Quiz

1
Quiz 6 Topics

• cut
• paste
• sort
• tail
• head

2
Numbering Systems
3
Converting Between Bases

• 42 base 10 converted to base 10,
• 42 base 10 converted to base 8,

4
Converting Between Bases

• 42 base 10 converted to base 2,
• 42 base 10 converted to base 16,

5
Number Base Conversion using bc

• To convert decimal to binary
• obase2
• 42
• 101010
• To convert octal to hexadecimal
• obase16
• ibase8
• 52
• 2A

6
Negative Number Representation
7
Converting a Negative Number into Binary using
2s Complement

• To convert -42 into a negative binary value
• Determine the number of bits to represent (e.g.
  16 bits)
• Convert the absolute value (42) into binary
• 0000000000101010
• Take the 1s complement of the number
• 1111111111010101
• Add 1 to the number
• 1111111111010110

8
Floating Point Representation

• Sets of bits are separated into
• mantissa
• exponent field

9
Another Example of Unformatted Output (e.pro)

• pro e
• openw, lun, 'e.dat', /get_lun
• e exp(1.0)
• writeu,lun, e
• free_lun, lun
• end

10
Octal Interpretation of e.dat (big endian)

• od -b e.dat
• 0000000 100 055 370 124
• 0000004

11
Binary Interpretation of e.dat (big endian)

• bc -l
• obase2
• ibase8
• 100
• 1000000
• 55
• 101101
• 370
• 11111000
• 124
• 1010100

12
Binary Interpretation of e.dat (big endian)

• 01000000 00101101 11111000 01010100

13
(SINGLE PRECISION) - IEEE (ANSI/IEEE Std
754-1985)

• --------------------------------
• S exp fraction
• --------------------------------
• Bit31 Bit0
• A formula that gives the value of this float is
• Value(-1)S X 1.fraction X 2(exp-127)

14
Binary Interpretation of e.dat (big endian)

• 01000000 00101101 11111000 01010100
• Bit 31
• S 0
• Bits 3023
• exp 1000000 0 128 (decimal)
• Bits 220
• fraction 0101101 11111000 01010100

15
(SINGLE PRECISION) - IEEE (ANSI/IEEE Std
754-1985)

• Value(-1)S X 1.fraction X 2(exp-127)
• 2(128-127) 2
• bc -l
• ibase2
• 1.0101101 11111000 01010100
• 1.35914087295532226562500
• ibase1010
• 21.35914087295532226562500
• 2.71828174591064453125000

16
Floating Point Interpretation of e.dat (big
endian)

• od -f e.dat
• 0000000 2.7182817e00
• 0000004

17
Debugging in IDL
18
Basic IDL Debugging Commands

• breakpoint
• .step (.s)
• .stepover (.so)
• .continue
• .return
• .out

19
mean.pro

• 10 -
• 11
• 12 function mean,x
• 13 n n_elements(x)
• 14 answer sum(x)/double(n)
• 15 return, answer
• 16 end

20
Breakpoints

• IDLgt breakpoint, mean.pro, 12
• IDLgt help,/breakpoint
• Breakpoints
• Index Module Line File
• ----- ------ ---- ----
• 0 MEAN 12 mean.pro
• IDLgt breakpoint, /clear, 0
• OR
• IDLgt breakpoint, /clear, mean.pro, 12

21
.step

• Used after a breakpoint is reached
• Single step through commands
• If statement is a function or procedure
• Will enter into it

22
.stepover

• Similar to .step
• Executes functions and procedures to completion
  without stopping inside the function

23
.return

• Continues execution until a return statement is
  encountered.
• Useful in checking the return value of an inner
  (nested) function in a function composition
• e.g.
• sqrt(variance(x))

24
.out

• Continues execution out of the current routine.
• Useful in leaving a routine that you stepped
  into, but have determined that all is working
  properly and want to return to the calling
  routine.

