CSCE 641:Computer Graphics Image Warping/Registration Jinxiang Chai.

CSCE 641:Computer Graphics Image Warping/Registration

Jinxiang Chai

Outline

Image warping

Image Registration

Image Warping

Image filtering: change range of image g(x) = h(f(x))

f

x

hf

x

f

x

hf

x

Image warping: change domain of imageg(x) = f(h(x))

Image Warping

Image filtering: change range of image g(x) = h(f(x))

Image warping: change domain of imageg(x) = f(h(x))

hf g

hf g

Image Warping

Why? - texture mapping

- image processing (rotation, zoom in/out, etc)

- image morphing/blending

- image editing

- image based-modeling & rendering

Parametric (global) Warping

Examples of image warps:

translation rotation aspect

affine perspective cylindrical

Transformation Function

f, g

Transform the geometry of an image to a desired geometry

Definition: Image Warping

Source Image: Image to be used as the reference. The geometry of this image is no changed

Target Image: this image is obtained by transforming the reference image.

(x,y): coordinates of points in the reference image

(u,v): coordinates of points in the target image

f,g or F,G: x and y components of a transformation function

Definition: Image Warping

Control points: Unique points in the reference and target images. The coordinates of corresponding control points in images are used to determine a transformation function.

Source Image Target Image

A Transformation Function

Used to computer the corresponding points

Source Image S(x,y) Target Image T(u,v)

u = f(x,y)

v = g(x.y)

x = F(u,v)

y = G(x.v)

Warping Types

Simple mappings: - Similarity - Affine mapping - Projective mapping

These can be applied globally over a subdivision of the plane:

- Piecewise affine over triangulation - Piecewise projective over quadrilaterization - Piecewise bilinear over a rectangular grid

Or other, arbitrary functions can be used, e.g. - Bieer-neely warp (popular for morphs) - Store u(x,y) and v(x,y) in large arrays

Similarity Transform

• A combination of 2-D scale, rotation, and translation transformations.

• Allows a square to be transformed into any rotated rectangle.

• Angle between lines is preserved

• 5 degrees of freedom (sx,sy,θ,tx,ty)

• Inverse is of same form (is also similarity). Given by inverse of 3X3 matrix above

Have the form: In matrix notation:

Affine Transform

• A combination of 2-D scale, rotation, shear, and translation transformations.

• Allows a square to be distorted into any parallelogram.• 6 degrees of freedom (a,b,c,d,e,f)• Inverse is of same form (is also affine). Given by

inverse of 3X3 matrix above• Good when controlling a warp with triangles, since 3

points in 2D determined the 6 degrees of freedom


Projective Transform (a.k.a “perspective”)

• Linear numerator & denominator• If g=h=0, then you get affine as a special case• Allow a square to be distorted into any quadrilateral• 8 degrees of freedom (a-h). We can choose i=1,

arbitrarily• Inverse is of same form (is also projective). • Good when controlling a warp with quadrilaterals, since

4 points in 2D determine the 8 degrees of freedom


Image Warpingx

y

u

v

Given a coordinate transform function f,g or F,G and source image S(x,y), how do we compute a transformed image T(u,v)?

S(x,y) T(u,v)

Forward Warpingx

y

u

v

S(x,y) T(u,v)

Forward warping algorithm:

for y = ymin to ymax

for x = xmin to xmax

u = f(x,y); v = g(x,y)

copy pixel at source S(x,y) to T(u,v)

Forward Warpingx

y

u

v

S(x,y) T(u,v)

Forward warping algorithm:



u = f(x,y); v = g(x,y)


- Any problems for forward warping?

Forward Warpingx

y

u

v

S(x,y) T(u,v)

Q: What if the transformed pixel located between pixels?

A: Distribute color among neighboring pixels

- known as “splatting”

Forward Warping• Iterate over source, sending pixels to destination• Some source pixel map to multiple dest. pixels• Some dest. pixels may have no corresponding source• Holes in reconstruction • Must splat etc.



u = f(x,y); v = g(x,y)


x

y

u

v

Forward Warping• Iterate over source, sending pixels to destination• Some source pixel map to multiple dest. pixels• Some dest. pixels may have no corresponding source• Holes in reconstruction • Must splat etc.



u = f(x,y); v = g(x,y)


x

y

u

v

- How to remove the holes?

