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Converting from an analog to a digital image.
1. Sampling image into pixels----> "picture element"
2. Quantize the pixel value to make discrete or finite
Digital imaging is the process of sampling and quantizing the analog image
(can store in computer)
SAMPLING in more detail
1. fit a grid over the image, create pixels
pixel location = (r,c)=(row,column)
2.Get value of each pixel
Pixel(r,c) =(R,G,B) = (red,green,blue)
RED = 
red(x,y)
dxdy
(over pixel area)
red(x,y) = function of red spectral energy at each contigous point in your sensor
Image size = (rows)*(colomns)
Resolution = size of image = #pixels
low resolution ---> # of pixels is small
high resolution ----> # of pixels is big
HOW DO YOU SELECT A RESOLUTION?
if grid is too large you will get jagged edges
QUANTIZATION in more detail
continuous value ---> discrete value
Z = decision level (Z1, Z2, Z3)
Qk = quantization level (Q0, Q1, Q2)
the above are all discrete values
The rule to convert
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How do we choose the Q's and Z's
The goal is to choose Q's & Z's to minimize error produced from quantization, Ewhere
E = Expected value{(pixel- newpixel)*(pixel-newpixel )}
= mean squared error
Note:
square error= (pixel value - new value)
expected value = a type of average or mean
If we try to minimize this error this leads to the following equation
Qk= (Zk+1+Zk) / 2
So, Steps to follow are:
- choose Zk
- calculate Q's via above equation
How Do We Choose Z's?
Basically, it is function of image content
You want more levels (Z values) in greyvalue or color ranges in which much
of the image pixels fall.
What do you loose in making a Digital Image?
1. Loss from sampling?
Over-Sampling = when you have more samples than you need
Under-Sampling = not enough samples are used
How many samples do you need?
You need 2 times the Nyquest rate.
Nyquest rate= function of highest frequency in the image
highest frequency in image = function of fastest varying spatial patter in
the image = f(how fast things change)
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2. Loss from Quantization?
There is always loss from process of quantization