Lecture 2 Digital Image Fundamentals. Lin ZHANG, PhD School of Software Engineering Tongji University Fall 2016

Transcription

1 Lecture 2 Digital Image Fundamentals Lin ZHANG, PhD School of Software Engineering Tongji University Fall 2016

2 Contents Elements of visual perception Light and the electromagnetic spectrum Image sensing and acquisition Image sampling and quantization Some basic relationships between pixels

3 Elements of Visual Perception Structure of the human eye visual axis sclera choroid blind spot

4 Elements of Visual Perception Structure of the human eye Three membranes enclose the eye: the cornea and sclera, choroid, and retina At its anterior extreme, the choroid is divided into the ciliary body and the iris; the later contracts or expands to control the amount of light that enters the eye

5 Elements of Visual Perception Structure of the human eye When the eye is properly focused, light from an object outside the eye is imaged on the retina Two kinds of light receptors distribute on the retina, cones and rods Cones are primarily located in the central portion of the retina, called fovea and are sensitive to color; they function best in relatively bright light; so, cone vision is called bright light vision Rods are distributed over the retinal surface; rods serve to give a general overall picture of the field of view; they are not involved in color vision and are sensitive to low levels of illumination; rod vision is called dim light vision Around the region of the emergence of the optic nerve, there is no receptors and results in the so called blind spot

6 Elements of Visual Perception Structure of the human eye (more on cone cell) Humans usually have three kinds of cones with different photopsins, which have different spectral response curves; thus, we have trichromatic vision. Interestingly, some people have four or more types of cones, giving them tetrachromatic vision

7 Elements of Visual Perception Structure of the human eye (more on cone cell) Three types of color sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue sensitive detectors.

8 Elements of Visual Perception Structure of the human eye (more on cone cell)

9 Elements of Visual Perception Structure of the human eye (more on rod cell) Rod cell

10 Elements of Visual Perception Structure of the human eye (more on rod cell) Wavelength responsiveness of rods compared to that of three types of cones. The dashed gray curve is for rods.

11 Elements of Visual Perception Structure of the human eye Distribution of rods and cones in the retina

12 Elements of Visual Perception Blind spot experiment Draw an image similar to that below on a piece of paper (the dot and cross are about 6 inches apart) Close your right eye and focus on the cross with your left eye Hold the image about 20 inches away from your face and move it slowly towards you The dot should disappear!

13 Elements of Visual Perception Image formation in the eye Muscles within the eye can be used to change the shape of the lens allowing us focus on objects that are near or far away An image is focused onto the retina causing rods and cones to become excited which ultimately send signals to the brain C is the optical center of the lens

14 Elements of Visual Perception Human eye VS camera VS Lens Iris Retina Related components?

15 Elements of Visual Perception Perceived brightness is not a simple function of intensity Visual system tends to undershoot or overshoot around the boundary of regions of different intensities, called as Mach bands A region s perceived brightness does not only depend simply on its intensity, but on its surrounding regions; such a phenomenon is called simultaneous contrast

16 Elements of Visual Perception Perceived brightness is not a simple function of intensity An example of Mach bands

17 Elements of Visual Perception Perceived brightness is not a simple function of intensity

18 Elements of Visual Perception Perceived brightness is not a simple function of intensity An example of simultaneous contrast

19 Elements of Visual Perception Perceived brightness is not a simple function of intensity

20 Elements of Visual Perception Perceived brightness is not a simple function of intensity

21 Elements of Visual Perception Optical illusions: our visual systems play lots of interesting tricks on us It is stilly not fully understood yet

22 Elements of Visual Perception

23 Contents Elements of visual perception Light and the electromagnetic spectrum Image sensing and acquisition Image sampling and quantization Some basic relationships between pixels