Inverse Warpingx

y

u

v

S(x,y) T(u,v)

Inverse warping algorithm:

for v = vmin to vmax

for u = umin to umax

x = F(u,v); y = G(u,v)


Inverse Warpingx

y

u

v

S(x,y) T(u,v)

Q: What if pixel comes from “between” two pixels?

A: Interpolate color values from neighboring pixels

Inverse Warping

• Iterate over dest., finding pixels from source• Non-integer evaluation source image, resample• May oversample source• But no holes• Simpler, better than forward mapping



x = F(u,v); y = G(u,v)


x

y

u

v

Resampling

x

y

u

v

This is a 2D signal reconstruction problem!

Resampling Filter

Review: Signal Reconstruction in Freq. Domain

T 2T…-2T -T… 0

x

fs(x)

f(x)

x

Fs(u)

u-fmax fmax

F(u)

u-fmax fmax

)()( uboxuFs

Inverse Fourier transform

Fourier transform

Review: Signal Reconstruction in Spatial Domain

)sin()( xxf s

T 2T…-2T -T… 0

x

x

fs(x) sinc(x)

Resampling

Compute weighted sum of pixel neighborhood - Weights are normalized values of kernel function

- Equivalent to convolution at samples with kernel

- Find good filters using ideas of previous lectures

x

y

u

v

Point Sampling

Nearest neighbor

x

y

u

v

- Copies the color of the pixel with the closest integer coordinate - A fast and efficient way to process textures if the size of the target is similar to the size of the reference- Otherwise, the result can be a chunky, aliased, or blurred image.

Bilinear Filter

Weighted sum of four neighboring pixels

x

y

u

v

Bilinear Filter

Sampling at S(x,y):

(i+1,j)

(i,j) (i,j+1)

(i+1,j+1)

S(x,y) = a*b*S(i,j) + a*(1-b)*S(i+1,j)

+ (1-a)*b*S(i,j+1) + (1-a)*(1-b)*S(i+1,j+1)

u

v

y

x

Bilinear Filter

Sampling at S(x,y):

(i+1,j)

(i,j) (i,j+1)

(i+1,j+1)

S(x,y) = a*b*S(i,j) + a*(1-b)*S(i+1,j)

+ (1-a)*b*S(i,j+1) + (1-a)*(1-b)*S(i+1,j+1)

Si = S(i,j) + a*(S(i,j+1)-S(i))

Sj = S(i+1,j) + a*(S(i+1,j+1)-S(i+1,j))

S(x,y) = Si+b*(Sj-Si)

To optimize the above, do the following

u

v

y

x

Bilinear Filter

(i+1,j)

(i,j) (i,j+1)

(i+1,j+1)

y

x

Anisotropic Filter

Anisotropic means "non-uniform shape" - Used in texture mapping

- A circle in screen space corresponds to an ellipse in texture space

- An Isotropic filter in screen space means an anisotropic filter

- Changing viewpoint results in different filters

- Calculated at run time

- Many calculations need to be done to draw a single pixel

Screen spaceTexture space

Comparison of Resampling Filters

Inverse Warping and Resampling

Inverse warping algorithm:



float x = F(u,v); float y = G(u,v);

T(u,v) = resample_souce(x,y,w);

x

y

u

v

(u,v)

(x,y)

Outline

Image warping

Image Registration

Image RegistrationImage warping: given h and f, compute g

g(x) = f(h(x))

hf

g?

Image registration: given f and g, compute h

h?f

g

Why Image Registration?

Lots of uses– Correct for camera jitter

(stabilization)– Align images (mosaics)– View morphing– Special effects– Image based modeling/rendering– Etc.

[Seitz 96]

http://www.cs.cmu.edu/~seitz/vmorph/mfig1.mpg

Image Registration

How do we align two images automatically?

Two broad approaches:– Feature-based alignment

• Find a few matching features in both images• compute alignment

– Direct (pixel-based) alignment• Search for alignment where most pixels agree

Outline

Image registration

- feature-based approach

- pixel-based approach

Feature-based Alignment

1. Find a few important features (aka Interest Points)

2. Match them across two images

3. Compute image transformation function h





How to choose features – Choose only the points (“features”) that are salient, i.e. likely to be there in

the other image– How to find these features?





How to choose features – Choose only the points (“features”) that are salient, i.e. likely to be there in

the other image– How to find these features?

