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DIGITAL IMAGE
PROCESSING
Dr. Divya Priya
School of Civil Engineering
SCE, VIT, Vellore
What is a Digital Image?
A digital image is a representation of the real world,
discretized in space with energy reflected / emitted /
transmitted by the objects in the image quantized to finite
number of levels
Digitization
• Digitization involves three steps:
–Sampling
–Quantization
–Coding
Sampling
• View area divided into cells
• Each cell is a picture element pixel
• The image now is a matrix of M rows, and N columns
• M = Length of View area / Length of Cell
• N = Width of View area / Width of Cell
• Smaller cell size better ability to distinguish between
closely spaced objects
Sampling
• In remotely sensed images the sampling is
essentially ground sampling – i.e., on the ground
a virtual grid is placed and the energy reflected /
transmitted / emitted from each grid cell is
collected by the sensors and stored as a pixel
value
Sampling
• Smaller the grid cell area better the details visible in the
image
• The grid cell corresponding to a predefined area on the
ground; e.g., 5.8m x 5.8m
• This is similar to dpi settings in desktop image scanners.
Higher dpi, smaller size of dot, more pixels or cells in the
image
Impact of Pixel Size
• Pixel size corresponds to the Instantaneous Field of View
(IFOV) of the sensing system
• Smaller the IFOV, better is the ability to resolve closely
spaced objects (RESOLUTION)
• Price to pay – larger size of data
• Noise sensitivity of the sensor determines the maximum
possible resolution
Point to Note
• IFOV is 10 metres x 10 metres square means that
objects that are of this size or larger are normally
seen as pixels representing these objects
• Even if an object is smaller, if it has very high or very
low reflectance relative to its background, such object
will be visible despite its size being smaller than the
pixel’s IFOV
Quantization
• Reflected / transmitted / emitted energy from the
object is converted into an electrical signal
• The electrical signal converted to a digital signal by
an analog-to-digital converter (ADC).
• Digital signal takes a range of values according to the
specification of the ADC
ADC
• 8-bit ADC 28 distinct values, represented in binary as
00000000 – 11111111, or 0 to 255 in decimal form or 00
to FF in hex
• 11-bit ADC 211 values, 0 to 2047
• The number of levels indicate the number of distinct
individually differentiable levels of received energy
Point to Note
• When an image contains regions of fine detail, high
spatial resolution (e.g. a stadium with large crowd) is
important
• When the image contains large regions of very little
change such as a close-up of a person, high number
of quantization levels is important
Encoding
• Normally the quantized image is binary encoded.
• If the number of quantization levels is between 0 and 255,
each pixel is represented by 1-byte.
• If the number of levels exceeds 255, each pixel is assigned
two-bytes.
• At present, American satellites Quickbird, Ikonos, Indian
satellites Cartosat and a few others have 11 bit and 10 bit
ADCs and store data in 2 bytes per pixel on disk.
Why Digital Image Processing for
Remote Sensing?
• Nature of data (inherently digital)
• Flexibility offered by computers
• Reducing the bias of human analysts
• Standardizing routine operations
• Rapid handling of large volumes of data
Why Digital Image Processing for
Remote Sensing?
• Certain operations cannot be done manually (removal
of distortions)
• Generation of different views
• Archival in compact/compressed mode
• Easy to share and disseminate
FCC
NCC
Black & White Image
• A black/white image is one that has no color but only
white, black and shades of gray.
• The smallest value at a pixel is 0 (black)
• The largest value is 2L-1 (white)
• L is the number of bits/pixel
• Intermediate values represent shades of gray, from black
increasing towards white
• For L=8, black = 0, white = 255
Gray Scale
Black and White Image
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx
3_1Digital Image.pptx

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3_1Digital Image.pptx

  • 1. DIGITAL IMAGE PROCESSING Dr. Divya Priya School of Civil Engineering SCE, VIT, Vellore
  • 2. What is a Digital Image? A digital image is a representation of the real world, discretized in space with energy reflected / emitted / transmitted by the objects in the image quantized to finite number of levels
  • 3.
  • 4. Digitization • Digitization involves three steps: –Sampling –Quantization –Coding
  • 5. Sampling • View area divided into cells • Each cell is a picture element pixel • The image now is a matrix of M rows, and N columns • M = Length of View area / Length of Cell • N = Width of View area / Width of Cell • Smaller cell size better ability to distinguish between closely spaced objects
  • 6. Sampling • In remotely sensed images the sampling is essentially ground sampling – i.e., on the ground a virtual grid is placed and the energy reflected / transmitted / emitted from each grid cell is collected by the sensors and stored as a pixel value
  • 7. Sampling • Smaller the grid cell area better the details visible in the image • The grid cell corresponding to a predefined area on the ground; e.g., 5.8m x 5.8m • This is similar to dpi settings in desktop image scanners. Higher dpi, smaller size of dot, more pixels or cells in the image
  • 8. Impact of Pixel Size • Pixel size corresponds to the Instantaneous Field of View (IFOV) of the sensing system • Smaller the IFOV, better is the ability to resolve closely spaced objects (RESOLUTION) • Price to pay – larger size of data • Noise sensitivity of the sensor determines the maximum possible resolution
  • 9. Point to Note • IFOV is 10 metres x 10 metres square means that objects that are of this size or larger are normally seen as pixels representing these objects • Even if an object is smaller, if it has very high or very low reflectance relative to its background, such object will be visible despite its size being smaller than the pixel’s IFOV
  • 10. Quantization • Reflected / transmitted / emitted energy from the object is converted into an electrical signal • The electrical signal converted to a digital signal by an analog-to-digital converter (ADC). • Digital signal takes a range of values according to the specification of the ADC
  • 11.
  • 12. ADC • 8-bit ADC 28 distinct values, represented in binary as 00000000 – 11111111, or 0 to 255 in decimal form or 00 to FF in hex • 11-bit ADC 211 values, 0 to 2047 • The number of levels indicate the number of distinct individually differentiable levels of received energy
  • 13.
  • 14. Point to Note • When an image contains regions of fine detail, high spatial resolution (e.g. a stadium with large crowd) is important • When the image contains large regions of very little change such as a close-up of a person, high number of quantization levels is important
  • 15.
  • 16.
  • 17.
  • 18.
  • 19. Encoding • Normally the quantized image is binary encoded. • If the number of quantization levels is between 0 and 255, each pixel is represented by 1-byte. • If the number of levels exceeds 255, each pixel is assigned two-bytes. • At present, American satellites Quickbird, Ikonos, Indian satellites Cartosat and a few others have 11 bit and 10 bit ADCs and store data in 2 bytes per pixel on disk.
  • 20. Why Digital Image Processing for Remote Sensing? • Nature of data (inherently digital) • Flexibility offered by computers • Reducing the bias of human analysts • Standardizing routine operations • Rapid handling of large volumes of data
  • 21. Why Digital Image Processing for Remote Sensing? • Certain operations cannot be done manually (removal of distortions) • Generation of different views • Archival in compact/compressed mode • Easy to share and disseminate
  • 22.
  • 23.
  • 24.
  • 25.
  • 26.
  • 27.
  • 28. FCC
  • 29.
  • 30. NCC
  • 31. Black & White Image • A black/white image is one that has no color but only white, black and shades of gray. • The smallest value at a pixel is 0 (black) • The largest value is 2L-1 (white) • L is the number of bits/pixel • Intermediate values represent shades of gray, from black increasing towards white • For L=8, black = 0, white = 255
  • 33.