source: trunk/src/3rdparty/libmng/doc/libmng.txt@ 497

libmng - Multiple-image Network Graphics (MNG) Reference Library 1.0.9

DESCRIPTION
The libmng library supports decoding, displaying, encoding, and various
other manipulations of the Multiple-image Network Graphics (MNG) format
image files. It uses the zlib compression library, and optionally the
JPEG library by the Independant JPEG Group (IJG) and/or
lcms (little cms), a color-management library by Marti Maria Saguer.


I. Introduction

This file describes how to use and modify the MNG reference library
(known as libmng) for your own use.  There are seven sections to this
file: introduction, callbacks, housekeeping, reading, displaying,
writing, and modification and configuration notes for various special
platforms. We assume that libmng is already installed; see the
INSTALL.README file for instructions on how to install libmng.

Libmng was written to support and promote the MNG specification.

The latest MNG specification (currently 1.0) is available at
  http://www.libpng.org/pub/mng/

Other information about MNG can be found at the MNG home page at
  http://www.libpng.org/pub/mng/

The latest version of libmng can be found at its own homepage at
  http://www.libmng.com/

In most cases the library will not need to be changed.
For standardization purposes the library contains both a Windows DLL
and a makefile for building a shared library (SO). The library is
written in C, but an interface for Borland Delphi is also available.

Libmng has been designed to handle multiple sessions at one time,
to be easily modifiable, to be portable to the vast majority of
machines (ANSI, K&R, 32-, and 64-bit) available, and to be easy
to use.

Libmng uses zlib for its compression and decompression of MNG files.
Further information about zlib, and the latest version of zlib, can be
found at the zlib home page, <http://www.zlib.org/>.
The zlib compression utility is a general purpose utility that is
useful for more than MNG/PNG files, and can be used without libmng.
See the documentation delivered with zlib for more details.

Libmng optionally uses the JPEG library by the Independant JPEG Group
(IJG). This library is used for the JNG sub-format, which is part of
the MNG specification, and allows for inclusion of JPEG decoded and
thus highly compressed (photographic) images.
Further information about the IJG JPEG library and the latest sources
can be found at <http://www.ijg.org/>.

Libmng can also optionally use the lcms (little CMS) library by
Marti Maria Saguer. This library provides an excellent color-management
system (CMS), which gives libmng the ability to provide full
color-correction for images with the proper color-information encoded.
Further information and the latest sources can be found at
<http://www.littlecms.com/>.

Libmng is thread safe, provided the threads are using different
handles as returned by the initialization call.
Each thread should have its own handle and thus its own image.
Libmng does not protect itself against two threads using the
same instance of a handle.

The libmng.h header file is the single reference needed for programming
with libmng:

#include <libmng.h>


II. Callbacks

Libmng makes extensive use of callback functions. This is meant to
keep the library as platform-independant and flexible as possible.
Actually, the first call you will make to the library, already contains
three parameters you can use to provide callback entry-points.

Most functions must return a mng_bool (boolean). Returning MNG_FALSE
indicates the library the callback failed in some way and the library
will immediately return from whatever it was doing back to the
application. Returning MNG_TRUE indicates there were no problems and
processing can continue.

Let's step through each of the possible callbacks. The sections on
reading, displaying and writing will also explain which callbacks are
needed when and where.

- mng_ptr mng_memalloc (mng_size_t iLen)

A very basic function which the library uses to allocate a memory-block
with the given size. A typical implementation would be:

    mng_ptr my_alloc (mng_size_t iLen) {
      return calloc (1, iSize);
    }

Note that the library requires you to zero-out the memory-block!!!

- void mng_memfree (mng_ptr    pPtr,
                    mng_size_t iLen)

Counterpart of the previous function. Typically:

    void my_free (mng_ptr pPtr, mng_size_t iLen) {
      free (pPtr);
    }

- mng_bool mng_openstream  (mng_handle hHandle)
- mng_bool mng_closestream (mng_handle hHandle)

These are called by the library just before it starts to process
(either read or write) a file and just after the processing stops.
This is the recommended place to do I/O initialization & finalization.
Whether you do or not, is up to you. The library does not put any
meaning into the calls. They are simply provided for your convenience.

