=head1 NAME

Prima Image Subsystem - How To Write A Codec

=head1 DESCRIPTION

=head1 Start simple

There are many graphical formats in the world, and yet more
libraries, that depend on them. Writing a codec that supports 
particular library is a tedious task, especially if one wants many
formats. Usually you never want to get into internal parts, the
functionality comes first, and who needs all those funky options that
format provides? We want to load a file and to show it. Everything
else comes later - if ever. So, in a way to not scare you off, we
start it simple.

=head2 Load

Define a callback function like:

   static Bool   
   load( PImgCodec instance, PImgLoadFileInstance fi)
   {
   }

Just that function is not enough for whole mechanism to work,
but bindings will come later. Let us imagine we work with an imaginary 
library libduff, that we want to load files of .duf format. 
I<[ To discern imaginary code from real, imaginary will be prepended
with _  - like, _libduff_loadfile ]>. So, we call _libduff_loadfile(),
that loads black-and-white, 1-bits/pixel images, where 1 is white and 0
is black. 


   static Bool   
   load( PImgCodec instance, PImgLoadFileInstance fi)
   {
      _LIBDUFF * _l = _libduff_load_file( fi-> fileName);
      if ( !_l) return false;

      // - create storage for our file
      CImage( fi-> object)-> create_empty( fi-> object,
        _l-> width, _l-> height, imBW);

      // Prima wants images aligned to 4-bytes boundary,
      // happily libduff has same considerations
      memcpy( PImage( fi-> object)-> data, _l-> bits, 
        PImage( fi-> object)-> dataSize);

      _libduff_close_file( _l);
         
      return true;
   }

Prima keeps an open handle of the file; so we can use it if
libduff trusts handles vs names:

   {
     _LIBDUFF * _l = _libduff_load_file_from_handle( fi-> f);
      ...
   // In both cases, you don't need to close the handle - 
   // however you might, it's ok:

      _libduff_close_file( _l);
      fclose( fi-> f);
   // You just assign it to null to indicate that you've closed it
      fi-> f = null;
      ...
   }

Together with load() you have to implement minimal open_load()
and close_load().

Simpliest open_load() returns non-null pointer - it's enough to report 'o.k'

   static void * 
   open_load( PImgCodec instance, PImgLoadFileInstance fi)
   {
      return (void*)1;
   }

It's result will be available in PImgLoadFileInstance-> instance,
just in case. If it was dynamically allocated, free it in close_load().
Dummy close_load() is doing simply nothing:

   static void
   close_load( PImgCodec instance, PImgLoadFileInstance fi)
   {
   }


=head2 Writing to PImage-> data

As mentioned above, Prima insists on keeping it's image data
in 32-bit aligned scanlines. If libduff allows reading from 
file by scanlines, we can use this possibility as well:


   PImage i = ( PImage) fi-> object; 
   // note - since this notation is more convenient than
   // PImage( fi-> object)-> , instead i-> will be used 

   Byte * dest = i-> data + ( _l-> height - 1) * i-> lineSize;
   while ( _l-> height--) {
      _libduff_read_next_scanline( _l, dest);
      dest -= i-> lineSize;
   }

Note that image is filled in reverse - Prima images are built
like classical XY-coordinate grid, where Y ascends upwards.

Here ends the simple part. You can skip down to "Register with 
image subsystem" part, if you want it fast.

=head1  Single-frame loading

=head2 Palette

Our libduff can be black-and-white in two ways -
where 0 is black and 1 is white and vice versa. While
0B/1W is perfectly corresponding to imbpp1 | imGrayScale
and no palette operations are needed ( Image cares 
automatically about these), 0W/1B is although black-and-white
grayscale but should be treated like general imbpp1 type.

     if ( l-> _reversed_BW) {
        i-> palette[0].r = i-> palette[0].g = i-> palette[0].b = 0xff;
        i-> palette[1].r = i-> palette[1].g = i-> palette[1].b = 0;
     }

NB. Image creates palette with size calculated by exponent of 2, since it can't know
beforehand of the actual palette size. If color palette for, say, 4-bit
image contains 15 of 16 possible for 4-bit image colors, code like

     i-> palSize = 15;

does the trick.