24 Light and the Electromagnetic Spectrum 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

25 Light and the Electromagnetic Spectrum The colours that we perceive are determined by the nature of the light reflected from an object In addition to frequency, three basic quantities are used to describe the quality of a chromatic light source Radiance. It is the total amount of energy that flows from the light source, measured in Watts Luminance. Gives a measure of the amount of energy an observer perceives, measured in lumens Brightness. It is a subjective descriptor of light perception that is practically impossible to measure

26 Light and the Electromagnetic Spectrum electromagnetic wave reflectance spectrum spectral power distribution spectral power distribution Red

27 Contents Elements of visual perception Light and the electromagnetic spectrum Image sensing and acquisition Image sampling and quantization Some basic relationships between pixels

28 Image Sensing and Acquisition Image creation based on two factors Illumination source Reflection or absorption of energy from that source by the elements of the scene being imaged Any examples for these two kinds?

29 Image Sensing and Acquisition Imaging sensors Single imaging sensor Line sensor Array sensor Single imaging sensor

30 Image Sensing and Acquisition Imaging sensors Single imaging sensor Line sensor Array sensor Line sensor Application scenario?

31 Image Sensing and Acquisition Imaging sensors Single imaging sensor Line sensor Array sensor Array sensor, used in ordinary digital camera

32 Image Sensing and Acquisition Imaging sensing using sensor strips Image acquisiton using a linear sensor strip and a circular sensor strip

33 Image Sensing and Acquisition Imaging sensing using sensor arrays An example of the digital image acquisition process

34 Contents Elements of visual perception Light and the electromagnetic spectrum Image sensing and acquisition Image sampling and quantization Some basic relationships between pixels

35 Image Sampling and Quantization Sampling and quantization will convert a continuous image signal f to a discrete digital form Digitizing the coordinate values is called sampling Digitizing the amplitude is called quantization

36 Image Sampling and Quantization Sampling and quantization will convert a continuous image signal f to a discrete digital form

37 Image Sampling and Quantization Sampling and quantization will convert a continuous image signal f to a discrete digital form Result of image sampling and quantization

38 Image Sampling and Quantization Representing images

39 Image Sampling and Quantization Spatial resolution The spatial resolution of an image is determined by how sampling was carried out DPI (dots per inch) is used to measure the spatial resolution Note: to say that an image has a resolution 1024*1024 is not a meaningful statement without stating the spatial dimensions encompassed by the image

40 Image Sampling and Quantization Spatial resolution an example

41 Image Sampling and Quantization Spatial resolution an example

42 Image Sampling and Quantization Spatial resolution an example

43 Image Sampling and Quantization Spatial resolution an example

44 Image Sampling and Quantization Spatial resolution an example

45 Image Sampling and Quantization Spatial resolution an example

46 Image Sampling and Quantization Intensity resolution Intensity resolution refers to the smallest discernible change in intensity level The more intensity levels used, the finer the level of detail discernable in an image The number of bits used to quantize intensity is often referred as the intensity resolution Number of Bits Number of Intensity Levels Examples 1 2 0, , 01, 10, , 0101, , ,

47 Image Sampling and Quantization Intensity resolution 8 bits per pixel 7 bpp 6 bpp 5 bpp 4 bpp 3 bpp 2 bpp 1 bpp

48 Image Sampling and Quantization Image interpolation It is a basic tool used in tasks such as zooming, shrinking, rotating, and geometric corrections Traditional methods include nearest neighbor, bilinear, and bicubic

49 Contents Elements of visual perception Light and the electromagnetic spectrum Image sensing and acquisition Image sampling and quantization Some basic relationships between pixels

50 Some Basic Relationships between Pixels Neighbors of a pixel p(x, y) is a pixel p p p N 4 (p): 4 neighbours of p N D (p): diagonal neighbours of p N 8 (p): 8 neighbours of p

51 Summary We have looked at: Elements of visual perception Light and the electromagnetic spectrum Image sensing and acquisition Image sampling and quantization Some basic relationships between pixels Next time we start to look at techniques for image enhancement

52 Thanks for your attention