- mng_bool mng_readdata (mng_handle  hHandle,
                         mng_ptr     pBuf,
                         mng_uint32  iBuflen,
                         mng_uint32p pRead)

This function is called when the library needs some more input while
reading an image. The reading process supports two modes:
Suspension-mode (SMOD) and non-suspension-mode (NSMOD).
See mng_set_suspensionmode() for a more detailed description.

In NSMOD, the library requires you to return exactly the amount of bytes
requested (= iBuflen). Any lesser amount indicates the input file
is exhausted and the library will return a MNG_UNEXPECTEDEOF errorcode.

In SMOD, you may return a smaller amount of bytes than requested.
This tells the library it should temporarily wait for more input to
arrive. The lib will return with MNG_NEEDMOREDATA, and will expect a
call to mng_read_resume() or mng_display_resume() next, as soon as
more input-data has arrived.

For NSMOD this function could be as simple as:

    mng_bool my_read (mng_handle  hHandle,
                      mng_ptr     pBuf,
                      mng_uint32  iBuflen,
                      mng_uint32p pRead) {
      *pRead = fread (pBuf, 1, iBuflen, myfile);
      return MNG_TRUE;
    }

- mng_bool mng_writedata (mng_handle  hHandle,
                          mng_ptr     pBuf,
                          mng_uint32  iBuflen,
                          mng_uint32p pWritten)

This function is called during the mng_write() function to actually
output data to the file. There is no suspension-mode during write,
so the application must return the exact number of bytes the library
requests to be written.

A typical implementation could be:

    mng_bool my_write (mng_handle  hHandle,
                       mng_ptr     pBuf, 
                       mng_uint32  iBuflen,
                       mng_uint32p pWritten) {
      *pWritten = fwrite (pBuf, 1, iBuflen, myfile);
      return MNG_TRUE;
    }

- mng_bool mng_errorproc (mng_handle  hHandle,
                          mng_int32   iErrorcode,
                          mng_int8    iSeverity,
                          mng_chunkid iChunkname,
                          mng_uint32  iChunkseq,
                          mng_int32   iExtra1,
                          mng_int32   iExtra2,
                          mng_pchar   zErrortext)

This function is called whenever an error is detected inside the
library. This may be caused by invalid input, callbacks indicating
failure, or wrongfully calling functions out of place.

If you do not provide this callback the library will still return
an errorcode from the called function, and the mng_getlasterror()
function can be used to retrieve the other parameters.

This function is currently only provided for convenience, but may
at some point be used to indicate certain errors may be acceptable,
and processing should continue.

- mng_bool mng_traceproc (mng_handle hHandle,
                          mng_int32  iFuncnr,
                          mng_int32  iFuncseq,
                          mng_pchar  zFuncname)

This function is provided to allow a functional analysis of the
library. This may be useful if you encounter certain errors and
cannot determine what the problem is.

Almost all functions inside the library will activate this
callback with an appropriate function-name at the start and end
of the function. Please note that large images may generate an
enormous amount of calls.

- mng_bool mng_processheader (mng_handle hHandle,
                              mng_uint32 iWidth,
                              mng_uint32 iHeight)

This function is called once the header information of an input-
image has been processed. At this point the image dimensions are
available and also some other properties depending on the type
of the image. Eg. for a MNG the frame-/layercount, playtime &
simplicity fields are known.

The primary purpose of this callback is to inform the application
of the size of the image, and for the application to initialize
the drawing canvas to be used by the library. This is also a good
point to set the canvas-style. Eg. mng_set_canvasstyle().

- mng_bool mng_processtext (mng_handle hHandle,
                            mng_uint8  iType,
                            mng_pchar  zKeyword,
                            mng_pchar  zText,
                            mng_pchar  zLanguage,
                            mng_pchar  zTranslation)

This callback is activated for each textual chunk in the input-
image. These are tEXt, zTXt & iTXt. It may be used to retain
specific comments for presentation to the user.