=head2 Data conversion

As mentioned before, Prima defines image scanline
size to be aligned to 32 bits, and the formula for 
lineSize calculation is

    lineSize = (( width * bits_per_pixel + 31) / 32) * 4;

Prima defines number of converting routines between different
data formats. Some of them can be applied to scanlines, and
some to whole image ( due sampling algorithms ). These are
defined in img_conv.h, and probably ones that you'll need
would be C<bc_format1_format2>, which work on scanlines
and probably ibc_repad, which combines some C<bc_XX_XX> with byte
repadding.
        For those who are especially lucky, some libraries do not
check between machine byte format and file byte format.
Prima unfortunately doesn't provide easy method for determining
this situation, but you have to convert your data in appropriate 
way to keep picture worthy of it's name. Note the BYTEORDER symbol
that is defined ( usually ) in sys/types.h 

=head2 Load with no data

If a high-level code just needs image information rather than
all it's bits, codec can provide it in a smart way. Old code
will work, but will eat memory and time. A flag 
PImgLoadFileInstance-> noImageData is indicating if image data
is needed. On that condition, codec needs to report only
dimensions of the image - but the type must be set anyway.
Here comes full code:

   static Bool
   load( PImgCodec instance, PImgLoadFileInstance fi)
   {
      _LIBDUFF * _l = _libduff_load_file( fi-> fileName);
      HV * profile = fi-> frameProperties;
      PImage i = ( PImage) fi-> frameProperties;
      if ( !_l) return false;

      CImage( fi-> object)-> create_empty( fi-> object, 1, 1, 
         _l-> _reversed_BW ? imbpp1 : imBW);

      // copy palette, if any
      if ( _l-> _reversed_BW) {
         i-> palette[0].r = i-> palette[0].g = i-> palette[0].b = 0xff;
         i-> palette[1].r = i-> palette[1].g = i-> palette[1].b = 0;
      }

      if ( fi-> noImageData) {
         // report dimensions
         pset_i( width,  _l-> width);
         pset_i( height, _l-> height);
         return true;
      } 

      // - create storage for our file
      CImage( fi-> object)-> create_empty( fi-> object,
           _l-> width, _l-> height, 
           _l-> _reversed_BW ? imbpp1 : imBW);

      // Prima wants images aligned to 4-bytes boundary,
      // happily libduff has same considerations
      memcpy( PImage( fi-> object)-> data, _l-> bits, 
        PImage( fi-> object)-> dataSize);


      _libduff_close_file( _l);
         
      return true;
   }

Newly introduced macros pset_i is a convenience operator, 
assigning integer (i) as a value to a hash key, given as a
first parameter - it becomes string literal upon macro 
expansion. Hash used for storage is a lexical of type HV*.
Code 

        HV * profile = fi-> frameProperties;
        pset_i( width, _l-> width);

is a prettier way for

        hv_store( 
            fi-> frameProperties, 
            "width", strlen( "width"),
            newSViv( _l-> width),
            0);

hv_store(), HV's and SV's along with other funny symbols are
described in perlguts.pod in Perl installation.

=head2  Return extra information

Image attributes are dimensions, type, palette and data.
However, it is only Prima point of view - different formats
can supply number of extra information, often irrelevant but
sometimes useful. From perl code, Image has a hash reference 'extras'
on object, where comes all this stuff. Codec can report also
such data, storing it in PImgLoadFileInstance-> frameProperties.
Data should be stored in native perl format, so if you're not 
familiar with perlguts, you better read it, especially if
you want return arrays and hashes. But just in simple, you can
return:

=over

=item 1 

integers:       pset_i( integer, _l-> integer);

=item 2

floats:         pset_f( float, _l-> float);

=item 3

strings:        pset_c( string, _l-> charstar); 
- note - no malloc codec from you required

=item 4

prima objects:  pset_H( Handle, _l-> primaHandle);

=item 5

SV's:           pset_sv_noinc( scalar, newSVsv(sv));

