Digital Imaging and Image Analysis

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Digital Imaging and Image Analysis Bozzola and
Russell Chapter 18
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A black and white photograph is nothing more than
a two dimensional arrangement of black (silver
grains) and white (paper backing from which
grains have been removed) arranged in such a way
that they create an image.
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Image Processing
Optical Mechanical manipulation (lenses,
enlargers, etc.) Analog Electronic
manipulation Digital Computer manipulation
Image as a data matrix
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Image Processing
Optical Brightness, contrast, enlargement,
cropping, dodging burning Analog Di
gital
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Image Processing
Optical Brightness, contrast, enlargement,
cropping, dodging burning Analog
Brightness, contrast. Digital
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Image Processing
Optical Brightness, contrast, enlargement,
cropping, dodging burning Analog
Brightness, contrast. Digital Brightness,
contrast, enlargement, cropping,
dodging burning, non- linear
display, contrast stretching,
erosion dilation, edge
enhancement, etc.
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Contrast vs. Brightness
Brightness The location of a visual perception
along the black to white continuum
Bright
Dark
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Contrast vs. Brightness
Contrast The range of optical density and tone
on a photographic image (or the extent to which
adjacent areas on a CRT differ in brightness.
High
Low
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Contrast vs. Brightness
As contrast is increased the overall brightness
of the image appears to increase in some areas
and decrease in others
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In photographic image processing brightness is
modified by exposure which is a function of
illumination intensity X time. The longer the
time and/or the more intense the illumination the
brighter the image will be. In analog
processing brightness is increased by increasing
the signal going to each pixel In digital
processing brightness is increased by increasing
the numerical value of each pixel by an equal
amount.
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In photographic image processing contrast is
controlled by the size of silver grains in the
emulsion, the larger the grains the greater the
contrast. In analog processing contrast is
increased by varying the ranges of signal to each
pixel. In digital processing contrast is
increased by expanding the differences in
numerical values between the maximum and minimum
pixels.
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Image Processing
Optical Stored as negatives (original) or
prints (second generation) Analog Stored as
video (original) or copy (second
generation) Digital Stored as computer data
file (original and multiple copies all first
generation)
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Image Processing
Optical Transmitted by distribution of prints
(second generation) or publication (fourth
generation) Analog Transmitted by distribution
of video tapes (second generation) or
broadcast (third generation) Digital
Transmitted by file copying and transfer (all
first generation)
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Digital Imaging
A digital image is nothing more than a data
matrix that assigns both a value and a location
to each picture element (pixel) in the image
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Digital Imaging
When considering the resolution of a digital
image we need to consider both of these aspects
value of the pixel and its position in the data
matrix
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Digital Imaging
Spatial resolution is defined as the number of
pixels used to create the image or more simply
the total number of pixels in the matrix.
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Spatial Resolution
100 x 200 25 x 50 13 x 25 3
x 6
Pixel resolution is typically given as X x Y
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Spatial Resolution
100 x 200 25 x 50 13 x 25 3
x 6
More is BETTER!
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A digital image from a typical SEM or TEM is
often 2K x 2K format whereas a photographic
negative from a TEM is closer to 8K x 6K if one
were to count silver grains as pixels.
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Pixels vs. Dots per Inch
Pixel resolution refers to the number of pixels
that comprise the image regardless of how large
the image is. A typical computer monitor has a
1024 x 768 pixel resolution whether it is a 15
or 21 Dots per Inch or DPI refers to the
number of pixels per linear inch in the final
image. Thus the DPI of a given image decreases as
the size of the final image increases.
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Digital Imaging
Grey level resolution refers to the range of
values that each pixel might have. The greater
the value range, the greater the grey level
resolution.
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Grey Level Resolution
Bit a unit of binary information either a
0 or a 1 Byte a string of
eight bits KiloByte 1000 Bytes or 8000
bits MegaByte 1000 KB
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Grey Level Resolution
One bit can code for only two values or states
(0) or (1) Two bits can code for four states
(0,0) (0,1) (1,0) or (1,1) Three bits for eight
and so on.... One Byte can code for 28 or 256
different states (0,0,0,0,0,0,0,0)
(0,0,0,0,0,0,0,1)...
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This is often referred to as the bit depth of
an image and it limits the range of pixel values
that can be displayed in the image.
1 8 24
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Shades of Grey
The average human eye can perceive fewer than 100
different shades of grey between absolute white
and absolute black. Thus an 8-bit image, with a
possible 256 different levels of grey is more
than sufficient to display any typical BW image.
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Range of Color
Color digital images can also be based on 8-bit
format but in this case a total of three separate
pixels (Red, Green, Blue) are combined for a 24
bit-depth range of colors. 28 x 28 x 28 224
RGB
16,777,216 different colors
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The best digital capture devices are capable of
capturing images in 12 or even 16-bit (216) grey
scale format and are cooled to reduce the effects
of noise from the electronics.
Micro Luminetics Cryocam
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The data storage demands increase dramatically
depending on the spatial and grey-scale
resolution
512 x 512 8-bit image 262 KB 1024 x 1024
8-bit 1,049 KB 1024 x 1024 12-bit 1,573
KB 2048 x 2048 16-bit 8,389 KB
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Grey Level Resolution
The number of possible pixel values increases
dramatically with relatively modest demands on
data storage.
8-bit image 256 shades of grey 12-bit image
4096 shades of grey 16-bit image 65,536
shades of grey
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If humans can only see 100 shades of grey why
bother with so much data?
8-bit 12-bit
16-bit
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Bit-depth can be an important aspect of color
images. The human eye can discern many different
colors and hues so the dynamic range of a color
image is important.