computer graphics - the university of edinburgh · taku komura computer graphics & vtk 11...
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![Page 1: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/1.jpg)
Taku Komura Computer Graphics & VTK 1
Visualisation : Lecture 2
Computer Graphics:
Visualisation – Lecture 3
Taku [email protected]
Institute for Perception, Action & Behaviour
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Taku Komura Computer Graphics & VTK 2
Visualisation : Lecture 2
Last lecture .....● Visualisation can be greatly enhanced through the
use of 3D computer graphics– computer graphics are our tool in visualisation
● In order to do effective visualisation we need:– to know some computer graphics (this lecture)
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Taku Komura Computer Graphics & VTK 3
Visualisation : Lecture 2
Computer Graphics : simulation of light behaviour in 3D
● Effective simulation requires to model:– object representation (geometry)– object illumination (lighting)– camera model (vision)
— world to image plane projection— rendering: converting
graphical data into an image
3D scene (e.g. real world)
Object with
surfaces
Light Source(s)
Camera Model
Renderingon image plane
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Taku Komura Computer Graphics & VTK 4
Visualisation : Lecture 2
Overview of Computer Graphics● Data representation● Lighting
– illumination— Ambient— Diffuse— Specular
– Shading – Lighting and surface shape perception
● Camera model
![Page 5: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/5.jpg)
Taku Komura Computer Graphics & VTK 5
Visualisation : Lecture 2
Data Representation : 3D shape● Approximate smooth surfaces with flat,
planar polygons– polygons formed of edges & vertices
– vertex: positional point (2D or 3D)– edge: joins 2 vertices– polygon: enclosed within N edges
— polygons share common edges
– mesh: set of connected polygons forming a surface (or object)
Hie
rarc
hy o
f Sur
face
Rep
rese
ntat
ion
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Taku Komura Computer Graphics & VTK 6
Visualisation : Lecture 2
Surface mesh : examples
(close up)
● Several primitives utilised, triangles generally fastest to draw
– modern graphics cards : 20-225 million + triangles per second
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Taku Komura Computer Graphics & VTK 7
Visualisation : Lecture 2
Different resolutions alter perception of surface smoothness
resolution higher
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Taku Komura Computer Graphics & VTK 8
Visualisation : Lecture 2
Mesh Based Representation● 3D file formats:
– set of vertices in ℝ3
– polygons reference into vertex set— implicitly define edges
– e.g. vertex 0 0 0
vertex 0 1 0
....
polyon 3 2 1 3
polyon 3 5 6 8
...
● perform transformations only on vertices
#VRML V1.0 ascii# Separator { Material { ambientColor 0.2 0.2 0.2 diffuseColor 1.0 1.0 1.0 } Coordinate3 { point [ 4.455030 -1.193380 1.930940, 4.581220 -1.506290 1.320410, 4.219560 -1.875190 1.918070, 3.535530 1.858740 -3.007500, 3.793260 1.185430 -3.034130, 4.045080 1.545080 -2.500000, 3.510230 3.468900 0.803110, 3.556410 3.514540 0.000000, 3.919220 3.078210 0.405431,
....
.... IndexedFaceSet { coordIndex [ 0, 1, 2, -1, 3, 4, 5, -1, 6, 7, 8, -1, 9, 10, 11, -1, 12, 13, 14, -1, 15, 16, 17, -1, 18, 19, 20, -1, 21, 22, 23, -1,
....
....
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Taku Komura Computer Graphics & VTK 9
Visualisation : Lecture 2
Light interaction with surfaces● Simple 3 parameter model
– The sum of 3 illumination terms:• Ambient : 'background' illumination • Specular : bright, shiny reflections• Diffuse : non-shiny illumination and shadows
Object
Light source(here point light source)
surface normal (specifies surface orientation)
'Virtual' camera
![Page 10: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/10.jpg)
Taku Komura Computer Graphics & VTK 10
Visualisation : Lecture 2
Ambient Lighting– Light from the environment– light reflected or scattered from other objects– simple approximation to complex 'real-world' process– Result: globally uniform colour for object
— Rc = resulting intensity curve
— Lc = light intensity curve
— Oc = colour curve of object
Object
Uniform Light source
c
c
O
L
=
=
Colour Object
Colour Light
Rc
ccc OLR =
On
camera
Example: sphere
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Taku Komura Computer Graphics & VTK 11
Visualisation : Lecture 2
Diffuse Lighting
● Also known as Lambertian reflection– considers the angle of incidence of light on surface
(angle between light and surface normal)
– Result: lighting varies over surface with orientation to light
Object
Infinite point light source
On
Ln
c
nn
c
OLO
L
=−•=
=
Colour Object )(cos
Colour Light
θ
Rcθ
Rc=Lc Oc cos No dependence on camera angle!
Example: sphere(lit from left)
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Taku Komura Computer Graphics & VTK 12
Visualisation : Lecture 2
Specular Lighting● Direct reflections of light source off shiny
object– specular intensity n = shiny reflectance of object– Result: specular highlight on object
Object
On
Rc = colour curveR
n = camera position
S (Reflection)θ
Infinite point light source
Ln
No dependence on object colour.
c
nn
c
OLO
L
=−•=
=
Colour Object )(cos
Colour Light
θα
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Taku Komura Computer Graphics & VTK 13
Visualisation : Lecture 2
Specular Light
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Taku Komura Computer Graphics & VTK 14
Visualisation : Lecture 2
Combined Lighting Models● R
c = w
a(ambient) + w
d(diffuse) +w
s(specular)
– for relative weights wa, w
d, w
s
– also specular power n
+ + =
Ambient(colour)
Diffuse(directional)
Specular(highlights)
Rc
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Taku Komura Computer Graphics & VTK 15
Visualisation : Lecture 2
Are we supposed to do the computation of lighting at all the
points over the surface?
