The Concepts of Digital Imaging Dr. Mohammed Alnafea alnafea@hotmail.com www.dralnafea.com

Overview of the topics: 5/17/2018 Overview of the topics: Image Definition or What is an image? Why are digital images (Digital System) interesting? Basic Concepts Scanning 2 2

Image definitions -- An image or picture is an artifact, usually two-dimensional, that has a similar appearance to some subject—usually a physical object or a person. The 2d images can be represented by 2 ways: A 2-dimensional representation of the function, I = f(x,y) where  f(x,y) is a continuous function for analog images and  f(x,y) is a discrete function for digital images. 3

The function f is a map of some value… light intensity, or X- ray intensity as a function of coordinates. The values of I can be either continuous or discrete. Most analog images are not really continuous functions either. For example, film has a finite grain size. Also, the noise in an analog signal will limit the extent of the “continuous” function. -- Images may be two-dimensional, such as a photograph, screen display, and as well as a three-dimensional, such as a statue. They may be captured by optical devices—such as cameras, mirrors, lenses, telescopes, microscopes, etc. and natural objects and phenomena, such as the human eye or water surfaces. What is an image? The function f is a map of some value… light intensity, SE, BSE, or X-ray intensity, height, current, magnetic force, or stiffness (in STM or AFM, etc.) as a function of coordinates. The values of I can be either continuous or discrete. Actually most analog images are not really continuous functions either. For example, film has a finite grain size. Also, the noise in an analog signal will limit the extent of the “continuous” function. The values of I can be either continuous or discrete. 4 4

What is the analog images ? Analog images can be photographic prints. This type of image can also be any continuous tone image.

Why are digital images interesting? Humans are visual creatures in a visual world Images are (often) the primary sense ”A picture is worth a 1000 words” Words are many times ambiguous Images in a computer are DIGITAL, as opposed to analog images using film 6

-- what is the digital images ? A digital image is a representation of a two-dimensional image using ones and zeros (binary). It is an image that is electronically produced using digital cameras, scanners, software programs. The images are files that can be displayed on a computer monitor, printed on paper, or stored as a file on media like CD-ROMs. Depending on whether or not the image resolution is fixed, it may be of vector or raster type.

-- Raster images have a finite set of digital values, called picture elements or pixels. The digital image contains a fixed number of rows and columns of pixels. Pixels are the smallest individual element in an image, holding quantized values by bits. An 8-bit grayscale image has pixel values that range from 0 to 255. that represent the brightness of a given color at any specific point.

-- In computer graphics, a raster graphics image or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium. A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. Bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent).

Imagine the smiley face in the top left corner as an RGB bitmap image Imagine the smiley face in the top left corner as an RGB bitmap image. When zoomed in, it might look like the big smiley face to the right. Every square represents a pixel. Zooming in further, the individual pixels can be analyzed, with their colors constructed by adding the values for red, green and blue

Digital Image Representation y x f(x,y) Origin Image is seen as a discrete function f(x,y) as opposed to a continuous function (show) x and y cannot take on any value! 11

Discrete image coordinate system x f(x,y) Origin y x f(0,0) f(?,?) f(2,6) 12

Pixels Most digital images are a grid of pixels, a checkerboard of colored squares, where each square is a single color Each pixel is described by a number 100% 400% 800% 13

Digital Image Representation Pixel representation (bits) A few words on bits and bytes: One bit: {0,1} One byte = eight bits One pixel: one byte = eight bits = one number: [0,255] (show) Grey-scale, intensity, black/white: 8 bits = [0,255] Binary image: 1 bit {0,1}. Black and white: visualized as: 8 bit {0,255} Colors: after the break 14

Digital Imaging Image Acquisition Image Storage Image Processing Image Printing 15

Where does an image come from? 16 Image elements, picture elements, pels, pixels

Where does an image come from? Under exposed Correct exposed Over exposed 17

Imaging system Image acquisition Illumination Camera lens Passive: sun Active: X-ray, gamma-ray Camera lens Focus the light on the CCD chip 18

-- Charged Coupled Device (CCD): Sensors that are light sensitive and arranged into a grid called an area array. They are found in digital cameras, scanners and video cameras. A charge-coupled device (CCD) is an analog shift register, that enables the conversion of light to electric charges through capacitors controlled by a clock signal.

