david luebke2/16/2016 cs 551 / 645: introductory computer graphics mathematical foundations the...
DESCRIPTION
David Luebke2/16/2016 XForms Intro l Xforms: a toolkit for easily building Graphical User Interfaces, or GUIs –See –Lots of widgets: buttons, sliders, menus, etc. –Plus, an OpenGL canvas widget that gives us a viewport or context to draw into with GL or Mesa. l Quick tour now l You’ll learn the details yourself in Assignment 1TRANSCRIPT
David Luebke 05/04/23
CS 551 / 645: Introductory Computer Graphics
Mathematical FoundationsThe Rendering Pipeline
David Luebke 05/04/23
Administrivia
Sorry about the canceled class OpenGL text: v1.1 or v1.2? Teddy link fixed Go over Assignment 1
David Luebke 05/04/23
XForms Intro
Xforms: a toolkit for easily building Graphical User Interfaces, or GUIs– See http://www.westworld.com/~dau/xforms– Lots of widgets: buttons, sliders, menus, etc.– Plus, an OpenGL canvas widget that gives us a
viewport or context to draw into with GL or Mesa. Quick tour now You’ll learn the details yourself in
Assignment 1
David Luebke 05/04/23
Mathematical Foundations
FvD appendix gives good review I’ll give a brief, informal review of some of the
mathematical tools we’ll employ– Geometry (2D, 3D)– Trigonometry– Vector and affine spaces
Points, vectors, and coordinates
– Dot and cross products– Linear transforms and matrices
David Luebke 05/04/23
2D Geometry
Know your high-school geometry:– Total angle around a circle is 360° or 2π radians– When two lines cross:
Opposite angles are equivalent Angles along line sum to 180°
– Similar triangles: All corresponding angles are equivalent Corresponding pairs of sides have the same length ratio
and are separated by equivalent angles Any corresponding pairs of sides have same length ratio
David Luebke 05/04/23
Trigonometry
Sine: “opposite over hypotenuse” Cosine: “adjacent over hypotenuse” Tangent: “opposite over adjacent” Unit circle definitions:
– sin () = x– cos () = y– tan () = x/y– Etc…
(x, y)
David Luebke 05/04/23
3D Geometry
To model, animate, and render 3D scenes, we must specify:– Location– Displacement from arbitrary locations– Orientation
We’ll look at two types of spaces:– Vector spaces– Affine spaces
We will often be sloppy about the distinction
David Luebke 05/04/23
Vector Spaces
Two types of elements:– Scalars (real numbers): … – Vectors (n-tuples): u, v, w, …
Supports two operations:– Addition operation u + v, with:
Identity 0 v + 0 = v Inverse - v + (-v) = 0
– Scalar multiplication: Distributive rule:(u + v) = (u) + (v)
( + )u = u + u
David Luebke 05/04/23
Vector Spaces
A linear combination of vectors results in a new vector:
v = 1v1 + 2v2 + … + nvn
If the only set of scalars such that 1v1 + 2v2 + … + nvn = 0
is 1 = 2 = … = 3 = 0 then we say the vectors are linearly independent The dimension of a space is the greatest number of linearly
independent vectors possible in a vector set For a vector space of dimension n, any set of n linearly
independent vectors form a basis
David Luebke 05/04/23
Vector Spaces: A Familiar Example Our common notion of vectors in a 2D plane
is (you guessed it) a vector space:– Vectors are “arrows” rooted at the origin– Scalar multiplication “streches” the arrow, changing
its length (magnitude) but not its direction– Addition uses the “trapezoid rule”:
u+vy
xu
v
David Luebke 05/04/23
Vector Spaces: Basis Vectors
Given a basis for a vector space:– Each vector in the space is a unique linear
combination of the basis vectors– The coordinates of a vector are the scalars from
this linear combination– Best-known example: Cartesian coordinates
Draw example on the board
