7.1 si31_2001 SI31 Advanced Computer Graphics AGR Lecture 7 Polygon Shading Techniques

7.1si31_2001

SI31Advanced Computer

GraphicsAGR

SI31Advanced Computer

GraphicsAGR

Lecture 7Polygon Shading Techniques

7.2si31_2001

Reflection ModelsReflection Models

We have seen how the reflected intensity at a point may be calculated– either by the Phong model or the

physically-based Cook and Torrance model

A reminder of the Phong reflection model...

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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

Note: R.V calculation replaced by H.N for speed - H = (L+V)/2

dist = distance attenuation factor

7.4si31_2001

Phong Reflection ModelPhong Reflection Model

Remember calculation depends on:– surface normal at a point– light source intensity and position– material properties– viewer position

L.N and H.N constant if L, V taken to be far away

7.5si31_2001

Viewing PolygonsViewing Polygons

We have also seen how a 3D polygon can be projected to screen space via a sequence of transformations

This lecture looksat how we shade the polygon, usingour reflection model

7.6si31_2001

Constant (or Flat) ShadingConstant (or Flat) Shading

Calculate normal (how?)

Assume L.N and R.V constant (light & viewer at infinity)

Calculate IRED, IGREEN, IBLUE using Phong reflection model

Use scan line conversion to fill polygon

N

lightviewer

7.7si31_2001

2D Graphics - Filling a Polygon

2D Graphics - Filling a Polygon

Scan line methods used to fill 2D polygons with a constant colour– find ymin, ymax of

vertices– from ymin to ymax do:– find intersection with

polygon edges– fill in pixels between

intersections using specified colour

See: Hearn&Baker, Ch 3- or try applet at:www.computing.edu.au/ ~chinkw/FillPoly

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Polygonal ModelsPolygonal Models

Recall that we use polygonal models to approximate curved surfaces

Constant shading will emphasise this approximation becauseeach facet will be constant shaded, with sudden change fromfacet to facet

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Flat ShadingFlat Shading

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Gouraud ShadingGouraud Shading

Gouraud shading attempts to smooth out the shading across the polygon facets

Begin by calculating the normal at each vertex

N

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Gouraud ShadingGouraud Shading

A feasible way to do this is by averagingaveraging the normals from surrounding facets

Then apply the reflection model to calculate intensitiesintensities at each vertex

N

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Gouraud ShadingGouraud Shading

We use linear linear interpolation interpolation to calculate intensity at edge intersection P

IPRED = (1-IP1

RED + IP2

RED

where P divides P1P2 in the ratio

Similarly for Q

P4

P2

P1

P3

PQ

1-

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Gouraud ShadingGouraud Shading

Then we do further linear interpolation to calculate colour of pixels on scanline PQ

P2

P1

P3

PQ

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Gouraud ShadingGouraud Shading

7.15si31_2001

Gouraud Shading Limitations - Specular

Highlights

Gouraud Shading Limitations - Specular

Highlights

Gouraud shading gives intensities within a polygon which are a weighted average of the intensities at vertices– a specular highlight at a vertex

tends to be smoothed out over a larger area than it should cover

– a specular highlight in the middle of a polygon will never be shown

7.16si31_2001

Gouraud Shading Limitations - Mach Bands

Gouraud Shading Limitations - Mach Bands

The rate of change of pixel intensity is even across any polygon, but changes as boundaries are crossed

This ‘discontinuity’ is accentuated by the human visual system, so that we see either light or dark lines at the polygon edges - known as Mach bandingMach banding

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Phong ShadingPhong Shading

Phong shading has a similar first step, in that vertex normals are calculated - typically as average of normals of surrounding faces

N

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Phong ShadingPhong Shading

However rather than calculate intensity at vertices and then interpolate intensities as we do in Gouraud shading ...

In Phong shading we interpolate normals at each pixel ...

P4

P2

P1

P3

P Q

N2

N1

N

7.19si31_2001

Phong ShadingPhong Shading

... and apply the reflection model at each pixel to calculate the intensity - IRED, IGREEN, IBLUE

P4

P2

P1

P3

P Q

N2

N1

N

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Phong ShadingPhong 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

But there are limitations to both Gouraud and Phong

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Gouraud versus PhongGouraud versus Phong

7.23si31_2001

Interpolated Shading Limitations - Perspective

Effects

Interpolated Shading Limitations - Perspective

Effects

Anomalies occur because interpolation is carried out in screen space, after the perspective transformation

Suppose P2 much more distant than P1. P is midway in screen space so gets 50 : 50 intensity (Gouraud) or normal (Phong)

... but in world coordinates it is much nearer P1 than P2

P4

P2

P1

P3

PQ

7.24si31_2001

Interpolated Shading Limitations - Averaging

Normals

Interpolated Shading Limitations - Averaging

Normals

Averaging the normals of adjacent faces usually works reasonably well

But beware corrugated surfaces where the averaging unduly smooths out the surface

7.25si31_2001

Wall LightsWall Lights

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Wall Lights with Fewer Polygons

Wall Lights with Fewer Polygons

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Final Note on NormalsFinal Note on Normals

If a sharp polygon boundary is required, we calculate two vertex normals for each side of the joint

NLEFT NRIGHT

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Simple Shading -Without Taking Account of

Normals

Simple Shading -Without Taking Account of

Normals

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Constant or Flat Shading -Each Polygon has Constant

Shade

Constant or Flat Shading -Each Polygon has Constant

Shade

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Gouraud ShadingGouraud Shading

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Phong ShadingPhong Shading

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Phong Shading with Curved Surfaces

Phong Shading with Curved Surfaces

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Better Illumination ModelBetter Illumination Model

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Further StudyFurther Study

Hearn and Baker, section 14-5 Watt, chapter 6 Think about the relative

computational costs of flat, Gouraud and Phong

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AcknowledgementsAcknowledgements

Thanks again to Alan Watt for the images

The following sequence is the famous Shutterbug from Foley et al