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DATE 2016-12-01

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Key: id Value: 543537

Key: archive_learn Value: 2016-12-01

MESSAGE
DATE 2016-12-06
FROM ruben safir
SUBJECT Subject: [Learn] png data format
From learn-bounces-at-nylxs.com Tue Dec 6 09:10:46 2016
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From: ruben safir
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Subject: [Learn] png data format
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Hello

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
Sizeof Byte 1
Character 0::
^-at-
Byte 0::
0
Character 1::
^-at-
Byte 1::
0
Character 2::
^-at-
Byte 2::
0
Character 3::
Byte 3::
13


As yet, when I break it down by single bytes, it returns 0 0 0 13, which
is correct. ddd seems to say the same thing, and I don't know why.
When evaluated as 4 bytes, you get this large number, but when you
evaluate them seperately, each byte, it comes out right.

The code snippet I'm using looks like this

in the .h file #ifndef PNGPRJ
#define PNGPRJ
#include
namespace png_proj{
typedef uint32_t CHUNK;



In the .cpp file
void Image::read_chunk()
{
char * cur = get_index();
CHUNK * tmp = reinterpret_cast(cur);
std::cout << std::endl << "Sizeof Chunk " << sizeof(*tmp) << std::endl;
for(int j = 0; j<4; j++){
std::cout << "Raw Chunk Number " << j << ": " << *tmp << std::endl;


switch ( j ) {
case 0:
std::cout << "***LENGTH****" << std::endl;
set_length(static_cast(*tmp));
std::cout << "Length value => " << static_cast(*tmp) << std::endl;
break;

case 1:
std::cout << "***TYPE****" << std::endl;
set_type(static_cast(*tmp));
break;

case 2:
{
std::cout << "***DATA****" << std::endl;
unsigned long int l = static_cast(get_length());
std::cout << "buffer size should be " << get_length() << std::endl;
int8_t * buffer = new int8_t[l];
std::cout << "buffer element size is " << *buffer << std::endl;
std::cout << "buffer size is " << l << std::endl;
for(unsigned int k = 0; k < get_length(); k++){
buffer[k] = static_cast(tmp[k]);
std::cout << "data " << *buffer << std::endl;
}
set_data(buffer);
}
break;

case 3:
std::cout << "***CRC****" << std::endl;
set_crc(static_cast(*tmp));
break;

default:
std::cout << "***NOMANDSLAND****" << std::endl;
break;
} /* ----- end switch ----- */

char * tmp2 = reinterpret_cast(tmp); //reading each byte
std::cout << "Sizeof Byte " << sizeof(*tmp2) << std::endl;
//std::cout << "Mark ==>>" << __LINE__ << std::endl;
for(int i=0; i<4; i++){
std::cout << "Character " << i << "::" << std::endl << "\t" << *tmp2
<< std::endl;
std::cout << "Byte " << i << "::" << std::endl << "\t" <<
static_cast(*tmp2) << std::endl;
tmp2++;
}
std::cout< std::cout< tmp++;
cur = ( reinterpret_cast(tmp) );
}
set_index(cur);
}



I dug through libpng since this seems to not being doing what I
expected. They seem to set it up as 4 byte array

void /* PRIVATE */
png_push_read_chunk(png_structrp png_ptr, png_inforp info_ptr)
{
png_uint_32 chunk_name;
#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
int keep; /* unknown handling method */
#endif

/* First we make sure we have enough data for the 4-byte chunk name
* and the 4-byte chunk length before proceeding with decoding the
* chunk data. To fully decode each of these chunks, we also make
* sure we have enough data in the buffer for the 4-byte CRC at the
* end of every chunk (except IDAT, which is handled separately).
*/
if ((png_ptr->mode & PNG_HAVE_CHUNK_HEADER) == 0)
{
png_byte chunk_length[4];
png_byte chunk_tag[4];

PNG_PUSH_SAVE_BUFFER_IF_LT(8)
png_push_fill_buffer(png_ptr, chunk_length, 4);
png_ptr->push_length = png_get_uint_31(png_ptr, chunk_length);
png_reset_crc(png_ptr);
png_crc_read(png_ptr, chunk_tag, 4);
png_ptr->chunk_name = PNG_CHUNK_FROM_STRING(chunk_tag);
png_check_chunk_name(png_ptr, png_ptr->chunk_name);
png_ptr->mode |= PNG_HAVE_CHUNK_HEADER;
}


I'm obviously not understanding something I'm evaluation here. So I'm
wondering if anyone can shed light on this.
http://www.nylxs.com/docs/grad_school/parallel/src/png/png_proj.h
http://www.nylxs.com/docs/grad_school/parallel/src/png/png_proj.cpp
http://www.nylxs.com/docs/grad_school/parallel/src/png/main_png.cpp
http://www.nylxs.com/docs/grad_school/parallel/src/png/makefile

ruben

let.me.in


Ruben
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  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
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  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
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