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Category: EECS 442

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1 Transformations [25pt]

We have seen that a 3D rotation can be formed as the product of three

matrices

R = Rx(α)Ry(β)Rz(γ),

where

Rx(α) =

1 0 0

0 cos α − sin α

0 sin α cos α

(1)

Ry(β) =

cos β 0 sin β

0 1 0

− sin β 0 cos β

(2)

Rz(γ) =

cos γ − sin γ 0

sin γ cos γ 0

0 0 1

(3)

are rotations around the x, y, and z axis, respectively. This is called the XY-Z Euler angle representation, indicating the order of matrix multiplication

However, representing rotations in this form can lead to ambiguities, since

the result depends on the order in which the transforms are performed. Let

1

p

0 be the point obtained by rotating a point p with a rotation matrix R, so

p

0 = Rp. Give an expression for p

0 obtained by rotating p in the following

two ways:

• First rotate β around y axis, then rotate γ around z axis.

• First rotate γ around z axis, then rotate β around y axis.

[10pt] Show that these rotations produce different values of p

0

.

[15pt] To avoid the representation ambiguity introduced in part a, we can

fix the rotation order. However, even if the order of rotation is fixed, this

rotation system may still result in a degenerate representation. For instance,

let α, β, and γ be three different angles. We can represent an arbitrary

rotation with respect to the X-Y-X axes as the product Rx(α)Ry(β)Rx(γ),

(note that the axes here are X-Y-X, not X-Y-Z). This representation of a

rotation matrix should have three degrees of freedom. However if we set

β = 0, something unusual happens. How many degrees of freedom are left?

Hint: some trigonometric identities may be useful.

2 Panoramic Imaging Theory [15pt]

Assume that we have two cameras with camera matrices M1 and M2

M1 = K1[R1T1] and M2 = K2[R2T2],

where

K1 = K2 = K

R1 = I = Identity matrix

R2 = R

T1 = T2 =

0 0 0T

Suppose that the cameras generate image 1 and 2, respectively, as shown

in the figure below, where p

0

1

, p0

2

, p0

3

, p0

4

and p1, p2, p3, p4 are corresponding

points across the two images.

Prove that the homographic transformation H defined by p

0

1

, p0

2

, p0

3

, p0

4

and

p1, p2, p3, p4 can be expressed as H = KRK−1

.

2

Computing H [30pt]

Write a function Homography that takes a set of at least 4 image point

correspondences between two images (manually selected) and returns an

estimate of the homographic transformation between the images. Use the

DTL algorithm to compute the homography.

What to return in this problem:

• Your code for the function Homography with comments.

• Numerical values for H given the point correspondences in file 4points.txt

(4points.txt file can be read using readPoints.m function).

Convolution [30pt]

In class we discussed Gaussian blurring and convolution. Implement in

MATLAB WITHOUT any built in functions (i.e. conv2, imconv, fspecial,

etc.). The construction of a Gaussian kernel with a variable σ parameter

and the code to apply that kernel to an image through convolution again

WITHOUT using the built in convolution functions. Apply your Gaussian

kernel to one of the images in the midterm folder (garden1,garden2) with 3

σ values and discuss the results.

You must return:

• The source code.

• The visual results (images) after blurring

• Comments about your results.

3

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