1gr2-00 gr2 advanced computer graphics agr lecture 9 adding realism through texture

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GR2Advanced Computer

GraphicsAGR

GR2Advanced Computer

GraphicsAGR

Lecture 9Adding Realism Through

Texture

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Adding RealismAdding Realism

Objects rendered using Phong reflection model and Gouraud or Phong interpolated shading often appear rather ‘plastic’ ‘plastic’ and ‘floating in ‘floating in air’air’

Addition of shadows (Lect 8) helps to plantplant the objects on a ground surface

In this lecture we look at how texture texture effects can be added to give more realistic looking surface appearance

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Adding Surface DetailAdding Surface Detail

The most obvious solution is not the best– breaking the scene into smaller and

smaller polygonal objects increases the detail

– ..BUT it is very hard to model and very time-consuming to render

Preferred solution is texture mapping – typically a 2D image ‘painted’ ‘painted’ onto

objects

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A Simple ExampleA Simple Example

Suppose we have a 2D image...

.. and a 3D box

.. we can paint the image on a face of the box

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Basic ConceptBasic Concept

Replace the shading calculation with a look-up into a texture map (ie 2D image) to get the colour of a pixel

May replace shaded value - or modulate it in some way

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QuestionQuestion

We could apply the texture in screen space (ie after projection)

... or we could apply it in object space (ie before projection)

Which is more sensible?

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Texture Mapping - Overview

Texture Mapping - Overview

screen space

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texture space

object space

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Texture Mapping - Screen to Object

Texture Mapping - Screen to Object

For each screen pixel (i,j), we calculate the corresponding object position (x,y,z)– we can do this during z-buffer rendering– apply inverse transformations to get

from projection co-ordinates back to modelling co-ordinates

– this gives the corresponding (x,y,z) position

How can we map the texture image to the object?

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Texture Mapping - Mapping Textures To

Objects

Texture Mapping - Mapping Textures To

Objects

We need to establish a mapping from texture space (u,v) to object space (x,y,z)– mapping functions

u = fu(x,y,z) and v = fv(x,y,z)– given a point (x,y,z) on an object,

these functions give us a position (u,v) in texture space - and hence an element in the texture image

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Texture Mapping - Mapping Textures to

Objects

Texture Mapping - Mapping Textures to

Objects

u = fu(x,y,z)

v = fv(x,y,z)

How can we calculate fu and fv?

texture space

object space

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Texture Mapping - Two Approaches

Texture Mapping - Two Approaches

There are two approaches: map texture image to entire

objectie fu, fv calculated per object

map texture image to each polygonal facet separatelyie fu, fv calculated per polygon

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Mapping Texture To Object

Mapping Texture To Object

This is achieved in two stages:

first: map texture to a simplesimple bounding shape

second: ‘project’ from bounding shape onto object itself

texture space

object spaceV

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Mapping to a CylinderMapping to a Cylinder

A simple bounding object for our bowl is a cylinder

We can wrap the texture around the cylinder as follows:– cylinder radius r, centre origin, has

equation

x = r cos , y = r sin , z– to wrap texture on to cylinder, we use

the mapping functions

u = = tan-1(y/x)

v = z

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ShrinkwrapShrinkwrap

We now need to ‘project’ from the bounding cylinder to the object

A common approach is shrinkwrappingshrinkwrapping

For an object position (x,y,z), we take the texture of the point (x’,y’,z’) on the bounding cylinder whose normal points at (x,y,z)

boundingcylinder

2d cross-section

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Intermediate Bounding Surfaces

Intermediate Bounding Surfaces

Other possible intermediate surfaces are:– box, sphere, plane

A simple default action is to calculate bounding box of object, map texture to box, and project from box to object

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Planar Texture MappingPlanar Texture Mapping

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Cylindrical Texture Mapping

Cylindrical Texture Mapping

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Spherical Texture MappingSpherical Texture Mapping

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Polygon Texture MappingPolygon Texture Mapping

This is now increasingly common approach

With each vertex of polygon, associate a texture co-ordinate

(x1,y1,z1)

(x2,y2,z2) (x3,y3,z3)

(u1,v1)

(u2,v2) (u3,v3)

So:u1=fu(x1,y1,z1)etc

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How to Texture Map Interior

How to Texture Map Interior

There are now two possibilities:– try to define fu, fv at every interior

point (x,y,z) by interpolation in the 3D object space

– relate the texture co-ordinates to the pixel positions (i,j) of the projected vertices and interpolate in 2D screen space

Discuss!

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Texture Mapping TrianglesTexture Mapping Triangles

(x1,y1,z1)

(x2,y2,z2) (x3,y3,z3)

(u1,v1)

(u2,v2) (u3,v3)

(i1,j1)

(i2,j2) (i3,j3)

Interpolation is doneduring scan conversion,similar as is done forGouraud interpolatedshading

But rather than interpolateto get RGB values, weget (u,v) values whichpoint to elements of texturemap.

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Texture ExtentTexture Extent

It is often useful to think of texture space having infinite extent

This can be achieved by replicating the image in texture space

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Summing UpSumming Up

We have seen how a 2D texture image can be mapped to an object, at the rendering stage– either per object or per polygon

The texture value is used to modifymodify the colour that would otherwise be drawn– options include replacing completely,

or modulating (eg by multiplying shaded value with texture value)

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AcknowledgementsAcknowledgements

Thanks to Alan Watt for the images again