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DATE 2016-12-01

LEARN

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Key: archive_learn Value: 2016-12-01

Key: id Value: 543563

MESSAGE
DATE 2016-12-12
FROM Ruben Safir
SUBJECT Re: [Learn] Fwd: Re: png data format
From learn-bounces-at-nylxs.com Mon Dec 12 12:04:35 2016
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To: Samir Iabbassen , learn-at-nylxs.com
References:
<885db5b8-cd28-1647-d931-d18f4bfb5e5a-at-mrbrklyn.com>
<1481484196598.70353-at-liu.edu>
From: Ruben Safir
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On 12/11/2016 02:23 PM, Samir Iabbassen wrote:
> I think the number you are getting is not huge as it's less than 2^31 - 1.
> did you use the following formula to calculate the value you are getting?
> uint32_t len = ((bytes[0] * 256 + bytes[1]) * 256 + bytes[2]) * 256 + bytes[3];


Right, it is not too large for the byte size, but it is larger than the
size of the entire image ;)

This post came from the PNG working group and includes am decent endian
explanation, so I forwarded it. What confuses me is that BYTEs are
bytes regardless of endianess. That makes no sense to me


> ________________________________________
> From: Learn on behalf of ruben safir
> Sent: Sunday, December 11, 2016 12:39 AM
> To: learn-at-nylxs.com
> Subject: [Learn] Fwd: Re: png data format
>
> -------- Forwarded Message --------
> Subject: Re: png data format
> Date: 7 Dec 2016 19:49:12 GMT
> From: Jens Thoms Toerring
> Organization: Freie Universitaet Berlin
> Newsgroups: comp.unix.programmer
> References:
>
> ruben safir wrote:
>> I'm having trouble with this imput of data from a PNG image. The
>> specification says that "chunks" have a 4 byte field that is the length of
>> the attached data segment. I tried to read the length in for a chunk that
>> has a length of 13, which was confirmed in a hexdump
>
>> 0000000 211 120 116 107 015 012 032 012 -->>000 000 000 015<<-- 111 110 104 122
>> 0000010 000 000 041 215 000 000 007 165 010 006 000 000 001 206 055 074
>> 0000020 336 000 000 000 004 147 101 115 101 000 000 261 217 013 374 141
>
>> I am storing the data in a uint32_t variable using the following code, but
>> the value keeps showing up with a huge number 218103808 which happens to be
>> the number that is evaluated by iostream for the value of the whole chunk
>
>
>> done reading header
>
>> Sizeof Chunk 4
>> Raw Chunk Number 0: 218103808
>> ***LENGTH****
>> Length value => 218103808
>
> The data are stored in big-endian order, i.e. with the most
> significant byte coming first, but you seem to have read it
> in as a 32-bit integer on a machine using little-endian byte
> order, e.g. with a machine with an Intel processor, where
> the least-significant byte comes first in memory. That's
> also what they expect when reading in binary data from a
> file. So simply read in the four bytes separately into e.
> g. an array
>
> uint8_t bytes[4]
>
> with the first byte you read from the file going into
> the first array element etc. Then do
>
> uint32_t len = ((bytes[0] * 256 + bytes[1]) * 256 + bytes[2]) * 256 +
> bytes[3];
>
> and you should get the correct value. The same applies, of
> course, also for other fields containing values that occupy
> multiple bytes! This then will work correctly on both big-
> and little-endian machines.
> Regards, Jens
> --
> \ Jens Thoms Toerring ___ jt-at-toerring.de
> \__________________________ http://toerring.de
> _______________________________________________
> Learn mailing list
> Learn-at-nylxs.com
> http://lists.mrbrklyn.com/mailman/listinfo/learn
>
>