– Note that a given vector v will have different coordinates for different bases
David Luebke 05/04/23
Vectors And Points
We commonly use vectors to represent:– Points in space (i.e., location)– Displacements from point to point– Direction (i.e., orientation)
But we want points and directions to behave differently– Ex: To translate something means to move it
without changing its orientation– Translation of a point = different point– Translation of a direction = same direction
David Luebke 05/04/23
Affine Spaces
To be more rigorous, we need an explicit notion of position
Affine spaces add a third element to vector spaces: points (P, Q, R, …)
Points support these operations– Point-point subtraction: Q - P = v
Result is a vector pointing from P to Q– Vector-point addition: P + v = Q
Result is a new point
– Note that the addition of two points is not definedP
Q
v
David Luebke 05/04/23
Affine Spaces
Points, like vectors, can be expressed in coordinates– The definition uses an affine combination– Net effect is same: expressing a point in terms of a
basis Thus the common practice of representing
points as vectors with coordinates (see FvD) Analogous to equating pointers & integers in C
– Be careful to avoid nonsensical operations
David Luebke 05/04/23
Affine Lines: An Aside
Parametric representation of a line with a direction vector d and a point P1 on the line:
P() = Porigin + d Restricting 0 produces a ray Setting d to P - Q and restricting 0 1
produces a line segment between P and Q
David Luebke 05/04/23
Dot Product
The dot product or, more generally, inner product of two vectors is a scalar:
v1 • v2 = x1x2 + y1y2 + z1z2 (in 3D) Useful for many purposes
– Computing the length of a vector: length(v) = sqrt(v • v)
– Normalizing a vector, making it unit-length– Computing the angle between two vectors:
u • v = |u| |v| cos(θ)
– Checking two vectors for orthogonality– Projecting one vector onto another
θu
v
David Luebke 05/04/23
Cross Product
The cross product or vector product of two vectors is a vector:
The cross product of two vectors is orthogonal to both
Right-hand rule dictates direction of cross product
1221
1221
1221
21 )(y x y xz x z x
z y z yvv
David Luebke 05/04/23
Linear Transformations
A linear transformation: – Maps one vector to another– Preserves linear combinations
Thus behavior of linear transformation is completely determined by what it does to a basis
Turns out any linear transform can be represented by a matrix
David Luebke 05/04/23
Matrices
By convention, matrix element Mrc is located at row r and column c:
By (OpenGL) convention, vectors are columns:
mnm2m1
2n2221
1n1211
MMM
MMMMMM
M
3
2
vvv
v1
David Luebke 05/04/23
Matrices
Matrix-vector multiplication applies a linear transformation to a vector:
Recall how to do matrix multiplication
z
y
x
vvv
MMMMMMMMM
vM333231
232221
131211
David Luebke 05/04/23
Matrix Transformations
A sequence or composition of linear transformations corresponds to the product of the corresponding matrices– Note: the matrices to the right affect vector first– Note: order of matrices matters!
The identity matrix I has no effect in multiplication
Some (not all) matrices have an inverse: vvMM 1
David Luebke 05/04/23
The Rendering Pipeline: A Whirlwind Tour
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
David Luebke 05/04/23
The Display You Know
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
David Luebke 05/04/23
The Framebuffer You Know
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
David Luebke 05/04/23
The Rendering Pipeline
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
Why do we call it a pipeline?