- mng_bool mng_processsave (mng_handle hHandle)
- mng_bool mng_processseek (mng_handle hHandle,
                            mng_pchar  zName)

The purpose of these callbacks is to signal the processing of the
SAVE & SEEK chunks in a MNG input-file. This may be used in the
future to specify some special processing. At the moment these
functions are only provided as a signal.

- mng_ptr mng_getcanvasline (mng_handle hHandle,
                             mng_uint32 iLinenr)
- mng_ptr mng_getbkgdline   (mng_handle hHandle,
                             mng_uint32 iLinenr)
- mng_ptr mng_getalphaline  (mng_handle hHandle,
                             mng_uint32 iLinenr)

These callbacks are used to access the drawing canvas, background
canvas and an optional separate alpha-channel canvas. The latter is
used only with the MNG_CANVAS_RGB8_A8 canvas-style.

If the getbkgdline() callback is not supplied the library will
composite full or partially transparent pixels in the image against
a specified background color. See mng_set_bgcolor() for more details.
If a chosen canvas-style includes an alpha-channel, this callback
is very likely not needed.

The application is responsible for returning a pointer to a line of
pixels, which should be in the exact format as defined by the call
to mng_set_canvasstyle() and mng_set_bkgdstyle(), without gaps between
the representation of each pixel.

- mng_bool mng_refresh (mng_handle hHandle,
                        mng_uint32 iX,
                        mng_uint32 iY,
                        mng_uint32 iWidth,
                        mng_uint32 iHeight)

This callback is called when the library has drawn a complete frame
onto the drawing canvas, and it is ready to be displayed.
The application is responsible for transferring the drawing canvas
from memory onto the actual output device.

- mng_uint32 mng_gettickcount (mng_handle hHandle)

This function should return the number of milliseconds on some internal
clock. The entire animation timing depends heavily on this function,
1and the number returned should be as accurate as possible.

- mng_bool mng_settimer (mng_handle hHandle,
                         mng_uint32 iMsecs)

This callback is activated every time the library requires a "pause".
Note that the function itself should NOT execute the wait. It should
simply store the time-field and allow the library to return. Libmng
will return with the MNG_NEEDTIMERWAIT code, indicating the callback
was called and it is now time to execute the pause.

After the indicated number of milliseconds have elapsed, the application
should call mng_display_resume(), to resume the animation as planned.

This method allows for both a real timer or a simple wait command in the
application. Whichever method you select, both the gettickcount() and
settimer() callbacks are crucial for proper animation timing.

- mng_bool mng_processgamma  (mng_handle hHandle,
                              mng_uint32 iGamma)
- mng_bool mng_processchroma (mng_handle hHandle,
                              mng_uint32 iWhitepointx,
                              mng_uint32 iWhitepointy,
                              mng_uint32 iRedx,
                              mng_uint32 iRedy,
                              mng_uint32 iGreenx,
                              mng_uint32 iGreeny,
                              mng_uint32 iBluex,
                              mng_uint32 iBluey)
- mng_bool mng_processsrgb   (mng_handle hHandle,
                              mng_uint8  iRenderingintent)
- mng_bool mng_processiccp   (mng_handle hHandle,
                              mng_uint32 iProfilesize,
                              mng_ptr    pProfile)
- mng_bool mng_processarow   (mng_handle hHandle,
                              mng_uint32 iRowsamples,
                              mng_bool   bIsRGBA16,
                              mng_ptr    pRow)

These callbacks are only required when you selected the MNG_APP_CMS
directive during compilation of the library. See the configuration
section for more details.

- mng_bool mng_iteratechunk (mng_handle  hHandle,
                             mng_handle  hChunk,
                             mng_chunkid iChunkid,
                             mng_uint32  iChunkseq)

This callback is only used for the mng_iterate_chunks() function.
It is called exactly once for each chunk stored.


