MESSAGE
| DATE | 2016-12-06 |
| FROM | Ruben Safir
|
| SUBJECT | Re: [Learn] png data format
|
From learn-bounces-at-nylxs.com Tue Dec 6 09:44:38 2016
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To: Christopher League , Hangout
, learn-at-nylxs.com
References: <66105244-4afa-4330-b0c2-0661bde965fd-at-mrbrklyn.com>
<87bmwpf857.fsf-at-contrapunctus.net> <878trtf7qw.fsf-at-contrapunctus.net>
From: Ruben Safir
Message-ID: <86dabb9c-361b-354b-1d44-1f4593cc70eb-at-mrbrklyn.com>
Date: Tue, 6 Dec 2016 09:44:35 -0500
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Subject: Re: [Learn] png data format
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On 12/06/2016 09:32 AM, Christopher League wrote:
>
> Ah... I *can* confirm the 218103808 number now. The number you're seeing
> in the dump *is* in octal: the three highest-order bytes are 000(oct)
> and the low order byte is 015(oct) == 8+5 == 13(dec) == D(hex).
>
> So the full 32-bit number in hexadecimal is 0x0000000D.
>
> Intel architecture stores those bytes in the order 0x0D, 0x00, 0x00,
> 0x00.
>
> This number, 0x0D000000 == 218103808.
damn
so your saying that the bytes are being stored left to right and the
array is right to left?
hexdup corrects the order of them automatically by itself?
>
> The D is in column 6 (counting from zero at the right), whose
> place-value is 16^6 == 16777216, and 13*16777216 == 218103808.
>
> CL
>
> Christopher League writes:
>
>> I can't really confirm this based on the numbers you've given (218103808
>> isn't how I think it would come out) but it sounds A LOT like an
>> endianness issue. That is, the 4 bytes are stored big-endian in the
>> file, but you're operating on a system (Intel) that is little-endian. So
>> if you read an entire uint32 at once, it doesn't come out correctly.
>> Reading one byte at a time, you can then swap the bytes into the right
>> order.
>>
>> Also, that's not a hex dump, right...? is it octal? decimal?
>>
>> CL
>>
>> ruben safir writes:
>>
>>> 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
>>> _______________________________________________
>>> Learn mailing list
>>> Learn-at-nylxs.com
>>> http://lists.mrbrklyn.com/mailman/listinfo/learn
>
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