KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 1 DIGITAL IMAGE FUNDAMENTALS KOM3212 Image Processing in Industrial Systems Some of the contents are adopted from R. C. Gonzalez, R. E. Woods, Digital Image Processing, 2nd edition, Prentice Hall, 2008 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Today’s lecture • Elements of visual perception • Visible Light Spectrum • Imaging with Sensors • Image Models 2 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 3 Elements of visual perception • Although the digital image processing field is built on a foundation of mathematical and probabilistic formulations mimicking the human vision system plays an essential role. Simplified Diagram of Human Eye KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 4 Elements of visual perception Three membranes enclose the eye 1. The cornea and sclera outer cover : Cornea is a transparent tissue. Sclera is an opaque membrane encloses the remainder of the optic globe 2. The choroid : directly below Sclera. Blood vessels. Divided into 3 sections, -Ciliary body: fibers and muscles -Iris diaphragm: contracts or expands to balance the amount of light entering -Lens : suspended by fibers that attach to the ciliary body. Helps focusing. 3. The retina : the innermost membrane -Cones vision (photopic / bright-light vision): centered at fovea, highly sensitive to color. ~7 million -Rods (scotopic /dim-light vision): general view. 75 to 150 million -Blind spot KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 5 Elements of visual perception Distribution of rods and cones on the retina • The fovea itself is a circular indentation in the retina about 1.5mm in diameter. • It can be regarded as 1.5mm x 1.5mm square sensor. • For a CCD sensor the same number of elements can be on a 5mm x 5mm chip • When the intelligence and light adaptivity is added human eye is outweighs the man-made sensors 6 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Elements of visual perception Cone distribution on the fovea (200,000 cones/mm2) •Models human visual system more precisely •Hexagonal sampling requires fewer samples than rectangular sampling KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 7 Image formation in the eye • The distance between the center of the lens and the retina (focal length) varies from 17mm to 14 mm as the refractive power of the lens increases from its max to its min • The shape of the lens is controlled by tension of the fibers in the ciliary body. • To focus on distant objects lens is flattened. • Lens is thickened to focus on near objects 8 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Image formation in the eye electrical impulses radiant energy Light receptor Brain KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Brightness adaptation • Subjective brightness is what human perceives. • Dynamic range of human visual system • 10-6 ~ 104 • Cannot accomplish this range simultaneously • For a given set of conditions current sensitivity level is called brightness adaption • Total adaption range is to large when compared to brightness adaption level. • The current sensitivity level of the visual system is called the brightness adaptation level 9 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Brightness discrimination • Weber ratio (the experiment) DIc/I • I: the background illumination to the opaque glass • DI increment of illumination in the center • DIc : the increment of illumination discriminable 50% of the time with background illumination • Small Weber ratio indicates good discrimination • Larger Weber ratio indicates poor discrimination 10 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 11 Perceived brightness The perceived brightness is not a simple function of intensity This side of the boundary is perceived darker than it is This side of the boundary is perceived brighter than it is KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 12 Perceived brightness All inner squares have the same intensity but they appear to be brighter from left to right when background is set darker KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 13 Light and Electromagnetic Spectrum • Electromagnetic spectrum can be expressed in terms of wavelength and frequency 𝑐 • Wavelength and frequency are related by: 𝜆 = 𝜈 𝑐 is the speed of the light, 𝜆 is the wavelength,𝜈 is the frequency. • The energy of EM waves is : 𝐸 = ℎ𝜈 (ℎ is Planck constant, 𝐸 is the energy) • Visible light represent a very small portion of the electromagnetic spectrum KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 14 Light and Electromagnetic Spectrum • Electromagnetic waves can be regarded as a stream of mass-less particles each travelling in a wavelike pattern and moving at the speed of light • Each particle contains a certain amount of energy KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 15 Light and Electromagnetic Spectrum • Light is a particular type of electromagnetic radiation • 0.43m-violet to about 0.79m-red • For convenience 6 bands are used to name the light in the visible spectrum • The colors that human perceive on an object is determined by the light it is reflected from it. • A body that reflects all of the light bands will be perceived as white to the user. If a body favors specific band and reflects it that we see it having that color. KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 16 Color Perception of Human Eye • Humans do not have a sensor for each frequency of light • Instead, rods and cones are employed. • Cones focus on recognizing brighter light (photopic vision) with more sensitivity about individual colors, • Rods on dim lighting scenarios (scotopic vision) with less color sensitivity, and a combination of the two for in-between lighting (mesopic vision). • cones detect light along something resembling a gaussian curve. There are three wavelengths at which the cones are centered, as shown blog.triplelift.com KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 17 Imaging Sensors An image sensor converts a optical image into an electronic signal. It is used mostly in imaging devices. Today, most digital still cameras use either a CCD image sensor or a CMOS sensor Neither technology has a clear advantage in image quality CMOS sensors can potentially be implemented with fewer components, use less power KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Imaging Sensors 18 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 19 Color Images in Cameras There are several main types of color image sensors, differing by the type of color-separation mechanism: One of them is Bayes filter sensor: An array of color filters is used. Original light is separated to its bands and the resulting patterns are registered to form a single image, en.wikipedia.org KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek A simple image formation model • f(x,y): the intensity is called the gray level for monochrome image • f(x, y) = i(x, y).r(x, y) • 0 < i(x, y) < inf, the illumination (lm/m2) • 0< r(x, y) < 1, the reflectance • Some illumination figures (lm/m2) • 90,000: full sun • 10,000: cloudy day • 0.1: full moon • 1,000: commercial office - 0.01: black velvet - 0.93: snow 20 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek A simple image formation model 21 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Sampling and Quantization 22 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Sampling and Quantization 23 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Sampling and Quantization 24 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek An effect of under-sampling, aliasing Aliasing (the Moire effect) www.wfu.edu/~matthews/misc/DigPhotog/alias/ 25 26 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 1x 1/2 x 1/4 x 1/8 x KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Uniform quantization • Digitized in amplitude (or pixel value) • PGM – 256 levels 4 levels 255 192 3 2 128 64 0 1 0 27 KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek 28 Uniform quantization Original 8 bits 4 levels (2 bits) 16 levels (4 bits) 2 levels (1 bit) KOM3212 Image Processing in Industrial Systems | Dr Muharrem Mercimek Image resolution • Spatial resolution • Line pairs per unit distance • Dots/pixels per unit distance • dots per inch - dpi • Intensity resolution • Smallest discernible change in intensity level • The more samples in a fixed range, the higher the resolution • The more bits, the higher the resolution 29
© Copyright 2024 Paperzz