=item 6

hashes:         pset_sv_noinc( scalar, ( SV *) newHV()); 
- hashes created through newHV() can be filled just in the same manner
as described here

=item 7

arrays:         pset_sv_noinc( scalar, ( SV *) newAV()); 
- arrays (AV) are described in perlguts also, but
most useful function here is av_push. To push 4 values, 
for example, follow this code:


    AV * av = newAV();
    for ( i = 0;i < 4;i++) av_push( av, newSViv( i));
    pset_sv_noinc( myarray, newRV_noinc(( SV *) av);

is a C equivalent to

      ->{extras}-> {myarray} = [0,1,2,3];

=back

High level code can specify if the extra 
information should be loaded. This behavior is determined by
flag PImgLoadFileInstance-> loadExtras. Codec may skip this 
flag, the extra information will not be returned, even if
PImgLoadFileInstance-> frameProperties was changed. However, 
it is advisable to check for the flag, just for an efficiency.
All keys, possibly assigned to frameProperties should
be enumerated for high-level code. These strings should be 
represented into char ** PImgCodecInfo-> loadOutput array.

   static char * loadOutput[] = { 
      "hotSpotX",
      "hotSpotY",
      nil
   };

   static ImgCodecInfo codec_info = {
      ...
      loadOutput 
   };

   static void * 
   init( PImgCodecInfo * info, void * param)
   {
      *info = &codec_info;
      ...
   }   

The code above is taken from codec_X11.c, where X11 bitmap can 
provide location of hot spot, two integers, X and Y. The type
of the data is not specified.

=head2 Loading to icons

If high-level code wants an Icon instead of an Image,
Prima takes care for producing and-mask automatically.
However, if codec knows explicitly about transparency
mask stored in a file, it might change object in the way
it fits better. Mask is stored on Icon in a -> mask field.

a) Let us imagine, that 4-bit image always
carries a transparent color index, in 0-15 range. In this case,
following code will create desirable mask:

      if ( kind_of( fi-> object, CIcon) && 
           ( _l-> transparent >= 0) &&
           ( _l-> transparent < PIcon( fi-> object)-> palSize)) {
         PRGBColor p = PIcon( fi-> object)-> palette;
         p += _l-> transparent;
         CIcon( fi-> object)-> set_maskColor( fi-> object, 
            ARGB( p->r, p-> g, p-> b));
         CIcon( fi-> object)-> set_autoMasking( fi-> object, amMaskColor);
      }   

Of course, 

      pset_i( transparentColorIndex, _l-> transparent);

would be also helpful.

b) if explicit bit mask is given, code will be like:

      if ( kind_of( fi-> object, CIcon) && 
           ( _l-> maskData >= 0)) {
         memcpy( PIcon( fi-> object)-> mask, _l-> maskData, _l-> maskSize);
         CIcon( fi-> object)-> set_autoMasking( fi-> object, amNone);
      }   

Note that mask is also subject to LSB/MSB and 32-bit alignment 
issues. Treat it as a regular imbpp1 data format.

=head2 open_load() and close_load()

open_load() and close_load() are used as brackets for load requests,
and although they come to full power in multiframe load
requests, it is very probable that correctly written
codec should use them. Codec that assigns C<false> to 
PImgCodecInfo-> canLoadMultiple claims that it cannot load
those images that have index different from zero. It may
report total amount of frames, but still be incapable of
loading them. 
There is also a load sequence, called null-load,
when no load() calls are made, just open_load() and close_load().
These requests are made in case codec can provide some file
information without loading frames at all. It can be any
information, of whetever kind. It have to be stored into the hash
PImgLoadFileInstance-> fileProperties, to be filled once on
open_load(). The only exception is PImgLoadFileInstance-> frameCount,
which can be filled on open_load(). Actually, frameCount could be 
filled on any load stage, except close_load(), to make sense in
frame positioning. Even single frame codec is advised to fill
this field, at least to tell whether file is empty ( frameCount == 0) or
not ( frameCount == 1). More about frameCount comes into chapters
dedicated to multiframe requests.
For strictly single-frame codecs it is therefore advised
to care for open_load() and close_load().

