1 CSCE 441 Computer Graphics: 2D Transformations Jinxiang Chai.

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CSCE 441 Computer Graphics:2D Transformations

Jinxiang Chai

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2D Transformations

x

yx

y

x

y

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2D Transformations

x

yx

y

x

y

Applications:- Animation

- Image/object manipulation

- Viewing transformation

- etc.

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2D Transformation

• Required readings: HB 5-1 to 5-5, 5-8 • Given a 2D object, transformation is to change the

object’s – Position (translation)– Size (scaling)– Orientation (rotation)– Shapes (shear)

• Apply a sequence of matrix multiplications to the object vertices

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Point Representation

• We can use a column vector (a 2x1 matrix) to represent a 2D point x

y• A general form of linear transformation can be

written as: x’ = ax + by + c

OR

y’ = dx + ey + f

X’ a b c xY’ = d e f * y1 0 0 1 1

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Translation• Re-position a point along a straight line • Given a point (x,y), and the translation distance

(tx,ty)

The new point: (x’, y’) x’ = x + tx y’ = y + ty (x,y)

(x’,y’)

OR P’ = P + T where P’ = x’ p = x T = tx y’ y ty

tx

ty

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3x3 2D Translation Matrix

x’ = x + tx y’ y ty

Use 3 x 1 vector

x’ 1 0 tx x y’ = 0 1 ty * y1 0 0 1 1

Note that now it becomes a matrix-vector multiplication

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Translation

• How to translate an object with multiple vertices?

Translate individualvertices

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2D Rotation

• Default rotation center: Origin (0,0)

Rotate counter clockwise

Rotate clockwise

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(x,y)

(x’,y’)

(x,y) -> Rotate about the origin by

(x’, y’)

How to compute (x’, y’) ?

r

2D Rotation

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(x,y)

(x’,y’)

(x,y) -> Rotate about the origin by

(x’, y’)

How to compute (x’, y’) ?

x = r cos () y = r sin ()

r

x’ = r cos () y’ = r sin ()

2D Rotation

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2D Rotation

(x,y)

(x’,y’)

r

x = r cos () y = r sin ()

x’ = r cos () y = r sin ()

x’ = r cos ()

= r cos() cos() – r sin() sin()

= x cos() – y sin()

y’ = r sin ()

= r sin() cos() + r cos()sin()

= y cos() + x sin()

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2D Rotation

(x,y)

(x’,y’)

r

x’ = x cos() – y sin()

y’ = y cos() + x sin()

Matrix form?

x’ cos() -sin() x y’ sin() cos() y

=

3 x 3?

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3x3 2D Rotation Matrix

x’ cos() -sin() x y’ sin() cos() y

=

(x,y)

(x’,y’)

r

x’ cos() -sin() 0 x y’ sin() cos() 0 y1 0 0 1 1

=

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How to rotate an object with multiple vertices?

Rotate individualVertices

2D Rotation

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2D Scaling

Scale: Alter the size of an object by a scaling factor (Sx, Sy), i.e.

x’ = x . Sx y’ = y . Sy

x’ Sx 0 xy’ 0 Sy y=

(1,1)

(2,2) Sx = 2, Sy = 2

(2,2)

(4,4)

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2D Scaling

(1,1)

(2,2) Sx = 2, Sy = 2

(2,2)

(4,4)

Not only the object size is changed, it also moved!! Usually this is an undesirable effect We will discuss later (soon) how to fix it

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3x3 2D Scaling Matrix

x’ Sx 0 xy’ 0 Sy y=

x’ Sx 0 0 x y’ = 0 Sy 0 * y1 0 0 1 1

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Put it all together

• Translation:

• Rotation:

• Scaling:

y

x

t

t

y

x

y

x

y

x

t

t

y

x

cossin

sincos

y

x

y

x

t

t

s

s

y

x

0

0

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Or, 3x3 Matrix Representations

• Translation:

• Rotation:

• Scaling:

x’ 1 0 tx x y’ = 0 1 ty * y1 0 0 1 1

x’ cos() -sin() 0 x y’ sin() cos() 0 * y1 0 0 1 1

=

x’ Sx 0 0 x y’ = 0 Sy 0 * y1 0 0 1 1

Why use 3x3 matrices?

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Why Use 3x3 Matrices?

• So that we can perform all transformations using matrix/vector multiplications

• This allows us to pre-multiply all the matrices together

• The point (x,y) needs to be represented as (x,y,1) -> this is called Homogeneous coordinates!• How to represent a vector (vx,vy)?

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Why Use 3x3 Matrices?

