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4e64c4d5
编写于
9月 05, 2011
作者:
A
Anton Khirnov
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AVOptions: add documentation.
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libavutil/opt.h
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libavutil/opt.h
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@@ -32,6 +32,189 @@
#include "dict.h"
#include "log.h"
/**
* @defgroup avoptions AVOptions
* @{
* AVOptions provide a generic system to declare options on arbitrary structs
* ("objects"). An option can have a help text, a type and a range of possible
* values. Options may then be enumerated, read and written to.
*
* @section avoptions_implement Implementing AVOptions
* This section describes how to add AVOptions capabilities to a struct.
*
* All AVOptions-related information is stored in an AVClass. Therefore
* the first member of the struct must be a pointer to an AVClass describing it.
* The option field of the AVClass must be set to a NULL-terminated static array
* of AVOptions. Each AVOption must have a non-empty name, a type, a default
* value and for number-type AVOptions also a range of allowed values. It must
* also declare an offset in bytes from the start of the struct, where the field
* associated with this AVOption is located. Other fields in the AVOption struct
* should also be set when applicable, but are not required.
*
* The following example illustrates an AVOptions-enabled struct:
* @code
* typedef struct test_struct {
* AVClass *class;
* int int_opt;
* char *str_opt;
* uint8_t *bin_opt;
* int bin_len;
* } test_struct;
*
* static const AVOption options[] = {
* { "test_int", "This is a test option of int type.", offsetof(test_struct, int_opt),
* AV_OPT_TYPE_INT, { -1 }, INT_MIN, INT_MAX },
* { "test_str", "This is a test option of string type.", offsetof(test_struct, str_opt),
* AV_OPT_TYPE_STRING },
* { "test_bin", "This is a test option of binary type.", offsetof(test_struct, bin_opt),
* AV_OPT_TYPE_BINARY },
* { NULL },
* };
*
* static const AVClass test_class = {
* .class_name = "test class",
* .item_name = av_default_item_name,
* .option = options,
* .version = LIBAVUTIL_VERSION_INT,
* };
* @endcode
*
* Next, when allocating your struct, you must ensure that the AVClass pointer
* is set to the correct value. Then, av_opt_set_defaults() must be called to
* initialize defaults. After that the struct is ready to be used with the
* AVOptions API.
*
* When cleaning up, you may use the av_opt_free() function to automatically
* free all the allocated string and binary options.
*
* Continuing with the above example:
*
* @code
* test_struct *alloc_test_struct(void)
* {
* test_struct *ret = av_malloc(sizeof(*ret));
* ret->class = &test_class;
* av_opt_set_defaults(ret);
* return ret;
* }
* void free_test_struct(test_struct **foo)
* {
* av_opt_free(*foo);
* av_freep(foo);
* }
* @endcode
*
* @subsection avoptions_implement_nesting Nesting
* It may happen that an AVOptions-enabled struct contains another
* AVOptions-enabled struct as a member (e.g. AVCodecContext in
* libavcodec exports generic options, while its priv_data field exports
* codec-specific options). In such a case, it is possible to set up the
* parent struct to export a child's options. To do that, simply
* implement AVClass.child_next() and AVClass.child_class_next() in the
* parent struct's AVClass.
* Assuming that the test_struct from above now also contains a
* child_struct field:
*
* @code
* typedef struct child_struct {
* AVClass *class;
* int flags_opt;
* } child_struct;
* static const AVOption child_opts[] = {
* { "test_flags", "This is a test option of flags type.",
* offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { 0 }, INT_MIN, INT_MAX },
* { NULL },
* };
* static const AVClass child_class = {
* .class_name = "child class",
* .item_name = av_default_item_name,
* .option = child_opts,
* .version = LIBAVUTIL_VERSION_INT,
* };
*
* void *child_next(void *obj, void *prev)
* {
* test_struct *t = obj;
* if (!prev && t->child_struct)
* return t->child_struct;
* return NULL
* }
* const AVClass child_class_next(const AVClass *prev)
* {
* return prev ? NULL : &child_class;
* }
* @endcode
* Putting child_next() and child_class_next() as defined above into
* test_class will now make child_struct's options accessible through
* test_struct (again, proper setup as described above needs to be done on
* child_struct right after it is created).
*
* From the above example it might not be clear why both child_next()
* and child_class_next() are needed. The distinction is that child_next()
* iterates over actually existing objects, while child_class_next()
* iterates over all possible child classes. E.g. if an AVCodecContext
* was initialized to use a codec which has private options, then its
* child_next() will return AVCodecContext.priv_data and finish
* iterating. OTOH child_class_next() on AVCodecContext.av_class will
* iterate over all available codecs with private options.
*
* @subsection avoptions_implement_named_constants Named constants
* It is possible to create named constants for options. Simply set the unit
* field of the option the constants should apply to to a string and
* create the constants themselves as options of type AV_OPT_TYPE_CONST
* with their unit field set to the same string.
* Their default_val field should contain the value of the named
* constant.
* For example, to add some named constants for the test_flags option
* above, put the following into the child_opts array:
* @code
* { "test_flags", "This is a test option of flags type.",
* offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { 0 }, INT_MIN, INT_MAX, "test_unit" },
* { "flag1", "This is a flag with value 16", 0, AV_OPT_TYPE_CONST, { 16 }, 0, 0, "test_unit" },
* @endcode
*
* @section avoptions_use Using AVOptions
* This section deals with accessing options in an AVOptions-enabled struct.
* Such structs in Libav are e.g. AVCodecContext in libavcodec or
* AVFormatContext in libavformat.
*
* @subsection avoptions_use_examine Examining AVOptions
* The basic functions for examining options are av_opt_next(), which iterates
* over all options defined for one object, and av_opt_find(), which searches
* for an option with the given name.
*
* The situation is more complicated with nesting. An AVOptions-enabled struct
* may have AVOptions-enabled children. Passing the AV_OPT_SEARCH_CHILDREN flag
* to av_opt_find() will make the function search children recursively.
*
* For enumerating there are basically two cases. The first is when you want to
* get all options that may potentially exist on the struct and its children
* (e.g. when constructing documentation). In that case you should call
* av_opt_child_class_next() recursively on the parent struct's AVClass. The
* second case is when you have an already initialized struct with all its
* children and you want to get all options that can be actually written or read
* from it. In that case you should call av_opt_child_next() recursively (and
* av_opt_next() on each result).
*
* @subsection avoptions_use_get_set Reading and writing AVOptions
* When setting options, you often have a string read directly from the
* user. In such a case, simply passing it to av_opt_set() is enough. For
* non-string type options, av_opt_set() will parse the string according to the
* option type.
*
* Similarly av_opt_get() will read any option type and convert it to a string
* which will be returned. Do not forget that the string is allocated, so you
* have to free it with av_free().
*
* In some cases it may be more convenient to put all options into an
* AVDictionary and call av_opt_set_dict() on it. A specific case of this
* are the format/codec open functions in lavf/lavc which take a dictionary
* filled with option as a parameter. This allows to set some options
* that cannot be set otherwise, since e.g. the input file format is not known
* before the file is actually opened.
* @}
*/
enum
AVOptionType
{
AV_OPT_TYPE_FLAGS
,
AV_OPT_TYPE_INT
,
...
...
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