=head2 Load input

So far codec is expected to respond for noImageData
hint only, and it is possible to allow a high-level code to alter
codec load behavior, passing specific parameters. 
PImgLoadFileInstance-> profile is a hash, that contains these
parameters. The data that should be applied to all frames and/or
image file are set there when open_load() is called. These data, 
plus frame-specific keys passed to every load() call.
However, Prima passes only those hash keys, which are
returned by load_defaults() function. This functions returns newly
created ( by calling newHV()) hash, with accepted keys and their
default ( and always valid ) value pairs.
Example below defines speed_vs_memory integer value, that 
should be 0, 1 or 2.

        
   static HV *
   load_defaults( PImgCodec c)
   {
      HV * profile = newHV();
      pset_i( speed_vs_memory, 1);
      return profile;
   }
   ...
   static Bool   
   load( PImgCodec instance, PImgLoadFileInstance fi)
   {
        ...
        HV * profile = fi-> profile;
        if ( pexist( speed_vs_memory)) {
           int speed_vs_memory = pget_i( speed_vs_memory);
           if ( speed_vs_memory < 0 || speed_vs_memory > 2) {
                strcpy( fi-> errbuf, "speed_vs_memory should be 0, 1 or 2");
                return false;
           }
           _libduff_set_load_optimization( speed_vs_memory);
        }
   }

The latter code chunk can be applied to open_load() as well.

=head2 Returning an error

Image subsystem defines no severity gradation for codec errors.
If error occurs during load, codec returns false value, which
is C<null> on open_load() and C<false> on load. It is advisable to 
explain the error, otherwise user gets common "Loading error"
string. To do so, error message is to be copied to 
PImgLoadFileInstance-> errbuf, which is char[256].
On an extreme severe error codec may call croak(),
which jumps to the closest G_EVAL block. If there is no G_EVAL 
blocks then program aborts. This condition could also happen if 
codec calls some Prima code with issues croak(). This condition 
is untrappable, - at least without calling perl functions. 
Understanding that that behavior is not acceptable, 
it is still under design.

=head1 Multiple-frame load

In order to indicate that a codec is ready to read
multiframe images, it have to set PImgCodecInfo-> canLoadMultiple
flag to true. This only means, that codec should respond to the
PImgLoadFileInstance-> frame field, which is integer that
can be in range from 0 to PImgLoadFileInstance-> frameCount - 1.
It is advised that codec should change the frameCount from
it's original value -1 to actual one, to help Prima filter range
requests before they go down to the codec. The only real problem that
may happen to the codec which it strongly unwilling to initialize
frameCount, worth an explanation, which is following.
If a loadAll request was made ( corresponding boolean
PImgLoadFileInstance-> loadAll flag is set for codec's information)
and frameCount is not initialized, then Prima starts loading all frames,
incrementing frame index until it receives an error. Assuming the
first error it gets is an EOF, it reports no error, so there's no
way for a high-level code to tell whether there was an loading error or
an end-of-file condition. 
Codec may initialize frameCount at any time during open_load()
or load(), even together with false return value.

=head1 Saving

Approach for handling saving requests is very similar to a load ones.
For the same reason and with same restrictions functions save_defaults()
open_save(), save() and close_save() are defined. Below shown a 
typical saving code and highlighted differences from load.
As an example we'll take existing codec_X11.c, which
defines extra hot spot coordinates, x and y.


   static HV *
   save_defaults( PImgCodec c)
   {
      HV * profile = newHV();
      pset_i( hotSpotX, 0);
      pset_i( hotSpotY, 0);
      return profile;
   }

   static void *
   open_save( PImgCodec instance, PImgSaveFileInstance fi)
   {
      return (void*)1;
   }

   static Bool   
   save( PImgCodec instance, PImgSaveFileInstance fi)
   {
      PImage i = ( PImage) fi-> object;
      Byte * l;
      ...
      
