x,y,z) to give position and ( to give direction

– in ‘free’ space, radiance constant along a line, so we have a 4D light field - we pre-compute the radiance along all lines in the space

18.8Si31_2001

Indexing the LinesIndexing the Lines

Use two parallel planes - think of these as between viewer and scene

u

v

s

tL(u,v,s,t)

For each pointon (u,v) grid, we havea line to every point on (s,t) grid- ie 4D set of lines - knownas light slab

viewer

scene

18.9Si31_2001

Constructing a Light FieldConstructing a Light Field

Place camera on (u,v) plane– for each point on grid ( ui, vj ),

render scene and store image as:

Imageij (sk, tl )– giving a 2D array of images!

Do this from all six surrounding directions of the scene - ie six light slabs

18.10Si31_2001

RenderingRendering

The rendering operation is now a linear look up operation on our 2D array of images

For example, any ray in a ray tracing approach will correspond to a particular 4D point (u,v,s,t) - we look up its value in the light field (using interpolation if it is not exactly on a grid point

18.11Si31_2001

CompressionCompression

Technique is only feasible because there is coherence between successive images

Hence the 2D array of images can be compressed by factors of over 100

18.12Si31_2001

Model-based versus Image-based Rendering

Model-based versus Image-based Rendering

virtual world

model

real-time interactive flythrough

real world

images

modelconstruction

real-timerendering

imageacquisition

image-basedrendering

off-linerendering

imageanalysis

18.13Si31_2001

Light Maps

18.14Si31_2001

The Problem with Gouraud….

The Problem with Gouraud….

Gouraud shading is established technique for rendering but has well known limitations– Vertex lighting only works well for

small polygons…– … but we don’t want lots of

polygons!

18.15Si31_2001

Pre-Compute the LightingPre-Compute the Lighting

Solution is to pre-compute some canonical light effects as texture maps

For example…

18.16Si31_2001

Rendering using Light Maps

Rendering using Light Maps

Suppose we want to show effect of a wall light– Create wall as a

single polygon– Apply vertex

lighting – Apply texture

map– In a second

rendering pass, apply light map to the wall

18.17Si31_2001

Light MapsLight Maps

Widely used in games industry

Latest graphics cards will allow multiple texture maps per pixel

18.18Si31_2001

What We Did Not Cover

18.19Si31_2001

Parametric Surface Representation

Parametric Surface Representation

Rather than require the user to represent curved surfaces as an ‘IndexedFaceSet’ of flat polygons, some modelling systems allow representation as Bezier or spline surfaces

Hearn & Baker Chap 10

18.20Si31_2001

Constructive Solid Geometry

Constructive Solid Geometry

Rather than model objects as surfaces, some systems work in terms of solid objects - field known as Constructive Solid Geometry (CSG)

Hearn & Baker, Chap 10

Primitive objects (sphere, cylinder, torus, ..) combined by operators (union, intersection, difference)

Result is always a solid

Rendering via ray tracing typically

18.21Si31_2001

Volume GraphicsVolume Graphics

A very new approach is to model using volumes - with varying transparency

OpenGL Volumizer adds this capability to OpenGL

See:www.sgi.com/software/volumizer

Mitsubishi Volume Pro 500 board– www.rtviz.com

18.22Si31_2001

Procedural ModellingProcedural Modelling

Objects can be defined procedurally - ie by mathematical functions

See Hearn & Baker, Chap 10

Fractals are well-known example

See The Fractory:library.advanced.org/

3288/

18.23Si31_2001

Other Important TopicsOther Important Topics

Colour Anti-aliasing Animation … and much more!

18.24Si31_2001

Learning MoreLearning More

Journals:– IEEE Computer Graphics and its

Applications– Computer Graphics Forum– Computers and Graphics

Conferences:– ACM SIGGRAPH (Proceedings as ACM

Computer Graphics)– Eurographics– Eurographics UK

18.25Si31_2001

The EndThe End