Rolando V. RaqueñoSunday, December 20, Mystery Images

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, Tools at your disposal for dissecting the image % file % anytopnm % more % ls -l % bc -l % od % dd % strings % rawtopgm or cat % xv

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, anytopnm Shell script that does a brute force test on images and converts to pnm format % anytopnm mystery_file > mystery_file.pnm

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, Can you guess the dimensions of the image? Try to guess the number of rows (width) first. –Possible guesses ^n 2^n+2^(n-1)

Rolando V. RaqueñoSunday, December 20, 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 image.raw > image.pgm

Rolando V. RaqueñoSunday, December 20, Square Gray Scale Image Used rawtopgm to create a pgm file and then use xv to display

Rolando V. RaqueñoSunday, December 20, A convenient little trick If you are just experimenting with a mystery image % rawtopgm image.raw | xv -

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, Square image no header Your guess on the width of the image is short by one pixel

Rolando V. RaqueñoSunday, December 20, Square image no header Your guess on the width of the image is long by one pixel

Rolando V. RaqueñoSunday, December 20, Square image no header You probably guessed a little too low on the width of this image (off by 6 pixels)

Rolando V. RaqueñoSunday, December 20, Square Image with no header Your probably guessed a little too high on the width of the image (off by 6 pixels)

Rolando V. RaqueñoSunday, December 20, A color image displayed as greyscale image Correct dimensions – 256x256x3 BIP Incorrectly assumed –443x443 image with header N.B /3=256^2 Indicates a multiband image

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, This is how a BSQ file would behave Square color image displayed as greyscale( 3:1 aspect ratio)

Rolando V. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, 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. RaqueñoSunday, December 20, You know one dimension… One dimension is 384 lines/samples You have pixels Find candidate integer dimension IDL> a=findgen(384) IDL> b= mod a IDL> c=where(b eq 0) IDL> print,a(c)

Rolando V. RaqueñoSunday, December 20, Mystery Image Examples Check Wiki for sources of examples. –Keyword: mystery image