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LeCroy 9350AM revisited

Yesterday I got a sort of crazy idea: "What if I could make my oscilloscope hardcopy to email instead of the broken built-in printer?". As it turns out, the LeCroy 93xx series is capable of both accepting remote control commands and dumping hardcopies to the same RS-232-C bus - write 'SCREEN_DUMP' and it will dump it in the same session, pretty sweet.

The 93xx series features quite a lot of output formats ranging from BMP and all the way to some odd format called HP 7550 which I assume is before my time - BMP and TIFF sadly taking 40 seconds at 19200 baud to fully process. HP 7550 took around 9 seconds, but after some googling I gave up on that format - it seemed to be very troublesome to get something working with that format. Finally I settled for the compressed TIFF format (12 seconds @ 19200 baud) - the problem was that the file received from the oscilloscope was not something libtiff gladly wanted to process.

When I gave the file to imagemagick's identify program, it didn't want to play along.
# identify -verbose filecomp.tiff
Image: filecomp.tiff
  Format: TIFF (Tagged Image File Format)
  Class: DirectClass
  Geometry: 816x696+0+0
  Resolution: 121.791x148.085
  Print size: 6.7x4.7
  Units: PixelsPerInch
  Type: Bilevel
  Base type: Bilevel
  Endianess: MSB
  Colorspace: RGB
  Rendering intent: Undefined
  Interlace: None
  Background color: white
  Border color: rgb(223,223,223)
  Matte color: grey74
  Transparent color: black
  Compose: Over
  Page geometry: 816x696+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: RLE
  Orientation: TopLeft
    date:create: 2011-03-09T17:48:01+01:00
    date:modify: 2011-03-09T17:48:01+01:00
    signature: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
    tiff:photometric: min-is-white
    tiff:rows-per-strip: 696
    verbose: true
  Tainted: False
  Filesize: 28.7KB
  Number pixels: 568KB
  Pixels per second: 0b
  User time: 0.010u
  Elapsed time: 0:01.000
  Version: ImageMagick 6.6.0-4 2010-11-16 Q16
identify: incorrect count for field "ColorMap" (768, expecting 6); tag trimmed. `filecomp.tiff' @ warning/tiff.c/TIFFWarnings/703.
identify: filecomp.tiff: Read error on strip 0; got 26911 bytes, expected 70992. `TIFFFillStrip' @ warning/tiff.c/TIFFErrors/493.

Fair enough. I tried to open it in Photoshop which to my great surprise actually worked - which meant that all the data was available but the format was corrupted.

Libtiff was helpful enough to provide me with a number: 70992 and 26911 - 70992 is located as a long at 0x7E in the TIFF header, but what about 26911? Replacing 70922 with 26911 fixed this one TIFF, but I needed to know how to calculate this number. This is where the utility called exiftool comes along.

# exiftool -g1 -D filecomp.tiff
---- ExifTool ----
    - ExifTool Version Number         : 8.15
---- System ----
    - File Name                       : filecomp.tiff
    - Directory                       : .
    - File Size                       : 28 kB
    - File Modification Date/Time     : 2011:03:09 17:48:01+01:00
    - File Permissions                : rw-r--r--
---- File ----
    - File Type                       : TIFF
    - MIME Type                       : image/tiff
    - Exif Byte Order                 : Big-endian (Motorola, MM)
---- IFD0 ----
  255 Old Subfile Type                : Full-resolution image
  256 Image Width                     : 816
  257 Image Height                    : 696
  258 Bits Per Sample                 : 1
  259 Compression                     : PackBits
  262 Photometric Interpretation      : WhiteIsZero
  273 Strip Offsets                   : 1746
  277 Samples Per Pixel               : 1
  278 Rows Per Strip                  : 696
  279 Strip Byte Counts               : 70992
  282 X Resolution                    : 121.7910448
  283 Y Resolution                    : 148.0851064
  284 Planar Configuration            : Chunky
  296 Resolution Unit                 : inches
  320 Color Map                       : (Binary data 1536 bytes, use -b option to extract)
---- Composite ----
    - Image Size                      : 816x696

Some quick math: file_size - 26911 = 1746 (Strip Offsets). In reality I did check how libtiff calculates the error message's "expect" value and then verified it with exiftool, but you get the idea.

import struct

def fix_tiff (f): (0, 2)
  file_size = f.tell()

  # Get 'Strip Offsets' (0x5A)
  real_size = file_size - struct.unpack ('>I', (4))[0]
  print 'Patching to', real_size
  # Write 'Strip Byte Counts' (0x7E)
  f.write (struct.pack ('>I', real_size))

This script has evolved and now contains an SMTP function which sends the hardcopy as a color and B/W attachment (PNG format) to my email when I press "Screen dump". Pretty neat if I may say so myself.


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