计算机图像处理 Computer Image Processing

Overview

The main content of this course is to process image with Matlab.

• Introduction to computer image and color model
• Image transform, enhancement and segmentation

Computer Graphics studies how to make computers display images, while Computer Image Processing focuses on processing image.

一、Image and Color Model

1. Color Image

Digital images has three types: color images, grayscale images and binary images.

1) Color Image

Each pixel of color image is composed of three primary colors. We use three matrices to store a color image.

2) Grayscale Image

Grayscale image has no color information.

For an 8-bit grayscale image, the grayscale value of each pixel is between 0 and 255.

3) Binary Image

Binary image has only two colors: black and white.

2. Color Model

We use color model to describe and use color scientifically and quantitatively.

Most commonly used color model: RGB.

R, G, and B respectively represent the three primary colors of red, green and blue.

These three colors can be mixed together to produce other colors.

The RGB of Pure Black:

R	G	B
0	0	0

Other Models: HSI

二、Image Processing

1. Image Transform

We often need to transform a picture from the spatial domain to the frequency domain.

Because in the frequency domain, the characteristics of the image are more prominent.

The edge and line information is reflected in the high-frequency components, and the image energy is reflected in the low-frequency components.

The most widely used transform is the Fourier transform.

In Matlab:

fft2()

2. Image Enhancement

It means highlighting certain information in an image.

Image enhancement can be performed in two domains:

• spatial domain
• frequency domain

1) Spatial Domain

We can use smoothing filter or sharpening filter to smooth or sharpen an image.

The principle is to change the grayvalue of each pixel through specific calculation.

2) Frequency Domain

We can use low-pass of high-pass filter to smooth or sharpen an image.

The principle is to block certain frequency.

3. Image Segmentation

Image segmentation is to segment the target from an image.

There are many basis for segmentation, such as grayvalue, edge and so on.

We mainly talk about the segmentation method based on threshold.

We select an appropriate threshold according to the difference in grayscale between the target and the background to determine whether a pixel belongs to the target or the background area.