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Mystery Images

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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

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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)

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Tools at your disposal for dissecting the image

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

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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

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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

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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

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anytopnm

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

% anytopnm mystery_file > mystery_file.pnm

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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

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Xv and NetPBM

• Xv want multi-byte NetPBM files in little-endian form

• N.B. NetPBM utilities behave inconsistently for multi-byte data

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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)

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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

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Square Gray Scale Image

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

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A convenient little trick

• If you are just experimenting with a mystery image

% rawtopgm 512 512 image.raw | xv -

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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.

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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

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Square image no header

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

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Square image no header

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

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Square image no header

• You probably guessed a little too low on the width of this image

• (off by 6 pixels)

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Square Image with no header

• Your probably guessed a little too high on the width of the image

• (off by 6 pixels)

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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

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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

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First band of color image file assumed to be BSQ

• Assume a square image for first band

• Color image file obviously not BSQ

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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

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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

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• This is how a BSQ file would behave

Square color image displayed as greyscale( 3:1 aspect ratio)

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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

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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

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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

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Mystery Image Examples

• Check Wiki for sources of examples.– Keyword: mystery image