David Luebke 05/04/23
2-D Rendering: Rasterization(Coming Soon)
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
David Luebke 05/04/23
The Rendering Pipeline: 3-D
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
David Luebke 05/04/23
The Rendering Pipeline: 3-D
ModelingTransforms
Scene graphObject geometry
LightingCalculations
ViewingTransform
Clipping
ProjectionTransform
Result:Result:• All vertices of scene in shared 3-D “world” coordinate All vertices of scene in shared 3-D “world” coordinate systemsystem
• Vertices shaded according to lighting modelVertices shaded according to lighting model
• Scene vertices in 3-D “view” or “camera” coordinate Scene vertices in 3-D “view” or “camera” coordinate systemsystem
• Exactly those vertices & portions of polygons in view Exactly those vertices & portions of polygons in view frustumfrustum
• 2-D screen coordinates of clipped vertices2-D screen coordinates of clipped vertices
David Luebke 05/04/23
The Rendering Pipeline: 3-D
Scene graphObject geometry
LightingCalculations
Clipping
Result:Result:• All vertices of scene in shared 3-D “world” coordinate All vertices of scene in shared 3-D “world” coordinate systemsystem
• Vertices shaded according to lighting modelVertices shaded according to lighting model
• Scene vertices in 3-D “view” or “camera” coordinate Scene vertices in 3-D “view” or “camera” coordinate systemsystem
• Exactly those vertices & portions of polygons in view Exactly those vertices & portions of polygons in view frustumfrustum
• 2-D screen coordinates of clipped vertices2-D screen coordinates of clipped vertices
ModelingTransforms
ViewingTransform
ProjectionTransform
David Luebke 05/04/23
Rendering: Transformations
So far, discussion has been in screen space But model is stored in model space
(a.k.a. object space or world space) Three sets of geometric transformations:
– Modeling transforms– Viewing transforms– Projection transforms
David Luebke 05/04/23
Rendering: Transformations
Modeling transforms– Size, place, scale, and rotate objects parts of the
model w.r.t. each other– Object coordinates world coordinates
Z
X
Y
X
Z
Y
David Luebke 05/04/23
Rendering: Transformations
Viewing transform– Rotate & translate the world to lie directly in front of
the camera Typically place camera at origin Typically looking down -Z axis
– World coordinates view coordinates
David Luebke 05/04/23
Rendering: Transformations
Projection transform– Apply perspective foreshortening
Distant = small: the pinhole camera model
– View coordinates screen coordinates
David Luebke 05/04/23
Rendering: Transformations
All these transformations involve shifting coordinate systems (i.e., basis sets)
That’s what matrices do… Represent coordinates as vectors, transforms
as matrices
Multiply matrices = concatenate transforms!
YX
YX
cossinsincos
David Luebke 05/04/23
Rendering: Transformations
Homogeneous coordinates: represent coordinates in 3 dimensions with a 4-vector– Denoted [x, y, z, w]T
Note that w = 1 in model coordinates
– To get 3-D coordinates, divide by w:[x’, y’, z’]T = [x/w, y/w, z/w]T
Transformations are 4x4 matrices Why? To handle translation and projection
David Luebke 05/04/23
Rendering: Transformations
OpenGL caveats:– All modeling transforms and the viewing transform
are concatenated into the modelview matrix– A stack of modelview matrices is kept– The projection transform is stored separately in
the projection matrix See Chapter 3 of the OpenGL book
David Luebke 05/04/23
The Rendering Pipeline: 3-D
ModelingTransforms
Scene graphObject geometry
LightingCalculations
ViewingTransform
Clipping
ProjectionTransform
Result:Result:• All vertices of scene in shared 3-D “world” coordinate All vertices of scene in shared 3-D “world” coordinate systemsystem
• Vertices shaded according to lighting modelVertices shaded according to lighting model
• Scene vertices in 3-D “view” or “camera” coordinate Scene vertices in 3-D “view” or “camera” coordinate systemsystem
• Exactly those vertices & portions of polygons in view Exactly those vertices & portions of polygons in view frustumfrustum
• 2-D screen coordinates of clipped vertices2-D screen coordinates of clipped vertices
David Luebke 05/04/23
Rendering: Lighting
Illuminating a scene: coloring pixels according to some approximation of lighting– Global illumination: solves for lighting of the whole
scene at once– Local illumination: local approximation, typically
lighting each polygon separately Interactive graphics (e.g., hardware) does
only local illumination at run time
David Luebke 05/04/23
The Rendering Pipeline: 3-D
ModelingTransforms
Scene graphObject geometry
LightingCalculations
ViewingTransform
Clipping
ProjectionTransform
Result:Result:• All vertices of scene in shared 3-D “world” coordinate All vertices of scene in shared 3-D “world” coordinate systemsystem
• Vertices shaded according to lighting modelVertices shaded according to lighting model
• Scene vertices in 3-D “view” or “camera” coordinate Scene vertices in 3-D “view” or “camera” coordinate systemsystem
• Exactly those vertices & portions of polygons in view Exactly those vertices & portions of polygons in view frustumfrustum
• 2-D screen coordinates of clipped vertices2-D screen coordinates of clipped vertices
David Luebke 05/04/23
Rendering: Clipping
Clipping a 3-D primitive returns its intersection with the view frustum:
See Foley & van Dam section 19.1
David Luebke 05/04/23
Rendering: Clipping
Clipping is tricky!
