MESSAGE
| DATE | 2016-12-06 |
| FROM | John Bowler
|
| SUBJECT | Re: [Hangout-NYLXS] [png-mng-implement] 4 byte length storage
|
These days almost all computer operating systems (maybe all) present
files as a sequence of numerical values each of which is in the range
0 to 255 ("bytes"). This is also the universal interchange format for
files that are transferred between systems using networks.
A PNG is a file so the only thing in it is a (single) sequence of such
numbers; this is all that can be read from any file.
The ISO/IEC PNG specification, here:
http://www.libpng.org/pub/png/spec/iso/index-object.html
describes how this stream of bytes is structured in section (5)
"Datastream structure", then it describes how this structure is
ENCODED in the stream in section (7) "Encoding the PNG image as a PNG
datastream." In particular in this case 7.1 explains how an integral
value bigger than 255 is encoded.
The encoding (section 7) has to be reversed to obtain the integral
values described in section 5 and elsewhere. Many operating systems
provide "serialization" primitives for doing this, but since the
encoding PNG uses is OS independent, it is necessary to do it in a
operating system independent fashion. The built in C++ << operators
are an example of such a thing; a particular serialization that does
not match the PNG one (in this case.)
libpng exports serialization primitives (e.g. get_uint_32 in this
case) that can be used. They take a buffer (array of bytes, as you
described it) with the original bytes read from the stream. Other
approaches are possible; in C++ you might choose to define a stream
class for a PNG with overloaded << operators that do the correct
decoding.
John Bowler
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