16.1 Si23_03 SI23 Introduction to Computer Graphics Lecture 16 – Some Special Rendering Effects.

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SI23Introduction to Computer

Graphics

SI23Introduction to Computer

Graphics

Lecture 16 – Some Special Rendering Effects

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Where were we…?Where were we…?

Phong reflection model tells us how light reflects from surfaces…

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Phong Reflection ModelPhong Reflection Model

lightsourceN

LR

Veye

surface

I() = Ka()Ia() + ( Kd()( L . N ) + Ks( R . V )n ) I*() / dist

In practice, we evaluate IRED, IGREEN, IBLUE for red, green, blue intensities:IRED= Ka

REDIaRED + ( Kd

RED( L . N ) + Ks( R . V )n ) I*RED/dist

dist = distance attenuation factor

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Interpolated ShadingInterpolated Shading

Polygons can be efficiently shaded using:

– Flat shading– Gouraud shading– Phong shading

Visible surfaces are determined using the z-buffer algorithm

flat

Gouraud

Phong

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Flat, Gouraud and Phong Shading

Flat, Gouraud and Phong Shading

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Phong versus Gouraud Shading

Phong versus Gouraud Shading

A major advantage of Phong shading over Gouraud is that specular highlights tend to be much more accurate– vertex highlight is much sharper– a highlight can occur within a polygon

Also Mach banding greatly reduced The cost is a substantial increase in

processing time because reflection model applied per pixel

OpenGL only supports Gouraud shading

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Adding Realism Through Texture Effects

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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 air’‘floating in air’

Texture exture effects can be added to give more realistic looking surface appearance– Simple texture– Bumps– Light maps

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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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… or a teapot… or a teapot

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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

I

J

object space

during scan conversionof each polygon, findcorresponding positionof pixel on object

texture space

V

U

X

Y

Z

paint textureon to object

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Mapping Texture to Polygons

Mapping Texture to Polygons

For polygon texture mapping, we explicitly define the (u,v) co-ordinates of the polygon vertices

That is, we pin the texture at the vertices

We interpolate within the triangle at the time of scan converting into screen space

X

Z

Y

object

texture space

V

U

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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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Interpolation in Texture Space

Interpolation in Texture Space

The interpolation in texture space has to be done carefully

Equal steps in screen space do not correspond to equal steps in object space (and hence texture space)

Why?

U

V

I

Jscreen

texture

A line is a line in all 3 spaces

X

Z

Y

object

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Interpolation in Texture Space

Interpolation in Texture Space

The rate of change in texture space will depend on the depth of the points from the viewer

Correct approach is to scale by the distance (zP, zQ) of the points from the viewer

U

Vtexture

I

Jscreen

P Q

P’Q’

If Q further away than P, then as we take equal steps from P towards Q, we want to take increasingly large steps in (U,V) space from P’ to Q’.

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Interpolation in Texture Space

Interpolation in Texture Space

Suppose (uP, vP) and (uQ,vQ) are texture co-ords at end-points P, Q

Linear interpolation would be:

– u = uQ + (1-)uP with increasing from 0 to

1 (similarly for v) Correct texture

interpolation is:u = [ uQ / zQ + (1-)uP / zP ] /

Dwhere D = [ / zQ + (1-)/ zP ]

U

Vtexture

P’Q’

I

Jscreen

P Q

Note: this is equivalentto a linear interpolationin projective space

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Check for YourselfCheck for Yourself

Suppose P is one unit from viewer, and Q is two units from viewer

Show that the mid-point in screen space is equivalent to one-third of the distance along the line 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– for a polygon, we pin texture to vertices

and interpolate (correctly!) at scan conversion time

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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Bump Mapping

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Bump MappingBump Mapping

This is another texturing technique

Aims to simulate a dimpled or wrinkled surface– for example, surface of an orange

Like Gouraud and Phong shading, it is a tricktrick– surface stays the same– but the true normal is perturbed, or

jittered, to give the illusion of surface ‘bumps’

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Bump MappingBump Mapping

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How Does It Work?How Does It Work?

Looking at it in 1D:

original surface P(u)

bump map b(u)

add b(u) to P(u)in surface normal direction, N(u)

new surface normalN’(u) for reflectionmodel

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Bump MappingA Bump Map

Bump MappingA Bump Map

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Bump MappingResulting ImageBump Mapping

Resulting Image

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Bump Mapping - Another Example

Bump Mapping - Another Example

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Bump MappingAnother ExampleBump Mapping

Another Example

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Bump MappingProcedurally Defined Bump

Map

Bump MappingProcedurally Defined Bump

Map

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Another pioneerAnother pioneer

Jim Blinn– Creator of bump

mapping and many other graphics effects

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Light Maps

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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!

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Pre-Compute the LightingPre-Compute the Lighting

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

For example…

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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

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Light MapsLight Maps

Widely used in games industry

Latest graphics cards will allow multiple texture maps per pixel