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1、图像与视频处理 Image and Video Processing,第二讲：图像处理基础 软件工程 School of Software Engineering 祝继华,Contents,Light and the electromagnetic spectrum Image sensing and acquisition Sampling, quantisation and resolution Image representation Basic relationships between pixels Mathematical tools,Contents,Light and the
2、electromagnetic spectrum Image sensing and acquisition Sampling, quantisation and resolution Image representation Basic relationships between pixels Mathematical tools,Light And The Electromagnetic Spectrum,In 1666 Sir Isaac Newton discovered that light passed through a prism splits into a continuou
3、s spectrum of colours Light is just a particular part of the electromagnetic spectrum that can be sensed by the human eye The electromagnetic spectrum is split up according to the wavelengths of different forms of energy,Light and EM Spectrum,频率Hz,波长m,光谱能量,0.43-0.79um,Electromagnetic waves,photon,Hi
4、gh frequency: shorter wave More energy,Light and EM Spectrum,The colors that humans perceive in an object are determined by the nature of the light reflected from the object. e.g. green objects reflect light with wavelengths primarily in the 500 to 570 nm range while absorbing most of the energy at
5、other wavelength,White Light,Colours Absorbed,Green Light,Light and EM Spectrum,Monochromatic light: void of color Intensity is the only attribute, from black to white Monochromatic images are referred to as gray-scale images Chromatic light bands: 0.43 to 0.79 um The quality of a chromatic light so
6、urce: Radiance（强度）: total amount of energy Luminance (光通量): the amount of energy an observer perceives from a light source Brightness: a subjective descriptor of light perception that is impossible to measure. It embodies the achromatic notion of intensity and one of the key factors in describing co
7、lor sensation.,Contents,Light and the electromagnetic spectrum Image sensing and acquisition Sampling, quantisation and resolution Image representation Basic relationships between pixels Mathematical tools,Sampling, Quantisation And Resolution,In the following slides we will consider what is involve
8、d in capturing a digital image of a real-world scene Image sensing and representation Sampling and quantisation Resolution,Image Sensing,Incoming energy lands on a sensor material responsive to that type of energy and this generates a voltage Collections of sensors are arranged to capture images,Ima
9、ging Sensor,Line of Image Sensors,Array of Image Sensors,Image Sensing,Photodiode: silicon materials, Output voltage waveform is proportional to light,High precision scanning,Low cost: high resolution,Image Sensing,Using Sensor Strips and Rings,Medical Industrial image,CT PET MRI,Image Acquisition,I
10、mages are typically generated by illuminating a scene and absorbing the energy reflected by the objects in that scene,Typical notions of illumination and scene can be way off: X-rays of a skeleton Ultrasound of an unborn baby Electro-microscopic images of molecules,A Simple Image Formation Model,Pro
11、portional to energy radiated by a physical source,Some Typical Ranges of illumination,Illumination Lumen A unit of light flow or luminous flux Lumen per square meter (lm/m2) The metric unit of measure for illuminance of a surface On a clear day, the sun may produce in excess of 90,000 lm/m2 of illum
12、ination on the surface of the Earth On a cloudy day, the sun may produce less than 10,000 lm/m2 of illumination on the surface of the Earth On a clear evening, the moon yields about 0.1 lm/m2 of illumination The typical illumination level in a commercial office is about 1000 lm/m2,Some Typical Range
13、s of Reflectance,Reflectance 0.01 for black velvet 0.65 for stainless steel 0.80 for flat-white wall paint 0.90 for silver-plated metal 0.93 for snow,Gray scale,Monochrome image,Contents,Light and the electromagnetic spectrum Image sensing and acquisition Sampling, quantisation and resolution Image
14、representation Basic relationships between pixels Mathematical tools,Image Sampling And Quantisation,A digital sensor can only measure a limited number of samples at a discrete set of energy levels Quantisation is the process of converting a continuous analogue signal into a digital representation o
15、f this signal,sampling,quantization,Image Sampling And Quantisation,Remember that a digital image is always only an approximation of a real world scene,Image quality is determined by the number of samples And discrete intensity levels used in sampling and quantization.,Sampling is determined by the
16、sensor arrangement used to generate the image; Sampling accuracy relates to quality of the optical components of the system.,Image Sampling And Quantisation,Contents,Light and the electromagnetic spectrum Image sensing and acquisition Sampling, quantisation and resolution Image representation Basic relationships between pixels Mathematical tools,Image Representation,Before we discuss image acquisition recall that a digital image is composed of M rows and N columns of pixels eac
