5th Intensive Course on Soil
Micromorphology
Naples 2001
12th - 14th September
Image Analysis
Lecture 2
Image Acquisition
Historic Aspects
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
The four I’s of Microfabric Analysis
• Image acquisition
• normal photographs (via scanner)
• optical microscope (via digital camera)
• electron microscope (directly via A/D converter)
•Image Processing
• Image enhancement
• Filtering
• Image reconstruction
• Image Analysis
•Quantitative analysis
• Interpretation
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Image Acquisition: - Requirements
•Spatial Resolution
•Intensity resolution
•Illumination issues
•Image distortion
•Image formats
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Image Acquisition: - Requirements
Spatial Resolution
Photographic Recording:
Photographic enlargement is possible often up to 10
times depending on grade of film.
Low magnification covers larger area, and detail can be
seen with enlargement.
Digital Recording:
NOT possible to enlarge to “see” detail.
Magnification / resolution must be selected at outset.
May be a compromise between area covered and detail.
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Spatial Resolution:- Digital Recording
Image divided into pixels which are the smallest element that
can be resolved. Area covered by each pixel depends on the
field of view and this will vary with the magnification in use.
Field of view - 0.1 mm ( ~ x1000 in the SEM).
Digital recording medium resolution - 512 x 512 pixels
>>>> each pixel will represent about 0.2 m.
Enlargement of (a) by x 4 gives no
additional information.
Isolated features (c) 1 pixel wide can
be resolved, but not if they touch (e).
Pixel size should be no more than ~
50% of smallest feature.
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Spatial Resolution:- Digital Recording
•Most modern frame stores /cameras have a minimum
resolution of around 400 - 500 pixels
•Ideally, for high quality analysis 1000 - 2000 pixel
device should be used.
•One SEM manufacturer has an option of a frame store
> 3000 pixels.
In the SEM
double number of pixels >>> 50% reduction in magnification
increases the area covered by a factor of 4.
[but may not be entirely true if beam diameter is adjusted with
magnification]
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Spatial Resolution:- Digital Recording
Scanners:
•
The resolution on these may be adjusted
•
Often set at default values between 50 and 200 dpi
•
options often up to 1200 dpi
For a 150 x 150 mm image and 100 dpi,
image is 600 x 600 pixels. Each pixel  0.25 mm
At 1200 dpi, the image will be very large at 7200 x 7200 pixels
Each pixel  0.02mm
For a black and white image (grey-level image):
storage 100 dpi >> 0.36Mbyte storage (for 150 x 150 mm image)
1200 dpi >> 50 Mbyte for a single image.
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Intensity resolution
Human eye can resolve about 16 grey levels at best
Monochrome publications can achieve 8 at best.
Digital Images:
Binary image: Black and White only
pores black - solids white or vice versa
Grey Level Image:
0 - 15 grey levels
0 - 255 grey levels
0 - 65535 or higher
False Colour:
0 - 255 pseudo colours
True Colour:
16+ million colour shades
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Intensity resolution
Binary images:
Binary images are basis of much image processing e.g.
particle/feature size analysis.
Each pixel is either 1 (foreground) or 0 background and
may be stored in a single bit.
Data storage is usually in form of BYTES - or 8 bits.
8 pixels stored in each byte. (32 kbytes needed to store
image.
Most images are not in binary form and must be
converted before most image processing/analysis
packages can be used.
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Intensity resolution
Grey Level Images:
Storage values for 512 x 512 image
16 grey levels (used by older devices).
store data from 2 pixels in each byte >>128 kBytes
BYTE: 256 grey levels [most common]
>> 256 kBytes
range 0 - 255 [must be integer values]
INTEGER: 65536 grey levels [2 bytes per pixel] >> 512 kBytes
range -32376 - 32375 [must be integer values]
REAL:
Higher grey level resolutions [REAL format]
storing as 4 bytes per pixel
>> 1024 kBytes
minimum intensity -1 x 1038 : maximum 1 x 1038
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Intensity resolution
False Colour:
Usually equivalent to 256 grey level images:
individual grey level shades are shown by colour shades to
overcome limitations of human eye.