      fprintf( fi-> f, "#define %s_width %d\n", name, i-> w);
      fprintf( fi-> f, "#define %s_height %d\n", name, i-> h);
      if ( pexist( hotSpotX))
         fprintf( fi-> f, "#define %s_x_hot %d\n", name, (int)pget_i( hotSpotX));
      if ( pexist( hotSpotY))
         fprintf( fi-> f, "#define %s_y_hot %d\n", name, (int)pget_i( hotSpotY));
      fprintf( fi-> f, "static char %s_bits[] = {\n  ", name);
      ...
      // printing of data bytes is omitted
   }   

   static void 
   close_save( PImgCodec instance, PImgSaveFileInstance fi)
   {
   }

Save request takes into account defined supported types, that
are defined in PImgCodecInfo-> saveTypes. Prima converts image
to be saved into one of these formats, before actual save() call
takes place.
Another boolean flag, PImgSaveFileInstance-> append
is summoned to govern appending to or rewriting a file, but
this functionality is under design. It's current value
is a hint, if true, for a codec not to rewrite but rather
append the frames to an existing file. Due to increased
complexity of the code, that should respond to the append hint, 
this behavior is not required.

Codec may set two of PImgCodecInfo flags, canSave and
canSaveMultiple. Save requests will never be called if canSave
is false, and append requests along with multiframe save requests
would be never invoked for a codec with canSaveMultiple set to false.
Scenario for a multiframe save request is the same as for a load one. All the
issues concerning palette, data converting and saving extra 
information are actual, however there's no corresponding flag like
loadExtras - codec is expected to save all information what it can extract from
PImgSaveFileInstance-> objectExtras hash. 


=head1 Registering with image subsystem

Finally, the code have to be registered. It's not as illustrative 
but this part better not to be oversimplified.
A codec's callback functions are set into ImgCodecVMT structure.
Those function slots that are unused should not be defined as
dummies - those are already defined and gathered under struct
CNullImgCodecVMT. That's why all functions in the illustration code
were defined as static.
A codec have to provide some information that Prima
uses to decide what codec should load this particular file.
If no explicit directions given, Prima asks those codecs whose
file extensions match to file's.
init() should return pointer to the filled struct, that describes 
codec's capabilities:

   // extensions to file - might be several, of course, thanks to dos...
   static char * myext[] = { "duf", "duff", nil };

   // we can work only with 1-bit/pixel
   static int    mybpp[] = { 
       imbpp1 | imGrayScale, // 1st item is a default type
       imbpp1, 
       0 };   // Zero means end-of-list. No type has zero value.

   // main structure
   static ImgCodecInfo codec_info = {
      "DUFF", // codec name 
      "Numb & Number, Inc.", // vendor
      _LIBDUFF_VERS_MAJ, _LIBDUFF_VERS_MIN,    // version
      myext,    // extension
      "DUmb Format",     // file type
      "DUFF",     // file short type
      nil,    // features 
      "",     // module
      true,   // canLoad
      false,  // canLoadMultiple 
      false,  // canSave
      false,  // canSaveMultiple
      mybpp,  // save types
      nil,    // load output 
   };

   static void * 
   init( PImgCodecInfo * info, void * param)
   {
      *info = &codec_info;
      return (void*)1; // just non-null, to indicate success
   }   

The result of init() is stored into PImgCodec-> instance, and
info into PImgCodec-> info. If dynamic memory was allocated
for these structs, it can be freed on done() invocation.
Finally, the function that is invoked from Prima,
is the only that required to be exported, is responsible for
registering a codec:

   void 
   apc_img_codec_duff( void )
   {
      struct ImgCodecVMT vmt;
      memcpy( &vmt, &CNullImgCodecVMT, sizeof( CNullImgCodecVMT));
      vmt. init          = init;
      vmt. open_load     = open_load;
      vmt. load          = load; 
      vmt. close_load    = close_load; 
      apc_img_register( &vmt, nil);
   }

This procedure can register as many codecs as it wants to, 
but currently Prima is designed so that one codec_XX.c file 
should be connected to one library only.

The name of the procedure is apc_img_codec_ plus
library name, that is required for a compilation with Prima.
File with the codec should be called codec_duff.c ( is our case)
and put into img directory in Prima source tree. Following
these rules, Prima will be assembled with libduff.a ( or duff.lib,
or whatever, the actual library name is system dependent) - if the library is present.