25
mean.pro

• 10 -
• 11
• 12 function mean,x
• 13 n n_elements(x)
• 14 answer sum(x)/double(n)
• 15 return, answer
• 16 end

26
Mystery Images
27
Given an image of unknown origin

• Dimensions of the image?
• Most importantly the number of samples
• Aspect ratio of the image?
• Number of bytes per pixel?
• Number of bands of the image?
• Number of bits per pixel?
• External or internal compression applied
• gzip
• tiff or jpeg

28
Know the imaging instrument

• Find out what kind of image capture device
• Framing Camera with filters
• Band Sequential Images (BSQ)
• Flatbed of Pushbroom scanner
• Band interleaved by line (BIL)
• Line Scanner
• Band interleaved by pixel (BIP)

29
Tools at your disposal for dissecting the image

• file
• anytopnm
• more
• ls -l
• bc -l
• od
• dd
• strings
• rawtopgm or cat
• xv

30
file

• Uses information in /etc/magic to determine the
  type of a file.
• Common outputs from the file command
• file irod.img.gz
• irod.img.gz gzip compressed data - deflate
  method , original file name , max compression
• file readme
• readme ascii text

31
File (more examples)

• file RIT_bip.img
• RIT_bip.img data
• file modtran4.bat
• modtran4.bat executable c-shell script
• bean file maincode.exe
• maincode.exe ELF 32-bit MSB executable SPARC
  Version 1, dynamically linked, not stripped

32
File (image examples)Dont always believe the
extensions

• bean file feep.tif
• feep.tif TIFF file, big-endian
• bean file feep.pgm
• feep.pgm PGM ascii file
• bean file AVIRIS_RIT.jpg
• AVIRIS_RIT.jpg JPEG file
• bean file rotate.pbm
• rotate.pbm PGM raw file

33
anytopnm

• Shell script that does a brute force test on
  images and converts to pnm format
• anytopnm mystery_file gt mystery_file.pnm

34
od

• Can be used to quickly check the minimum and
  maximum values in a file
• Stupid UNIX Trick
• od d v image.raw cut c10- tr s
  \012 sort nu head

35
Xv and NetPBM

• Xv want multi-byte NetPBM files in little-endian
  form
• N.B. NetPBM utilities behave inconsistently for
  multi-byte data

36
Can you guess the dimensions of the image?

• Try to guess the number of rows (width) first.
• Possible guesses
• 256
• 512
• 2n
• 2n2(n-1)

37
Can you guess the aspect ratio of the image?

• wc -c or ls -l
• determine how many bytes are in the image
• Square root of the image size
• Multiple of a perfect square
• Assume that the image is greyscale square
• use rawtopgm
• rawtopgm 512 512 image.raw gt image.pgm

38
Square Gray Scale Image

• Used rawtopgm to create a pgm file and then use
  xv to display

39
A convenient little trick

• If you are just experimenting with a mystery
  image
• rawtopgm 512 512 image.raw xv -

40
Does it have a header?

• Not a perfect square
• go ahead and try to display it as if it were a
  square image to see if it has a header.

41
Square Grey Scale image with a header

• This structure means you have a header in the
  file that is causing a uniform shift
• You can fix this by using dd

42
Square image no header

• Your guess on the width of the image is short by
  one pixel

43
Square image no header

• Your guess on the width of the image is long by
  one pixel

44
Square image no header

• You probably guessed a little too low on the
  width of this image
• (off by 6 pixels)

45
Square Image with no header

• Your probably guessed a little too high on the
  width of the image
• (off by 6 pixels)

46
A color image displayed as greyscale image

• Correct dimensions
• 256x256x3 BIP
• Incorrectly assumed
• 443x443 image with header
• N.B.
• 196608/32562
• Indicates a multiband image

47
Multiband Assumption

• Once multiband image is assumed
• Need to determine interleaving
• Can be deduced by displaying first band
• This assumes you have a guess of image dimension
• Extract the first band out using dd

48
First band of color image file assumed to be BSQ

• Assume a square image for first band
• Color image file obviously not BSQ

49
Square color image displayed as a greyscale (31
aspect ratio)

• Original image
• 3x256x256
• Displayed it as a greyscale
• 768x256
• To get insight into interleaving structure.
• BIP file would look like the left image

50
Square color image displayed as greyscale( 31
aspect ratio)

• This is how a BIL image would behave
• What we have here is the individual bands side by
  side

51
Square color image displayed as greyscale( 31
aspect ratio)

• This is how a BSQ file would behave

52
Multibyte pixel image

• Need to be careful about the endian of the
  machine on which the image was written
• Can use dd to swap bytes for short integer images
  (2-bytes per pixel) or the swap_endian function
  in IDL

53
Mystery Image Examples

• Check Webpage for sources of examples.
• An example will be given for the midterm exam.