III. Housekeeping


> Memory management

The library can use internal memory allocation/deallocation or use
provided callbacks for its memory management. The choice is made at
compilation time. See the section on customization for details.

If internal management has been selected, the memory callback functions
need not be supplied. Even if you do supply them they will not be used.
The actual code used is similar to the code discussed in the callback
section:

      pPtr = calloc (1, iSize);

      free (pPtr);

If your compiler does not support these functions, or you wish to monitor
the library's use of memory for certain reasons, you can choose to
compile the library with external memory management. In this case the
memory callback functions MUST be supplied, and should function as if the
above code was used.


> Initialization

The basic initialization of the library is short and swift:

    myhandle = mng_initialize (myuserdata, my_alloc, 
                               my_free, MNG_NULL);
    if (myhandle == MNG_NULL)
      /* process error */;

The first field is an application-only parameter. It is saved in
libmng's internal structures and available at all times through the
mng_get_userdata() function. This is especially handy in callback functions
if your program may be handling multiple files at the same time.

The second and third field supply the library with the memory callback
1function entry-points. These are described in more detail in the callback
section and the previous paragraph.

The fourth and last field may be used to supply the library with the
entry-point of a trace callback function. For regular use you will not
need this!

The function returns a handle which will be your ticket to MNG-heaven.
All other functions rely on this handle. It is the single fixed unique
reference-point between your application and the library.

You should call the initialization function for each image you wish to
process simultaneously. If you are processing images consecutively, you can
reset the internal status of the library with the mng_reset() function.
This function will clear all internal state variables, free any stored
chunks and/or objects, etc, etc. Your callbacks and other external parameters
will be retained.

After you successfully received the handle it is time to set the required
callbacks. The sections on reading, displaying & writing indicate which
callbacks are required and which are optional.
To set the callbacks simply do:

    myretcode = mng_setcb_xxxxxx (myhandle, my_xxxxxx);
    if (myretcode != MNG_NOERROR)
      /* process error */;

Naturally you'd replace the x's with the name of the callback.


> Cleanup

Once you've gotten hold of that precious mng_handle, you should always,
and I mean always, call the cleanup function when you're done.
Just do:

    mng_cleanup (myhandle);

And you're done. There shouldn't be an ounce of memory spilled after
that call.

Note that if you would like to process multiple files consecutively
you do not need to do mng_cleanup() / mng_initialize() between each file
but simply

    myretcode = mng_reset (myhandle);
    if (myretcode != MNG_NOERROR)
      /* process error */;

will suffice. Saves some time and effort, that.


> Error handling

From the examples in the previous paragraphs you may have noticed a
meticulous scheme for error handling. And yes, that's exactly what it is.
Practically each call simply returns an errorcode, indicating success,
eg. MNG_NOERROR or failure, anything else but MNG_NEEDMOREDATA and
MNG_NEEDTIMERWAIT. These latter two will be discussed in more detail in
their respective fields of interest: the reading section and displaying
section respectively.

It is the application's responsibility to check the returncode after
each call. You can call mng_getlasterror() to receive the details of
the last detected error. This even includes a discriptive error-message
if you enabled that option during compilation of the library.

Note that after receiving an error it is still possible to call the
library, but it's also very likely that any following call will fail.
The only functions deemed to work will be mng_reset() and mng_cleanup().
Yes, if you abort your program after an error, you should still call
mng_cleanup().


IV. Reading

Reading a MNG, JNG or PNG is fairly easy. It depends slightly on your
ultimate goal how certain specifics are to be handled, but the basics
are similar in all cases.

For the read functioins to work you must have compiled the library with
the MNG_READ_SUPPRT directive. The standard DLL and Shared Library
have this on by default!


> Setup

Naturally you must have initialized the library and be the owner of
a mng_handle. The following callbacks are essential:

    mng_openstream, mng_readdata, mng_closestream

You may optionally define:

    mng_errorproc, mng_traceproc
    mng_processheader, mng_processtext
    mng_processsave, mng_processseek

The reading bit will also fail if you are already creating or
displaying a file. Seems a bit obvious, but I thought I'd mention it,
just in case.


