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
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From: Ruben Safir
Message-ID: <5318dcc5-5ed2-9012-b42b-3c16a8a7a83f-at-mrbrklyn.com>
Date: Mon, 12 Dec 2016 12:04:33 -0500
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Subject: Re: [Learn] Fwd: Re: png data format
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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
> _______________________________________________
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>
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