● Depends on the shading model– Flat Shading (once per polygon)– Gouraud shading (for all the vertex of the polygon)– Phong Shading (all the points)
![Page 16: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/16.jpg)
Taku Komura Computer Graphics & VTK 16
Visualisation : Lecture 2
Local Shading Models
● Flat Shading (less computation needed)● Gouraud shading ● Phong Shading (heavy computation needed)
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Taku Komura Computer Graphics & VTK 17
Visualisation : Lecture 2
How to compute the normal vectors?
● The mesh is a set of polygons● We can compute the normal vectors for each polygon
(triangle)● We can color the whole polygon using the same normal
vector (flat shading)
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Taku Komura Computer Graphics & VTK 18
Visualisation : Lecture 2
Flat Shading● Compute the color at the middle of the polygon● All points in the same polygon are colored by
the same color
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Taku Komura Computer Graphics & VTK 19
Visualisation : Lecture 2
Computing the normal vectors of the vertices
● We can compute the normals of the vertices by averaging the normal vectors of the surrounding triangles
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Taku Komura Computer Graphics & VTK 20
Visualisation : Lecture 2
Gouraud Shading (Smooth Shading)● Compute the color at each vertex first● Compute the color inside the polygon by interpolating the
colors of the vertices composing the polygon
Ln
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Taku Komura Computer Graphics & VTK 21
Visualisation : Lecture 2
Phong Shading ● interpolating the normal vectors at the vertices ● Do the computation of illumination at each point in the
polygon
Ln
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Flat shaded Flat shaded Utah Teapot Utah Teapot
Phong shaded Phong shaded Utah Teapot Utah Teapot
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Flat shaded spot lights Flat shaded spot lights Phong shaded Phong shaded spot lightsspot lights
• Gouraud shading is not good when the polygon count is low
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Comparison
• Summary– Phong shading is good but computationally costly– Flat shading is easy but results are too bad– Gouraud shading is usually used for simple applications
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Taku Komura Computer Graphics & VTK 25
Visualisation : Lecture 2
Surface Shape Perception - 1
3D surface of the skin from a medical scanner.
Diffuse lighting only.Light is coming from the top front
Perpendicular to light
Area perpendicular to the light can be recognized well
![Page 26: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/26.jpg)
Taku Komura Computer Graphics & VTK 26
Visualisation : Lecture 2
Surface Shape Perception - 2
3D surface of the skin from a medical scanner.
Diffuse + specular lighting.
Specular Power = 4.0
Edge of highlight
Area at the Edge of the highlight can be recognized well
![Page 27: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/27.jpg)
Taku Komura Computer Graphics & VTK 27
Visualisation : Lecture 2
Surface Shape Perception - 3
3D surface of the skin from a medical scanner.
Diffuse + specular lighting.
Specular Power = 200.0
Edge of highlight
![Page 28: Computer Graphics - The University of Edinburgh · Taku Komura Computer Graphics & VTK 11 Visualisation : Lecture 2 Diffuse Lighting Also known as Lambertian reflection – considers](https://reader031.vdocuments.mx/reader031/viewer/2022022521/5b246db37f8b9ad2478b4a65/html5/thumbnails/28.jpg)
Taku Komura Computer Graphics & VTK 28
Visualisation : Lecture 2
Perception of Shape
● Specular highlights– improve perception of surface shape features (e.g. nose)– ... but only where the highlight occurs
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Taku Komura Computer Graphics & VTK 29
Visualisation : Lecture 2
Enhancing the Perception of Shape
● Specular highlights– We can
— dynamically change the specular power, — Rotate the light — Rotate the viewpoint to enhance the perception of the shape >> changing the edge of the highlight
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Taku Komura Computer Graphics & VTK 30
Visualisation : Lecture 2
Other cues to shape● Texture
● The motion/direction of lines or patterns on the surface of the shape
● Stereo● Viewing depth with 2 eyes● Stereo displays frequently used for visualisation
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Taku Komura Computer Graphics & VTK 31
Visualisation : Lecture 2
Scene Coordinate Systems
Object A’sCoordinate System
3D (x,y,z)
Object B’sCoordinate System
3D (x,y,z)
Object CoordinatesWorld Coordinates-Light Position-Camera Position-Object Position(s)3D (x,y,z)
Object A’sTransform
Objects B’sTransform
Display transform-Window size-Position2D (x,y)
Camera transform- 4x4 Matrix (3D→2D)
-1 1
View Coordinates
1
-1
Display Coordinates
● Object / World co-ordinates: transformations and rotations in ℝ3
● Camera transform: 3D→2D matrix transformation (perspective projection).
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Taku Komura Computer Graphics & VTK 32
Visualisation : Lecture 2
Camera Model
Position
View Up
View angle
Front Clipping Plane
Direction of Projection
● Perspective projection (3D world → 2D image plane rendering)
– all rays into camera go through a common point● View Frustrum – 3D space viewable to camera
– bound by clipping planes (front, back); by camera view angle (top, bottom)
– clipping planes eliminate data that is too near or too distant from camera
Back Clipping Plane
View Frustrum
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Taku Komura Computer Graphics & VTK 33
Visualisation : Lecture 2
Summary● Computer Graphics (basics)
– representing object geometry as polygon meshes– illumination models (ambient, diffuse, specular)– Shading models (flat, Gouraud, Phong) – camera model & projection
● Next lecture: systems architectures for visualisation