--
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DRM is THEFT - We are the STAKEHOLDERS - RI Safir 2002
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  1. 2016-12-01 Ruben Safir <ruben.safir-at-my.liu.edu> Subject: [Learn] Fwd: Patrolling the Dark Net,
  2. 2016-12-02 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Tutorial on threaded binary tree part 1: simple
  3. 2016-12-03 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Witner Labs
  4. 2016-12-04 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Nice lecture on Quantum Mechanics
  5. 2016-12-05 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] information access, copyright wars and DRM
  6. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  7. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  8. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  9. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] png data format
  10. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  11. 2016-12-06 Ruben Safir <ruben.safir-at-my.liu.edu> Re: [Learn] png data format
  12. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] png data format
  13. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  14. 2016-12-06 Ruben Safir <mrbrklyn-at-panix.com> Re: [Learn] png data format
  15. 2016-12-06 Christopher League <league-at-contrapunctus.net> Re: [Learn] png data format
  16. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] [Hangout-NYLXS] png data format
  17. 2016-12-06 John Bowler <john.cunningham.bowler-at-gmail.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  18. 2016-12-06 John Bowler <john.cunningham.bowler-at-gmail.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  19. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  20. 2016-12-06 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Ocaml
  21. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: [luny-talk] Humble Bundle O'Reilly UNIX books
  22. 2016-12-06 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] JT's words
  23. 2016-12-06 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] png data format
  24. 2016-12-07 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  25. 2016-12-08 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: png data format
  26. 2016-12-08 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  27. 2016-12-08 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  28. 2016-12-10 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] references to pointers
  29. 2016-12-10 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] C++ returning lvalue references and pointers and refs
  30. 2016-12-10 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] const puzzle and literal type arguments
  31. 2016-12-11 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: png data format
  32. 2016-12-12 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Fwd: Re: png data format
  33. 2016-12-12 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] gene phylogienics of homonids
  34. 2016-12-13 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Bit Depth
  35. 2016-12-13 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] C++ Threads Workshop
  36. 2016-12-13 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Summer Jobs
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  38. 2016-12-14 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: [dinosaur] Ceratopsid (Centrosaurinae:
  39. 2016-12-15 Ruben Safir <ruben.safir-at-my.liu.edu> Re: [Learn] Happy Holidays
  40. 2016-12-15 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] zlib demo with vector::resize
  41. 2016-12-15 John Bowler <john.cunningham.bowler-at-gmail.com> Re: [Learn] [png-mng-implement] 4 byte length storage
  42. 2016-12-15 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: [dinosaur] Elephant bird nuclear genome fragments
  43. 2016-12-15 Christopher League <league-at-contrapunctus.net> Subject: [Learn] zlib demo with vector::resize
  44. 2016-12-16 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] zlib demo with vector::resize
  45. 2016-12-16 Christopher League <league-at-contrapunctus.net> Re: [Learn] zlib demo with vector::resize
  46. 2016-12-16 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] PNG Parallel Programming problem
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  50. 2016-12-18 ruben safir <ruben-at-mrbrklyn.com> Re: [Learn] Threads and Object Methods
  51. 2016-12-18 ruben safir <ruben-at-mrbrklyn.com> Re: [Learn] Threads and Object Methods
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  60. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Noobdy is home
  61. 2016-12-21 Samir Iabbassen <Samir.Iabbassen-at-liu.edu> Re: [Learn] thread concurancy
  62. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [Learn] thread concurancy
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  70. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  71. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  72. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  73. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  74. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Re: Threads and Object Methods
  75. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) thread concurancy
  76. 2016-12-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [Learn] (fwd) Threads and Object Methods
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  79. 2016-12-21 Ruben Safir <ruben-at-mrbrklyn.com> Subject: [Learn] thread concurancy
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  82. 2016-12-22 ruben safir <ruben-at-mrbrklyn.com> Subject: [Learn] Fwd: Re: thread concurancy
  83. 2016-12-23 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] HOPL (History of Programming Languages)
  84. 2016-12-23 Ruben Safir <ruben-at-mrbrklyn.com> Re: [Learn] Noobdy is home
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  88. 2016-12-27 Samir Iabbassen <Samir.Iabbassen-at-liu.edu> Re: [Learn] thread concurancy

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