The following problems appeared in the exercises in the **Coursera course Image Processing (by Northwestern University)**. The following descriptions of the problems are taken directly from the exercises’ descriptions.

## 1. Analysis of an Image quality after applying an *nxn* Low Pass Filter (LPF) for different n

The next figure shows the problem statement. Although it was originally implemented in *MATLAB*, in this article a *python* implementation is going to be described.

The following figure shows how the images get more and more **blurred** after the application of the **nxn LPF** as *n* increases.

The following figure shows how the quality of the transformed image decreases when compared to the original image, when an *nxn LPF* is applied and how the quality (measured in terms of *PSNR*) degrades as *n* (*LPF kernel* width) increases.

## 2. Changing Resolution of an Image with Down/Up-Sampling

The following figure describes the problem:

The following steps are needed to be followed:

- Smooth the original image with a 3×3 LPF (box1)
*kernel*. - Downsample (choose pixels corresponding to every odd rows and columns)
- Upsample the image (double the width and height)
- Use the kernel and use it for convolution with the upsampled image to obtain the final image.

Although in the original implementation *MATLAB* was used, but the following results come from a *python* implementation.

As we go on increasing the kernel size, the quality fo the final image obtained by down/up sampling the original image decreases as *n* increases, as shown in the following figure.

## 3. Motion Estimation in Videos using Block matching between consecutive video frames

The following figure describes the problem:

For example we are provided with the input image with known location of an object (**face marked with a rectangle**) as shown below.

Now we are provided with another image which is the **next frame** extracted from the same video but with the **face unmarked**) as shown below. The problem is that we have to locate the face in this next frame and mark it using simple **block matching** technique (and thereby estimate the motion).

As shown below, using just the **simple block matching**, we can mark the face in the very next frame.

Now let’s play with the following two videos. The first one is the video of some students working on a university corridor, as shown below (obtained from *youtube*), extract some consecutive frames, mark a face in one image and use that image to mark all thew faces om the remaining frames that are consecutive to each other, thereby mark the entire video and estimate the motion using the simple **block matching technique** only.

The following figure shows the *frame* with the **face marked**, now we shall use this image and **block matching** technique to* estimate the motion* of the student in the video, by marking his face in all the consecutive frames and reconstructing the video, as shown below..

*Google CEO Mr. Pichai*talking at Google I/O 2017, as shown below (obtained from

*youtube*), again extract some

*consecutive frames*, mark his face in

*one*image and use that image to mark all the faces in the remaining frames that are

*consecutive*to each other, thereby mark the entire video and

*estimate the motion*using the simple

**block matching technique**only.

**face marked**, now we shall use this image and

**block matching**technique to estimate the motion of the

*Google CEO*in the video, by marking his face in all the consecutive frames and reconstructing the video, as shown below.

*image features*such as

*HOG /***, still it did a pretty descent job with simple**

*SIFT***.**

*block matching*## 4. Using Median Fiter to remove salt and paper noise from an image

The following figure describes the problem:

The following figure shows the *original image*, the *noisy image* and images obtained after applying the *median filter of different sizes (*nxn*, for different values of *n):

As can be seen from the following figure, the *optimal median filter size* is *5×5*, which generates the highest quality output, when compared to the *original image*.

## Using Inverse Filter to Restore noisy images with Motion Blurs

The following figure shows the description of the problem:

The following figure shows the theory behind the **inverse filters** in the **(frequency) spectral domain**.

*restoring*an image with

**motion blurs**and noise-corruption using the

**Inverse Filter**:

- Generate
*restoration filter*in the frequency domain (with) from**Fast Fourier Transform***frequency response*of*motion blur*and using the threshold**T**. - Get the
*spectrum*of*blurred*and*noisy-corrupted*image (the input to restoration). - Compute
*spectrum*of*restored image*by*convolving*the*restoration filter*with the*blurred noisy image*in the*frequency domain*. - Genrate the
*restored image*from its*spectrum (with*.**inverse Fourier Transform**)

The following figures show the **inverse filter** is applied with different threshold value *T* (starting from **0.1** to **0.9** in that order) to *restore* the noisy / blurred image.

The following figure shows how the *Restored PSNR* and the *ISNR* varies with different values if the threshold *T*.