1. Rolando V. RaqueñoSunday, December 20, 2015 1 Mystery Images

2. Rolando V. RaqueñoSunday, December 20, 2015 2 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

3. Rolando V. RaqueñoSunday, December 20, 2015 3 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)

4. Rolando V. RaqueñoSunday, December 20, 2015 4 Tools at your disposal for dissecting the image % file % anytopnm % more % ls -l % bc -l % od % dd % strings % rawtopgm or cat % xv

5. Rolando V. RaqueñoSunday, December 20, 2015 5 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

6. Rolando V. RaqueñoSunday, December 20, 2015 6 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

7. Rolando V. RaqueñoSunday, December 20, 2015 7 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

8. Rolando V. RaqueñoSunday, December 20, 2015 8 anytopnm Shell script that does a brute force test on images and converts to pnm format % anytopnm mystery_file > mystery_file.pnm

9. Rolando V. RaqueñoSunday, December 20, 2015 9 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

10. Rolando V. RaqueñoSunday, December 20, 2015 10 Xv and NetPBM Xv want multi-byte NetPBM files in little- endian form N.B. NetPBM utilities behave inconsistently for multi-byte data

11. Rolando V. RaqueñoSunday, December 20, 2015 11 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)

12. Rolando V. RaqueñoSunday, December 20, 2015 12 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

13. Rolando V. RaqueñoSunday, December 20, 2015 13 Square Gray Scale Image Used rawtopgm to create a pgm file and then use xv to display

14. Rolando V. RaqueñoSunday, December 20, 2015 14 A convenient little trick If you are just experimenting with a mystery image % rawtopgm 512 512 image.raw | xv -

15. Rolando V. RaqueñoSunday, December 20, 2015 15 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.

16. Rolando V. RaqueñoSunday, December 20, 2015 16 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

17. Rolando V. RaqueñoSunday, December 20, 2015 17 Square image no header Your guess on the width of the image is short by one pixel

18. Rolando V. RaqueñoSunday, December 20, 2015 18 Square image no header Your guess on the width of the image is long by one pixel

19. Rolando V. RaqueñoSunday, December 20, 2015 19 Square image no header You probably guessed a little too low on the width of this image (off by 6 pixels)

20. Rolando V. RaqueñoSunday, December 20, 2015 20 Square Image with no header Your probably guessed a little too high on the width of the image (off by 6 pixels)

21. Rolando V. RaqueñoSunday, December 20, 2015 21 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

22. Rolando V. RaqueñoSunday, December 20, 2015 22 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

23. Rolando V. RaqueñoSunday, December 20, 2015 23 First band of color image file assumed to be BSQ Assume a square image for first band Color image file obviously not BSQ

24. Rolando V. RaqueñoSunday, December 20, 2015 24 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

25. Rolando V. RaqueñoSunday, December 20, 2015 25 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

26. Rolando V. RaqueñoSunday, December 20, 2015 26 This is how a BSQ file would behave Square color image displayed as greyscale( 3:1 aspect ratio)

27. Rolando V. RaqueñoSunday, December 20, 2015 27 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

28. Rolando V. RaqueñoSunday, December 20, 2015 28 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

29. Rolando V. RaqueñoSunday, December 20, 2015 29 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 a IDL> 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

30. Rolando V. RaqueñoSunday, December 20, 2015 30 Mystery Image Examples Check Wiki for sources of examples. –Keyword: mystery image