Rolando V. Raqueño Friday, April 21, 202311
Mystery Images
Rolando V. Raqueño Friday, April 21, 202322
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
Rolando V. Raqueño Friday, April 21, 202333
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)
Rolando V. Raqueño Friday, April 21, 202344
Tools at your disposal for dissecting the image
% file% anytopnm % more% ls -l% bc -l% od% dd% strings% rawtopgm or cat% xv
Rolando V. Raqueño Friday, April 21, 202355
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
Rolando V. Raqueño Friday, April 21, 202366
File (more examples)
% file RIT_bip.imgRIT_bip.img: data
% file modtran4.batmodtran4.bat: executable c-shell script
bean% file maincode.exemaincode.exe: ELF 32-bit MSB executable SPARC Version 1, dynamically linked, not stripped
Rolando V. Raqueño Friday, April 21, 202377
File (image examples)Don’t 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
Rolando V. Raqueño Friday, April 21, 202388
anytopnm
• Shell script that does a brute force test on images and converts to pnm format
% anytopnm mystery_file > mystery_file.pnm
Rolando V. Raqueño Friday, April 21, 202399
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
Rolando V. Raqueño Friday, April 21, 20231010
Xv and NetPBM
• Xv want multi-byte NetPBM files in little-endian form
• N.B. NetPBM utilities behave inconsistently for multi-byte data
Rolando V. Raqueño Friday, April 21, 20231111
Can you guess the dimensions of the image?
• Try to guess the number of rows (width) first.– Possible guesses
•256•512•2^n•2^n+2^(n-1)
Rolando V. Raqueño Friday, April 21, 20231212
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 > image.pgm
Rolando V. Raqueño Friday, April 21, 20231313
Square Gray Scale Image
• Used rawtopgm to create a pgm file and then use xv to display
Rolando V. Raqueño Friday, April 21, 20231414
A convenient little trick
• If you are just experimenting with a mystery image
% rawtopgm 512 512 image.raw | xv -
Rolando V. Raqueño Friday, April 21, 20231515
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.
Rolando V. Raqueño Friday, April 21, 20231616
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
Rolando V. Raqueño Friday, April 21, 20231717
Square image no header
• Your guess on the width of the image is short by one pixel
Rolando V. Raqueño Friday, April 21, 20231818
Square image no header
• Your guess on the width of the image is long by one pixel
Rolando V. Raqueño Friday, April 21, 20231919
Square image no header
• You probably guessed a little too low on the width of this image
• (off by 6 pixels)
Rolando V. Raqueño Friday, April 21, 20232020
Square Image with no header
• Your probably guessed a little too high on the width of the image
• (off by 6 pixels)
Rolando V. Raqueño Friday, April 21, 20232121
A color image displayed as greyscale image
• Correct dimensions– 256x256x3 BIP
• Incorrectly assumed– 443x443 image with
header
• N.B. 196608/3=256^2
• Indicates a multiband image
Rolando V. Raqueño Friday, April 21, 20232222
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
Rolando V. Raqueño Friday, April 21, 20232323
First band of color image file assumed to be BSQ
• Assume a square image for first band
• Color image file obviously not BSQ
Rolando V. Raqueño Friday, April 21, 20232424
Square color image displayed as a greyscale (3:1 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
Rolando V. Raqueño Friday, April 21, 20232525
Square color image displayed as greyscale( 3:1 aspect ratio)
• This is how a BIL image would behave• What we have here is the individual bands
side by side
Rolando V. Raqueño Friday, April 21, 20232626
• This is how a BSQ file would behave
Square color image displayed as greyscale( 3:1 aspect ratio)
Rolando V. Raqueño Friday, April 21, 20232727
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
Rolando V. Raqueño Friday, April 21, 20232828
Useful IDL command for image dimension analysis
• Key goal is to find integer dimensions
• Example– You know a given size (pixels, bytes, lines)– Need to find an integer number that “fits”
with the sizes you are observing
• Minimize trial and error using the following IDL commands
Rolando V. Raqueño Friday, April 21, 20232929
You know one dimension…
• One dimension is 384 lines/samples• You have 588640 pixels• Find candidate integer dimension
IDL> a=findgen(384)IDL> b=588640 mod aIDL> c=where(b eq 0)IDL> print,a(c) 1.00000 2.00000 4.00000 5.00000 8.00000 10.0000 13.0000 16.0000 20.0000 26.0000 32.0000 40.0000 52.0000 65.0000 80.0000 104.000 130.000 160.000 208.000 260.000 283.000
Rolando V. Raqueño Friday, April 21, 20233030
Mystery Image Examples
• Check Wiki for sources of examples.– Keyword: mystery image