EC-433 Digital Image Processing

Transcription

1 EC-433 Digital Image Processing Lecture 2 Digital Image Fundamentals Dr. Arslan Shaukat 1

2 Fundamental Steps in DIP

3 Image Acquisition An image is captured by a sensor (such as a monochrome or color TV camera) and digitized. If the output of the camera or sensor is not already in digital form, an analog-to-digital converter digitizes it. Pre-processing such as scaling.

4 Image Enhancement To bring out detail which is obscured, or simply to highlight certain features of interest in an image.

5 Image Enhancement

6 Enhancing objects of interest

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8 Image Restoration Improving the appearance of an image Tend to be based on mathematical or probabilistic models of image degradation Distorted Image Restored Image

9 Color Image Processing Gaining in importance because of the significant increase in the use of digital images over the Internet

10 Wavelets Foundation for representing images in various degrees of resolution. Used in image data compression and pyramidal representation (images are subdivided successively into smaller regions)

11 Compression Reducing the storage required to save an image or the bandwidth required to transmit it. Example is JPEG (Joint Photographic Experts Group) image compression standard.

12 Morphological Processing Tools for extracting image components that are useful in the representation and description of shape.

13 Image Segmentation Image processing system tries to separate objects from the image background Output of the segmentation stage is raw pixel data, constituting either the boundary of a region or all the points in the region itself.

14 Segmentation

15 Segmentation

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17 Representation & Description Representation make a decision whether the data should be represented as a boundary or as a complete region. Boundary representation focus on external shape characteristics, such as corners and inflections. Region representation focus on internal properties, such as texture or skeleton shape. Description, also called feature selection, deals with extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another.

18 Representation & Description

19 Recognition Recognition the process that assigns a label to an object based on the information provided by its descriptors.

20 Knowledge Base A problem domain detailing regions of an image where the information of interest is known to be located. Help to limit the search

21 Not All the Processes are Required Example: Postal Code Problem

22 Human Visual Perception Developing a basic understanding of human visual perception Interest lies in the mechanics and parameters related to how images are formed in the eye.

23 Human Eye The eye's lens, iris & cornea form the optical system to projects an upside down image onto retina The iris is a membrane in the eye, responsible for controlling the diameter and size of the pupil and the amount of light reaching the retina. The pupil is an opening located in the centre of the iris of the eye that allows light to enter the retina Retina contains the light sensors

24 Retina The Sensor Chip Cones -- Responsible for bright-light vision (Photopic Vision) 6 to 7 million Resolve fine details High visual resolution Rods -- Responsible for dim-light vision (Scotopic Vision) 75 to 150 million Lesser details overall picture Monochrome vision

25 Image Formation in the Eye The distance between the lens and the retina is fixed. Focal length needed to achieve proper focus is obtained by varying the shape of the lens.

26 Brightness Discrimination The ability of the eye to discriminate between changes in brightness is of considerable interest. I is a uniform illumination on a flat area large enough to occupy the entire field of view. ΔI c is the change in the object brightness required to just distinguish the object from the background Weber's ratio: ΔI c /I If ΔI c /I is small Good brightness discrimination If ΔI c /I is large Poor brightness discrimination

27 Weber Ratio Brightness discrimination is poor (the Weber ratio is large) at low levels of illumination, and it improves significantly (the Weber ratio decreases) as background illumination increases. At low levels of illumination vision is carried out by rods, whereas at high levels (showing better discrimination) vision is the function of cones.

28 Brightness Discrimination Brightness is not a simple function of intensity. Visual system tends to undershoot or overshoot around the boundary of regions of different intensities. The intensity of the stripes is constant but we actually perceive a different brightness pattern.

29 Simultaneous Contrast All the small squares have exactly the same intensity, but they appear to the eye progressively darker as the background becomes brighter. Region s perceived brightness does not depend simply on its intensity.

30 Human Perception Phenomena

31 Components of an Image Formation Imaging Source --- Illumination Sunlight, X-Rays Reflectance Certain wavelengths absorbed; certain reflected Imaging Surface Sensor or Eye The reflected or transmitted energy is focused onto a photo converter (e.g., a phosphor screen), which converts the energy into visible light

32 Image Sensing and Acquisition Incoming energy is transformed into a voltage by the combination of input electrical power and sensor material The output voltage waveform is the response of the sensor(s), and a digital quantity is obtained from each sensor by digitizing its response.

33 Image Acquisition

34 Image Acquisition Collect the incoming energy and focus it onto an image plane. If the illumination is light, the front end of the imaging system is a lens, which projects the viewed scene onto the lens focal plane. The sensor array, which is coincident with the focal plane, produces output voltage. Digital and analog circuitry sweep these outputs and convert them to an analog signal, which is then digitized by another section of the imaging system. The output is a digital image.