In: 3 verticesIn: 3 verticesOut: 6 verticesOut: 6 verticesClip
Clip In: 1 polygonIn: 1 polygonOut: 2 polygonsOut: 2 polygons
David Luebke 05/04/23
The Rendering Pipeline: 3-D
Transform
Illuminate
Transform
Clip
Project
Rasterize
Model & CameraModel & CameraParametersParameters Rendering PipelineRendering Pipeline FramebufferFramebuffer DisplayDisplay
David Luebke 05/04/23
Modeling: The Basics
Common interactive 3-D primitives: points, lines, polygons (i.e., triangles)
Organized into objects– Collection of primitives, other objects– Associated matrix for transformations
Instancing: using same geometry for multiple objects – 4 wheels on a car, 2 arms on a robot
David Luebke 05/04/23
Modeling: The Scene Graph
The scene graph captures transformations and object-object relationships in a DAG
Objects in black; blue arrows indicate instancing and each have a matrix
Robot
BodyHead
ArmTrunkLegEyeMouth
David Luebke 05/04/23
Modeling: The Scene Graph
Traverse the scene graph in depth-first order, concatenating transformations
Maintain a matrix stack of transformations
ArmTrunkLegEyeMouth
Head Body
Robot
Foot
MatrixMatrixStackStack
VisitedVisited
UnvisitedUnvisited
ActiveActive
David Luebke 05/04/23
Modeling: The Camera
Finally: need a model of the virtual camera– Can be very sophisticated
Field of view, depth of field, distortion, chromatic aberration…
– Interactive graphics (OpenGL): Pinhole camera model
Field of view Aspect ratio Near & far clipping planes
Camera pose: position & orientation
David Luebke 05/04/23
Modeling: The Camera
These parameters are encapsulated in a projection matrix– Homogeneous coordinates 4x4 matrix!– See OpenGL Appendix F for the matrix
The projection matrix premultiplies the viewing matrix, which premultiplies the modeling matrices– Actually, OpenGL lumps viewing and modeling
transforms into modelview matrix
David Luebke 05/04/23
Rigid-Body Transforms
Goal: object coordinatesworld coordinates Idea: use only transformations that preserve
the shape of the object– Rigid-body or Euclidean transforms – Includes rotation, translation, and scale
To reiterate: we will represent points as column vectors:
zyx
zyx ),,(
David Luebke 05/04/23
Vectors and Matrices
Vector algebra operations can be expressed in this matrix form– Dot product:
– Cross product: Note: use
right-handrule!
z
y
x
zyx
bbb
aaaba
0cb0ca
cba
z
y
x
z
y
x
xy
xz
yz
ccc
bbb
aaaa
aa
00
0
David Luebke 05/04/23
Translations
For convenience we usually describe objects in relation to their own coordinate system– Solar system example
We can translate or move points to a new position by adding offsets to their coordinates:
– Note that this translates all points uniformly
z
y
x
ttt
zyx
zyx
'''
David Luebke 05/04/23
3-D Rotations
Rotations in 2-D are easy:
3-D is more complicated– Need to specify an axis of rotation– Common pedagogy: express rotation about this
axis as the composition of canonical rotations Canonical rotations: rotation about X-axis, Y-axis, Z-axis
yx
yx
cossinsincos
''
David Luebke 05/04/23
3-D Rotations
Basic idea:– Using rotations about X, Y, Z axes, rotate model until
desired axis of rotation coincides with Z-axis– Perform rotation in the X-Y plane (i.e., about Z-axis)– Reverse the initial rotations to get back into the initial
frame of reference Objections:
– Difficult & error prone– Ambiguous: several combinations about the canonical
axis give the same result– For a different approach, see McMillan’s lecture 12
David Luebke 05/04/23
The End
Next: Basic OpenGL