True Colour:
Often as RGB (red: green: blue) - [most common]
each colour has an intensity range 0 - 255 giving
16+ million possible colours. 3 bytes (24 bit) are needed
for each pixel
Sometimes CYM (cyan: yellow: magenta) is used
Alternatively: HSI (Hue: Saturation: Intensity)
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Illumination issues
Binary Images often needed for analysis
BUT
Illumination Settings at acquisition can often bias the results
•brightness and contrast of illumination >>>
saturation
•brightness and contrast setting on SEM >>> saturation
•intermediate photographic developing and printing
>>>
saturation
May want high contrast (and saturation for binary images)
BUT in other cases full dynamic range should be retained.
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Illumination issues
full 0 - 255
range
saturated
above 150
Saturated
above 200
saturated
above 150
and below 50
Though (b) or (c) may look better, (a) contains more information
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Illumination issues
full 0 - 255
range
Oversaturated
Some image acquisitions systems when using byte (0 - 255)
range - reflect range when saturation occurs.
256 >>> 0, 257 >>> 1 etc. - see effect in bright areas
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Illumination issues
Normal Image
Non-uniform
illumination will create
problems in later
thresholding and analysis
Centralised
Illumination
Gradation from
top left
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Image Distortion
Optics of microscope can distort image
>>> problems with measurement
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Image Distortion
Microscope should be calibrated with a grid
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Image Formats
BMP: TIFF:
JPG:
GIF ------Widespread
Some formats require key information to be stored in another
file problems if associated file is removed
Most formats have key header information in file itself.
Most compact formats use BYTE (0 - 255) format
can be swapped easily between platforms
BUT
INTEGER format (2 bytes) may be problematic
- swapping between PC/Macintosh/UNIX (as high and low
bytes are swapped).
Even more problems if REAL/ COMPLEX format is used
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Image acquisition
Image Formats
Typical Image 512 x 512 x 8 >>>> 256 kBytes
True Colour Image 4096 x 4096 x (24bit) would occupy more
than 50 Mbytes
Special Formats - (RLE)
for Binary Images and selected Classified Images arising from Image Analysis
- more efficiently stored in a Run-Length-Encoded format (RLE).
Applicable for BYTE and INTEGER format if large areas have the same
“intensity value” - compression up to 90+% is possible.
May need to re-expand image back to normal BYTE or INTEGER format
before using for Analysis purposes.
Cannot be used on grey-level images
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Historic Development of Image Quantification
Optical Methods
Optical Diffraction was
used as a method to assess
orientation in 1970s.
Related technique of
Convolution Square was
also used
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Historic Development of Image Quantification
Upturned plates on sand grain
The diffraction pattern indicates preferred orientation
direction, degree of orientation and spacing of features
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Historic Development of Image Quantification
A high degree of orientation in consolidated clay shown by
shape of diffraction pattern.
Note: inverse spacing relationship between diffraction pattern
and image
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Digital equivalent of Optical Diffraction
The Optical Diffraction Pattern of images may be computed digitally as
Fourier Transform:
- this is exploited in image reconstruction in Lecture 10
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Digital equivalent of Convolution
Image
Convolution of image with
itself (cross-correlation)
Note: unlike diffraction the spacing of features in convolution
is directly related to spacings in image
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Stereoscopy/Photogrammetry
SEM is ideal for photogrammetric measurement using
stereo-photographs
Geometry of SEM should be considered with viewer at the
electron source and illumination at the electron collector - i.e.
and apparent reversal of convention.
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Geometry of SEM
At low magnification
< 500
central projection
At high magnification
> 1000x
parallel projection
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Unconsolidated Kaolin - picture width = 6.2 m
5th Intensive Course on Soil Micromorphology - Naples 2001
Image Analysis - Lecture 2 Historic Aspects
Stereo-photogrammetry
Stereo photograph and
associated Pole Diagram showing
preferred 3 - D orientation