> To suspend or not to suspend

There is one choice you need to make before calling the read function.
Are you in need of suspension-mode or not?

If you're reading from a disk you most certainly do not need
suspension-mode. Even the oldest and slowest of disks will be fast
enough for straight reading.

However, if your input comes from a really slow device, such as a
dialup-line or the likes, you may opt for suspension-mode. This is done
by calling

    myretcode = mng_set_suspensionmode (myhandle,
                                        MNG_TRUE);
    if (myretcode != MNG_NOERROR)
      /* process error */;

Suspension-mode will force the library to use special buffering on the
input. This allows your application to receive data of arbitrarily length
and return this in the mng_readdata() callback, without disturbing the
chunk processing routines of the library.

Suspension-mode does require a little extra care in the main logic of the
1application. The read function may return with MNG_NEEDMOREDATA when the
mng_readdata() callback returns less data then it needs to process the
next chunk. This indicates the application to wait for more data to arrive
and then resume processing by calling mng_read_resume().


> The read HLAPI

The actual reading is just plain simple. Since all I/O is done
1outside the library through the callbacks, the library can focus on
its real task. Understanding, checking and labelling the input data!

All you really need to do is this:

    myretcode = mng_read (myhandle);
    if (myretcode != MNG_NOERROR)
      /* process error */;

Of course, if you're on suspension-mode the code is a little more
complicated:

    myretcode = mng_read (myhandle);

    while (myretcode == MNG_NEEDMOREDATA) {
      /* wait for input-data to arrive */
      myretcode = mng_read_resume (myhandle);
    } 
   
    if (myretcode != MNG_NOERROR)
      /* process error */;

This is rather crude and more sophisticated programming methods may
dictate another approach. Whatever method you decide on, it should
act as if the above code was in its place.

There is also the mng_readdisplay() function, but this is discussed
in the displaying section. It functions pretty much as the mng_read()
function, but also immediately starts displaying the image.
mng_read_resume() should be replaced by mng_display_resume() in that
case!


> What happens inside

What actually happens inside the library depends on the configuration
options set during the compilation of the library.

Basically the library will first read the 8-byte file header, to determine
its validity and the type of image it is about to process. Then it will
repeatedly read a 4-byte chunk-length and then the remainder of the chunk
until it either reaches EOF (indicated by the mng_readdata() callback) or
implicitly decides EOF as it processed the logically last chunk of the
image.

Applications that require strict conformity and do not allow superfluous
data after the ending chunk, will need to perform this check in their
mng_closestream() callback.

Each chunk is then checked on CRC, after which it is handed over to the
appropriate chunk processing routine. These routines will disect the
chunk, check the validity of its contents, check its position with respect
to other chunks, etc, etc.

If everything checks out, the chunk is further processed as follows:

If display support has been selected during compilation, certain pre-display
initialization will take place.

If chunk-storage support has been selected during compilation, the chunks
data may be stored in a special internal structure and held for future
reference.


> Storing and accessing chunks

One of the compilation options activates support for chunk storage.
This option may be useful if you want to examine an image. The directive
is MNG_STORE_CHUNKS. You must also turn on the MNG_ACCESS_CHUNKS
directive.

The actual storage facility can be turned on or off with the
mng_set_storechunks() function. If set to MNG_TRUE, chunks will be
stored as they are read.

At any point you can then call the mng_iterate_chunks() function
to iterate through the current list of chunks. This function requires
a callback which is called for each chunk and receives a specific
chunk-handle. This chunk-handle can be used to call the appropriate
mng_getchunk_xxxx() function, to access the chunks properties.

