General Imaging System

Transcription

1 General Imaging System Lecture Slides ME 4060 Machine Vision and Vision-based Control Chapter 5 Image Sensing and Acquisition By Dr. Debao Zhou 1 2 Light, Color, and Electromagnetic Spectrum Penetrate glass UV protection sunglasses? Human visual system (HVS) is sensitive to photons of wavelengths between 400 and 700 nm, where 1 nm = 10 9 m 3 4 Types of Images Types of Images Reflection Images: Emission Images: Absorption Images: 5 6 1

2 Light and Color Perception Light and Color Perception Light is a particular type of EM radiation that can be sensed by the human eye. Colors perceived by humans are determined by the nature of the light reflected/radiated by the object, which is a function of the spectral properties of the light source as well as the absorption and reflectance properties of the object. Sunlight is actually composed of many different colors of light rather than just one Ozone CO Human Vision System (HVS) Color Perception Human Vision System (HVS) Colon Perception See PowerPoint Color demo Brightness: The subjective perception of (achromatic) luminous intensity, or the attribute of a visual sensation according to which an area appears to emit more or less light. Hue: The attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors, red, yellow, green and blue, or a combination of two of them. From a spectral viewpoint, hue can be associated with the dominant wavelength of an SPD Human Vision System (HVS) Colon Perception Saturation: The colorfulness of an area judged in proportion to its brightness, which usually translates into a description of the whiteness of the light source. From a spectral viewpoint, the more an Spectrum of Power Density (SPD) is concentrated at one wavelength, the more saturated will be the associated color. The addition of white light, that is, light that contains power at all wavelengths, causes color desaturation. Image Sensors EM energy - electrical signals to computer Two main technologies CCDs (charge-coupled devices) CMOS (complementary metal oxide semiconductor)

3 CCD Image Sensor CCDs A CCD image sensor on a flexible circuit board Image sensor on the motherboard of a Nikon Coolpix L2 6 MP Photoactive region (an epitaxial layer of silicon), and a transmission region made out of a shift register (the CCD, properly speaking). An image causing each capacitor to accumulate an electric charge proportional to the light intensity at that location Charge-Coupled Devices Active Pixel Sensor Bayer Pattern 3-CCD color cameras

4 Foveon's scheme of vertical filtering for color sensing Camera Optics Magnification factor (m), the ratio between image size and object size: Focal length, f (in millimeters), the distance from the lens to the point at which parallel incident rays converge Camera Optics Focal Length and FOV Aperture: f number a dimensionless value that represents the ratio between focal length and aperture diameter: Filed of View (FOV) Angle of View angular extent of a given scene that is imaged by a camera Distortion Due to Lenses Aberrations Image Digitization Digitization involves two processes: sampling (in time or space) and Quantization (in amplitude)

5 Sampling Measuring the value of a 2D function at discrete intervals along the x and y dimensions. square sampling and nonsquare sampling. Sampling rate aliasing - Nyquist criterion), Quantization Gray levels Spatial resolution Spatial resolution A way of expressing the density of pixels in an image: the greater the spatial resolution, the more pixels are used to display the image within a certain fixed physical size Unit: dots per inch or dpi Gray-level resolution Smallest change in intensity level that the HVS can discern or a image can show. Requantization in MATLAB grayslice function Example 5.2 (code) I1 = imread('ml_gray_640_by_480_256.png'); I2 = grayslice(i1,128); figure, imshow(i2,gray(128)); I3 = grayslice(i1,64); figure, imshow(i3,gray(64)); I4 = grayslice(i1,32); figure, imshow(i4,gray(32)); I5 = grayslice(i1,16); figure, imshow(i5,gray(16)); I6 = grayslice(i1,8); figure, imshow(i6,gray(8)); I7 = grayslice(i1,4); figure, imshow(i7,gray(4)); I8 = grayslice(i1,2); figure, imshow(i8,gray(2)); 29 5

6 Requantization in MATLAB Example 5.2 (results) Frame Grabber Process the info from various image sources Store image information quickly and efficiently Offer a graphics user interface (GUI) Be flexible concerning various applications What is used for? How to connect to computer? 32 Components Video Input Unit Video input unit: CCD and storage unit Frame buffer: Digital signal processing Video output unit Image Buffer Video Output Unit

7 Homework #1 (Chapters 1-5) Page 19, 1.3 Page 59, 3.4 Page 80, 4.1 Page 101: 5.3 Read and practice Tutorials 3.1 and 3.2. Describe the working principle of CCD camera sensors. Due Friday Sept