• So that we can perform all transformations using matrix/vector multiplications

• This allows us to pre-multiply all the matrices together

• The point (x,y) needs to be represented as (x,y,1) -> this is called Homogeneous coordinates!• How to represent a vector (vx,vy)? (vx,vy,0)

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Shearing

• Y coordinates are unaffected, but x coordinates are translated linearly with y

• That is:– y’ = y – x’ = x + y * h

x 1 h 0 xy = 0 1 0 * y1 0 0 1 1

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x 1 0 0 xy = g 1 0 * y1 0 0 1 1

A 2D rotation is three shears Shearing will not change the area of the

object Any 2D shearing can be done by a rotation,

followed by a scaling, and followed by a rotation

Interesting Facts:

Shearing in Y

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Reflection

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Reflection

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Reflection

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Reflection about X-axis

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Reflection about X-axis

x 1 0 0 xy = 0 -1 0 * y1 0 0 1 1

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Reflection about Y-axis

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Reflection about Y-axis

x -1 0 0 xy = 0 1 0 * y1 0 0 1 1

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What’s the Transformation Matrix?

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What’s the Transformation Matrix?

x -1 0 0 xy = 0 -1 0 * y1 0 0 1 1

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The standard rotation matrix is used to rotate about the origin (0,0)

cos() -sin() 0 sin() cos() 0 0 0 1

What if I want to rotate about an arbitrary center?

Rotation Revisit

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Arbitrary Rotation Center

• To rotate about an arbitrary point P (px,py) by :

(px,py)

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• To rotate about an arbitrary point P (px,py) by : – Translate the object so that P will coincide with the

origin: T(-px, -py)

(px,py)

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origin: T(-px, -py) – Rotate the object: R()

(px,py)

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origin: T(-px, -py) – Rotate the object: R()– Translate the object back: T(px,py)

(px,py)

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Translate the object so that P will coincide with the origin: T(-px, -py)

Rotate the object: R() Translate the object back: T(px,py)

Put in matrix form: T(px,py) R() T(-px, -py) * P

x’ 1 0 px cos() -sin() 0 1 0 -px x y’ = 0 1 py sin() cos() 0 0 1 -py y1 0 0 1 0 0 1 0 0 1 1


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The standard scaling matrix will only anchor at (0,0)

Sx 0 0 0 Sy 0 0 0 1

What if I want to scale about an arbitrary pivot point?

Scaling Revisit

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To scale about an arbitrary fixed point P (px,py):

(px,py)

Arbitrary Scaling Pivot

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To scale about an arbitrary fixed point P (px,py): Translate the object so that P will coincide

with the origin: T(-px, -py)

(px,py)


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with the origin: T(-px, -py) Scale the object: S(sx, sy)

(px,py)


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with the origin: T(-px, -py) Scale the object: S(sx, sy) Translate the object back: T(px,py)

(px,py)


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Affine Transformation

• Translation, Scaling, Rotation, Shearing are all affine transformation

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• Affine transformation – transformed point P’ (x’,y’) is a linear combination of the original point P (x,y), i.e.

11001232221

131211

y

x

mmm

mmm

y

x

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• Affine transformation – transformed point P’ (x’,y’) is a linear combination of the original point P (x,y), i.e.

• Any 2D affine transformation can be decomposed into a rotation, followed by a scaling, followed by a shearing, and followed by a translation.

Affine matrix = translation x shearing x scaling x rotation

11001232221

131211

y

x

mmm

mmm

y

x

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Composing Transformation

• Composing Transformation – the process of applying several transformation in succession to form one overall transformation

• If we apply transforming a point P using M1 matrix first, and then transforming using M2, and then M3, then we have:

(M3 x (M2 x (M1 x P )))

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• Composing Transformation – the process of applying several transformation in succession to form one overall transformation

• If we apply transforming a point P using M1 matrix first, and then transforming using M2, and then M3, then we have:

(M3 x (M2 x (M1 x P ))) = M3 x M2 x M1 x P

M

(pre-multiply)

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• Matrix multiplication is associative M3 x M2 x M1 = (M3 x M2) x M1 = M3 x (M2 x M1)

• Transformation products may not be commutative A x B != B x A • Some cases where A x B = B x A A B translation translation scaling scaling rotation rotation uniform scaling rotation

(sx = sy)

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Transformation Order Matters!

• Example: rotation and translation are not commutative

Translate (5,0) and then Rotate 60 degree

OR

Rotate 60 degree and then translate (5,0)??

Rotate and then translate !!

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Finding Affine Transformations• How many points determines affine transformation

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Finding Affine Transformations• How many points determines affine transformation

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Finding Affine Transformations• Image of 3 points determines affine transformation

P

q

rp’

q’r’

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11100232221

131211

y

x

y

x

p

p

p

p

MMM

MMM

p’P

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11100232221

131211

y

x

y

x

p

p

p

p

MMM

MMM

p’P

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q

rp’

q’r’

P

What’s the corresponding point in the right image?

pnew

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q

rp’

q’r’

P

What’s the corresponding point in the right image?

11001232221

131211

new

new

new

new

p

p

MMM

MMM

p

p

pnew

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Next Lecture

• 2D coordinate transformations

• 3D transformations

• Lots of vector and matrix operations!

1 CSCE 441 Computer Graphics: 2D Transformations Jinxiang Chai.

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