A typical implementation may look like this:

    mng_bool my_iteratechunk (mng_handle  hHandle,
                              mng_handle  hChunk,
                              mng_chunkid iChunkid,
                              mng_uint32  iChunkseq) {
      switch (iChunkid) {
        case MNG_UINT_MHDR : { /* process MHDR */;
                               break; }
        case MNG_UINT_FRAM : { /* process FRAM */;
                               break; }

            ...etc...

        case MNG_UINT_HUH  : { /* unknown chunk */; 
                               break; }
        default : { /* duh; forgot one */; }
      }

      return MNG_TRUE; /* keep'm coming */
    }

To get to the actual chunk fields of lets say a SHOW chunk you would do:

    mng_bool isempty;
    mng_uint16 firstid, lastid;
    mng_uint8 showmode;

    myretcode mng_getchunk_show (hHandle, hChunk, 
                                 isempty, firstid,
                                 lastid, showmode);
    if (myretcode != MNG_NOERROR)
      /* process error */;


V. Displaying


> Setup

Assuming you have initialized the library and are the owner of
a mng_handle. The following callbacks are essential:

    mng_getcanvasline, mng_refresh
    mng_gettickcount, mng_settimer

If you wish to use an application supplied background you must supply:

    mng_getbkgdline

If you wish to use the MNG_CANVAS_RGB8_A8 canvas style you must supply:

    mng_getalphaline

You may optionally define:

    mng_errorproc, mng_traceproc
    mng_processheader, mng_processtext
    mng_processsave, mng_processseek

Note that the mng_processheader() callback is optional but will
be quite significant for proper operation!

Displaying an image will fail if you are creating a file or already
displaying one. Yes, you can't display it twice!


> A word on canvas styles

The canvas style describes how your drawing canvas is made up.
You must set this before the library actually starts drawing, so
the mng_processheader() callback is a pretty good place for it.

Currently only 8-bit RGB canvas styles are supported, either with
or without an alpha channel.

If you like to do alpha composition yourself you can select one of
the canvas styles that include an alpha channel. You can even have
a separate alpha canvas by selecting the MNG_CANVAS_RGB8_A8 style.

All styles require a compact model. Eg. MNG_CANVAS_BGR8 requires
your canvas lines in bgrbgrbgr... storage, where each letter
represents an 8-bit value of the corresponding color, and each
threesome makes up the values of one(1) pixel.

The library processes a line at a time, so the canvas lines do not
actually need to be consecutive in memory.


> Alpha composition and application backgrounds

All Network Graphics can be partially transparent. This requires
special processing if you need to display an image against some
background. Note that the MNG header (MHDR chunk) contains a
simplicity field indicating whether transparency information in
the file is critical or not. This only applies to embedded images,
which means the full image-frame of the MNG may still contain fully
transparent pixels!

Depending on your needs you can supply a single background color,
a background canvas or tell the library to return the alpha-channel
and do alpha composition yourself.

This is different from the BACK chunk in a MNG, or the bKGD chunk
in an (embedded) PNG or JNG. The BACK chunk indicates an optional or
mandatory background color and/or image. The bKGD chunk only indicates
an optional background color. These chunks indicate the Authors
preferences. They may be absent in which case you need to supply
some sort of background yourself.

> Composing against a background color

This is the easiest method. Call the mng_set_bgcolor() function to
set the values of the red, green and blue component of your preferred
background color.

Use one of the canvas styles that do not have an alpha-channel, and
which matches your output requirements.

> Composing against a background canvas

This is somewhat more complicated. You will need to set the
mng_getbkgdline() callback. This will be called whenever the library
needs to compose a partially transparent line.

This canvas must hold the background against which the image should
be composed. Its size must match exactly with the image dimensions
and thus the drawing canvas!

Use one of the canvas styles that do not have an alpha-channel, and
which matches your output requirements. The canvas style of the
background canvas may even differ from the drawing canvas. The library's
composing will still function properly.

> Composing within the application

If you have the option in your application to draw a (partially)
transparent canvas to the output device, this option is preferred.

Select one of the canvas styles that do have an alpha-channel.
The library will now supply the appropriate alpha information,
allowing the application to compose the image as it sees fit.


> Color information and CMS

Network Graphics may, and usually will, contain color-correction
information. This information is intended to compensate for the
difference in recording and display devices used.