• windows where has two large eigenvalues• Harris Corner detector• Better features (SIFT features)

Feature Detection

-Two images taken at the same place with different angles

- Projective transformation H3X3

Feature Matching

?



Feature Matching

?



How do we match features across images? Any criterion?

Feature Matching

• Intensity/Color similarity– The intensity of pixels around the

corresponding features should have similar intensity

Feature Matching

• Intensity/Color similarity– The intensity of pixels around the


– Sum of squared differences (SSD), Normalized cross-correlation

http://www.cse.yorku.ca/~kosta/CompVis_Notes/ssd_cross-correlation.pdf

http://en.wikipedia.org/wiki/Cross-correlation

Feature Matching

• Feature similarity (Intensity or SIFT signature)– The intensity of pixels around the


– Cross-correlation, SSD

• Distance constraint– The displacement of features should be

smaller than a given threshold

http://en.wikipedia.org/wiki/Cross-correlation

http://www.cse.yorku.ca/~kosta/CompVis_Notes/ssd_cross-correlation.pdf

Feature-space Outlier Rejection

bad

Good


Can we now compute H3X3 from the blue points?


Can we now compute H3X3 from the blue points?

– No! Still too many outliers…


Can we now compute H3X3 from the blue points?– No! Still too many outliers… – What can we do?


Can we now compute H3X3 from the blue points?– No! Still too many outliers… – What can we do?

Robust estimation!

Robust Estimation: A Toy Example

How to fit a line based on a set of 2D points?

http://en.wikipedia.org/wiki/Robust_statistics

Robust Estimation: A Toy Example

How to fit a line based on a set of 2D points?

RANSAC: an iterative method to estimate parameters of a mathematical model from a set of observed data which contains outliers

http://en.wikipedia.org/wiki/Robust_statistics

http://en.wikipedia.org/wiki/RANSAC

http://en.wikipedia.org/wiki/Outliers

RANSAC: RANdom SAmple Consensus

ObjectiveRobust fit of model to data set S which contains outliers

Algorithm

(i) Randomly select a sample of s data points from S and instantiate the model from this subset.

(ii) Determine the set of data points Si which are within a distance threshold t of the model. The set Si is the consensus set of samples and defines the inliers of S.

(iii) If the subset of Si is greater than some threshold T, re-estimate the model using all the points in Si and terminate

(iv) If the size of Si is less than T, select a new subset and repeat the above.

(v) After N trials the largest consensus set Si is selected, and the model is re-estimated using all the points in the subset Si


RANSAC

Repeat M times:– Sample minimal number of matches to

estimate two view relation (affine, perspective, etc).

– Calculate number of inliers or posterior likelihood for relation.

– Choose relation to maximize number of inliers.


RANSAC Line Fitting Example

Task:

Estimate best line


Sample two points


Fit Line


Total number of points within a threshold of line.


Repeat, until get a good result

How Many Samples?

Choose N so that, with probability p, at least one random sample is free from outliers. e.g. p=0.99

sepN 11log/1log

peNs 111

proportion of outliers es 5% 10% 20% 25% 30% 40% 50%2 2 3 5 6 7 11 173 3 4 7 9 11 19 354 3 5 9 13 17 34 725 4 6 12 17 26 57 1466 4 7 16 24 37 97 2937 4 8 20 33 54 163 5888 5 9 26 44 78 272 1177

How Many Samples?


sepN 11log/1log

peNs 111


Affine transform

How Many Samples?


sepN 11log/1log

peNs 111


Projective transform

RANSAC for Estimating Projective Transformation

RANSAC loop:1. Select four feature pairs (at random)

2. Compute the transformation matrix H (exact)

3. Compute inliers where SSD(pi’, H pi) < ε

4. Keep largest set of inliers

5. Re-compute least-squares H estimate on all of the inliers

RANSAC

Feature-based Registration

Works for small or large motion

Model the motion within a patch or whole image using a parametric transformation model

Feature-based Registration

Works for small or large motion

Model the motion within a patch or whole image using a parametric transformation model

How to deal with motions that cannot be described by a small number of parameters?

http://www.cs.cmu.edu/~seitz/vmorph/mfig1.mpg

Outline

Image registration

- feature-based approach

- pixel-based approach

Next Lecture

More on image registration

Image mosaicing

CSCE 641:Computer Graphics Image Warping/Registration Jinxiang Chai.

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