NAME
Prima::codecs - How to write a codec for Prima image subsystem
DESCRIPTION
How to write a codec for Prima image subsystem
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.
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. [ 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 is 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().
Simplest open_load() returns non-null pointer - it is enough to report
’o.k’
static void *
open_load( PImgCodec instance, PImgLoadFileInstance fi)
{
return (void*)1;
}
Its 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)
{
}
Writing to "PImage-> data"
As mentioned above, Prima insists on keeping its 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 "Registering with image
subsystem" part, if you want it fast.
Single-frame loading
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.
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 "bc_format1_format2", which
work on scanlines and probably ibc_repad, which combines some
"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 its
name. Note the BYTEORDER symbol that is defined ( usually ) in
sys/types.h
Load with no data
If a high-level code just needs image information rather than all its
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;
}
The newly introduced macro "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 the 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.
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:
1. integers: pset_i( integer, _l-> integer);
2. floats: pset_f( float, _l-> float);
3. strings: pset_c( string, _l-> charstar); - note - no malloc
codec from you required
4. prima objects: pset_H( Handle, _l-> primaHandle);
5. SV’s: pset_sv_noinc( scalar, newSVsv(sv));
6. hashes: pset_sv_noinc( scalar, ( SV *) newHV()); - hashes
created through newHV() can be filled just in the same manner as
described here
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];
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.
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;
PIcon( fi-> object)-> maskColor = ARGB( p->r, p-> g, p-> b);
PIcon( fi-> object)-> autoMasking = 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);
PIcon( fi-> object)-> autoMasking = amNone;
}
Note that mask is also subject to LSB/MSB and 32-bit alignment issues.
Treat it as a regular imbpp1 data format.
c) A format supports transparency information, but image does not
contain any. In this case no action is required on the codec’s part;
the high-level code specifies if the transparency mask is created (
iconUnmask field ).
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 "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 whatever 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().
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.
Returning an error
Image subsystem defines no severity gradation for codec errors. If
error occurs during load, codec returns false value, which is "null" on
open_load() and "false" on load. It is advisable to explain the error,
otherwise the user gets just "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 that issues croak(). This condition is untrappable, - at
least without calling perl functions. Understanding that that behavior
is not acceptable, it is still under design.
Multiple-frame load
In order to indicate that a codec is ready to read multiframe images,
it must 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 its 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, is as follows. 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.
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. Its 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.
Registering with image subsystem
Finally, the code have to be registered. It is 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.
AUTHOR
Dmitry Karasik, <dmitry@karasik.eu.org>.
SEE ALSO
Prima, Prima::Image, Prima::internals, Prima::image-load