Digital Image Representation An image f(x,y) is represented as an Array. In computer science an array is a data structure consisting of a group of elements that are accessed by indexing. In computer science an array is a data structure consisting of a group of elements that are accessed by indexing. Width = number of pixels in x-direction Height = number of pixels in y-direction Size (width x height, width > height) ROI = region of interest To reduce the amount of data Width ROI Height 20

Spatial Image Resolution: The display resolution of a digital television or computer display typically refers to the number of distinct pixels in each dimension that can be displayed Different Resolutions are possible (256x256….16x16) Lower resolution leads to data reduction! The resolution can be measured by PPI (Pixels Per Inch. A measure used in image resolution ) An image that is high in resolution shows more detail than an image that is low in resolution. Pictures that are high in resolution have a higher number of PPI. 21

How do you acquire a digital image? Film / scanner Video camera / frame store Digital camera Digital scan generator These are some of the typical methods for acquiring digital images. We live in an analog world. At some point we need to convert the analog image to the digital domain. In each of these methods, the transition from the analog domain to the digital takes place at different points in the imaging process. 22 22

Film Film, while an old technology, has many benefits as a recording medium. High spatial resolution ) The measure of how closely lines can be resolved in an image is called spatial resolution see the next figure “Parallel” image recording Long storage life No special reader needed Cheap Film, while an old technology, still has a lot to offer in terms of a recording medium. Film records at a high spatial resolution and records the image in “parallel”. It has a long storage life and can easily be examined without using a special reader. In terms of cost per image, it is one of the least expensive formats. It also has some disadvantages. The intensity response curve is not linear and film has a limited dynamic range. Negatives, once filed, are easily lost unless great care is taken to index them. 23 23

The 1951 USAF Resolution Test Target is a classic test target used to determine spatial resolution of imaging sensors and imaging systems.

Film is characterized by: Film grain and speed -- resolution is determined by the grain size. 1- Film grain or granularity is the random optical texture of processed photographic film due to the presence of small grains of a metallic silver developed from silver halide that have received enough photons 2- Film speed is the measure of a photographic film sensitivity to light. Film with lower sensitivity (lower ISO/ASA speed) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity can shoot the same scene with a shorter exposure and is called a fast film 3- Contrast The contrast ratio is a measure of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing Film is usually characterized by its speed (light sensitivity) and by its graininess (i.e. the size of the light-sensitive silver halide particles). Generally film that is more sensitive to light has larger grains and hence a somewhat worse spatial resolution. SO-163, a TEM film, has about a 12 m grain size. A film’s resolution may also be expressed in terms of its MTF (modulation transfer function). This is basically an expression of the resolution in terms of spatial frequency (1/distance). Film can also be characterized by its contrast, i.e. how much light it takes to change the film from clear to full exposure. The density range of some films can range up to 4. 25 25

4- Optical Density Optical density, also known as Dynamic Range, is the scanner's ability to "see" all tones available between the black and white which mean the gray scale. Another way to think of Optical Density is contrast. A scanner with limited Optical Density ability yields a higher-contrast image that has lost highlight and shadow detail. Conversely, a scanner with a high Optical Density ability yields a full range of tonal detail from shadow to highlight areas.

Image With Full Tonal Density

Image With Compromised Tonal Density Notice the lost detail in the highlight and shadow areas.

In summary, Optical Density represents how much of the original's (scanned source) total tonal information is represented by the scanner.

Dynamic range is a term used frequently in numerous fields to describe the ratio between the smallest and largest possible values of a changeable quantity, such as in sound and light. An example of a rendering of an HDRI tone-mapped image in a New York City nighttime cityscape.