# CSE559A Lecture 17

## Local Features

### Types of local features

#### Edge

Goal: Identify sudden changes in image intensity

Generate edge map as human artists.

An edge is a place of rapid change in the image intensity function.

Take the absolute value of the first derivative of the image intensity function.

For 2d functions, $\frac{\partial f}{\partial x}=\lim_{\Delta x\to 0}\frac{f(x+\Delta x)-f(x)}{\Delta x}$

For discrete images data, $\frac{\partial f}{\partial x}\approx \frac{f(x+1)-f(x)}{1}$

Run convolution with kernel $[1,0,-1]$ to get the first derivative in the x direction, without shifting. (generic kernel is $[1,-1]$)

Prewitt operator:

$$
M_x=\begin{bmatrix}
1 & 0 & -1 \\
1 & 0 & -1 \\
1 & 0 & -1 \\
\end{bmatrix}
\quad 
M_y=\begin{bmatrix}
1 & 1 & 1 \\
0 & 0 & 0 \\
-1 & -1 & -1 \\
\end{bmatrix}
$$
Sobel operator:

$$
M_x=\begin{bmatrix}
1 & 0 & -1 \\
2 & 0 & -2 \\
1 & 0 & -1 \\
\end{bmatrix}
\quad 
M_y=\begin{bmatrix}
1 & 2 & 1 \\
0 & 0 & 0 \\
-1 & -2 & -1 \\
\end{bmatrix}
$$
Roberts operator:

$$
M_x=\begin{bmatrix}
1 & 0 \\
0 & -1 \\
\end{bmatrix}
\quad 
M_y=\begin{bmatrix}
0 & 1 \\
-1 & 0 \\
\end{bmatrix}
$$

Image gradient:

$$
\nabla f = \left(\frac{\partial f}{\partial x}, \frac{\partial f}{\partial y}\right)
$$

Gradient magnitude:

$$
||\nabla f|| = \sqrt{\left(\frac{\partial f}{\partial x}\right)^2 + \left(\frac{\partial f}{\partial y}\right)^2}
$$

Gradient direction:

$$
\theta = \tan^{-1}\left(\frac{\frac{\partial f}{\partial y}}{\frac{\partial f}{\partial x}}\right)
$$

The gradient points in the direction of the most rapid increase in intensity.

> Application: Gradient-domain image editing
>
> Goal: solve for pixel values in the target region to match gradients of the source region while keeping the rest of the image unchanged.
>
> [Poisson Image Editing](http://www.cs.virginia.edu/~connelly/class/2014/comp_photo/proj2/poisson.pdf)

Noisy edge detection:

When the intensity function is very noisy, we can use a Gaussian smoothing filter to reduce the noise before taking the gradient.

Suppose pixels of the true image $f_{i,j}$ are corrupted by Gaussian noise $n_{i,j}$ with mean 0 and variance $\sigma^2$.
Then the noisy image is $g_{i,j}=(f_{i,j}+n_{i,j})-(f_{i,j+1}+n_{i,j+1})\approx N(0,2\sigma^2)$

To find edges, look for peaks in $\frac{d}{dx}(f\circ g)$ where $g$ is the Gaussian smoothing filter.

or we can directly use the Derivative of Gaussian (DoG) filter:

$$
\frac{d}{dx}g(x,\sigma)=\frac{1}{\sqrt{2\pi\sigma^2}}e^{-\frac{x^2}{2\sigma^2}}
$$

##### Separability of Gaussian filter

A Gaussian filter is separable if it can be written as a product of two 1D filters.

$$
\frac{d}{dx}g(x,\sigma)=\frac{1}{\sqrt{2\pi\sigma^2}}e^{-\frac{x^2}{2\sigma^2}}
\quad \frac{d}{dy}g(y,\sigma)=\frac{1}{\sqrt{2\pi\sigma^2}}e^{-\frac{y^2}{2\sigma^2}}
$$

##### Separable Derivative of Gaussian (DoG) filter

$$
\frac{d}{dx}g(x,y)\propto -x\exp\left(-\frac{x^2+y^2}{2\sigma^2}\right)
\quad \frac{d}{dy}g(x,y)\propto -y\exp\left(-\frac{x^2+y^2}{2\sigma^2}\right)
$$

##### Derivative of Gaussian: Scale

Using Gaussian derivatives with different values of 𝜎 finds structures at different scales or frequencies

(Take the hybrid image as an example)

##### Canny edge detector

1. Smooth the image with a Gaussian filter
2. Compute the gradient magnitude and direction of the smoothed image
3. Thresholding gradient magnitude
4. Non-maxima suppression
   - For each location `q` above the threshold, check that the gradient magnitude is higher than at adjacent points `p` and `r` in the direction of the gradient
5. Thresholding the non-maxima suppressed gradient magnitude
6. Hysteresis thresholding
   - Use two thresholds: high and low
   - Start with a seed edge pixel with a gradient magnitude greater than the high threshold
   - Follow the gradient direction to find all connected pixels with a gradient magnitude greater than the low threshold

##### Top-down segmentation

Data-driven top-down segmentation:

#### Interest point

Key point matching:

1. Find a set of distinctive keypoints in the image
2. Define a region of interest around each keypoint
3. Compute a local descriptor from the normalized region
4. Match local descriptors between images

Characteristic of good features:

- Repeatability
  - The same feature can be found in several images despite geometric and photometric transformations 
- Saliency
  - Each feature is distinctive
- Compactness and efficiency
  - Many fewer features than image pixels
- Locality
  - A feature occupies a relatively small area of the image; robust to clutter and occlusion

##### Harris corner detector





### Applications of local features

#### Image alignment

#### 3D reconstruction

#### Motion tracking

#### Robot navigation

#### Indexing and database retrieval

#### Object recognition



