HWContextType算是ffmpeg中为硬解码第三方接口的一个辅助类，它自己有两个辅助子类
AVHWDeviceContext和AVHWFramesContext。
AVHWDeviceContext主要表示硬件上下文
AVHWFramesContext主要表示硬件Frame的一些参数，比如你解码后的YUV数据还在硬件上，那么就通过这个类去存储相关参数。
那么它辅助硬解码器做什么呢？我们还是通过英伟达的例子来看
c
const HWContextType ff_hwcontext_type_cuda = {
.type = AV_HWDEVICE_TYPE_CUDA,
.name = "CUDA",
.device_hwctx_size = sizeof(AVCUDADeviceContext),
.frames_priv_size = sizeof(CUDAFramesContext),
.device_create = cuda_device_create,
.device_derive = cuda_device_derive,
.device_init = cuda_device_init,
.device_uninit = cuda_device_uninit,
.frames_get_constraints = cuda_frames_get_constraints,
.frames_init = cuda_frames_init,
.frames_get_buffer = cuda_get_buffer,
.transfer_get_formats = cuda_transfer_get_formats,
.transfer_data_to = cuda_transfer_data,
.transfer_data_from = cuda_transfer_data,
.pix_fmts = (const enum AVPixelFormat[]){ AV_PIX_FMT_CUDA, AV_PIX_FMT_NONE },
};仔细看上面函数就发现，全部是device mem操作，大白话说就是ffmpeg通过这套机制来实现D2H或者H2D的操作，别无其它。
具体看看结构体定义吧，一大堆函数指针，这些就是你要实现的，不一定全部要实现，实现你自己想要的就可以了，主要的有transfer_data_to/transfer_data_from，map_to/map_from。
c
typedef struct HWContextType {
enum AVHWDeviceType type;
const char *name;
/**
* An array of pixel formats supported by the AVHWFramesContext instances
* Terminated by AV_PIX_FMT_NONE.
*/
const enum AVPixelFormat *pix_fmts;
/**
* size of the public hardware-specific context,
* i.e. AVHWDeviceContext.hwctx
*/
size_t device_hwctx_size;
/**
* size of the private data, i.e.
* AVHWDeviceInternal.priv
*/
size_t device_priv_size;
/**
* Size of the hardware-specific device configuration.
* (Used to query hwframe constraints.)
*/
size_t device_hwconfig_size;
/**
* size of the public frame pool hardware-specific context,
* i.e. AVHWFramesContext.hwctx
*/
size_t frames_hwctx_size;
/**
* size of the private data, i.e.
* AVHWFramesInternal.priv
*/
size_t frames_priv_size;
int (*device_create)(AVHWDeviceContext *ctx, const char *device,
AVDictionary *opts, int flags);
int (*device_derive)(AVHWDeviceContext *dst_ctx,
AVHWDeviceContext *src_ctx,
AVDictionary *opts, int flags);
int (*device_init)(AVHWDeviceContext *ctx);
void (*device_uninit)(AVHWDeviceContext *ctx);
int (*frames_get_constraints)(AVHWDeviceContext *ctx,
const void *hwconfig,
AVHWFramesConstraints *constraints);
int (*frames_init)(AVHWFramesContext *ctx);
void (*frames_uninit)(AVHWFramesContext *ctx);
int (*frames_get_buffer)(AVHWFramesContext *ctx, AVFrame *frame);
int (*transfer_get_formats)(AVHWFramesContext *ctx,
enum AVHWFrameTransferDirection dir,
enum AVPixelFormat **formats);
int (*transfer_data_to)(AVHWFramesContext *ctx, AVFrame *dst,
const AVFrame *src);
int (*transfer_data_from)(AVHWFramesContext *ctx, AVFrame *dst,
const AVFrame *src);
int (*map_to)(AVHWFramesContext *ctx, AVFrame *dst,
const AVFrame *src, int flags);
int (*map_from)(AVHWFramesContext *ctx, AVFrame *dst,
const AVFrame *src, int flags);
int (*frames_derive_to)(AVHWFramesContext *dst_ctx,
AVHWFramesContext *src_ctx, int flags);
int (*frames_derive_from)(AVHWFramesContext *dst_ctx,
AVHWFramesContext *src_ctx, int flags);
} HWContextType;AVHWFramesContext结构体表示
c
/**
* This struct describes a set or pool of "hardware" frames (i.e. those with
* data not located in normal system memory). All the frames in the pool are
* assumed to be allocated in the same way and interchangeable.
*
* This struct is reference-counted with the AVBuffer mechanism and tied to a
* given AVHWDeviceContext instance. The av_hwframe_ctx_alloc() constructor
* yields a reference, whose data field points to the actual AVHWFramesContext
* struct.
*/
typedef struct AVHWFramesContext {
/**
* A class for logging.
*/
const AVClass *av_class;
/**
* Private data used internally by libavutil. Must not be accessed in any
* way by the caller.
*/
AVHWFramesInternal *internal;
/**
* A reference to the parent AVHWDeviceContext. This reference is owned and
* managed by the enclosing AVHWFramesContext, but the caller may derive
* additional references from it.
*/
AVBufferRef *device_ref;
/**
* The parent AVHWDeviceContext. This is simply a pointer to
* device_ref->data provided for convenience.
*
* Set by libavutil in av_hwframe_ctx_init().
*/
AVHWDeviceContext *device_ctx;
/**
* The format-specific data, allocated and freed automatically along with
* this context.
*
* Should be cast by the user to the format-specific context defined in the
* corresponding header (hwframe_*.h) and filled as described in the
* documentation before calling av_hwframe_ctx_init().
*
* After any frames using this context are created, the contents of this
* struct should not be modified by the caller.
*/
void *hwctx;
/**
* This field may be set by the caller before calling av_hwframe_ctx_init().
*
* If non-NULL, this callback will be called when the last reference to
* this context is unreferenced, immediately before it is freed.
*/
void (*free)(struct AVHWFramesContext *ctx);
/**
* Arbitrary user data, to be used e.g. by the free() callback.
*/
void *user_opaque;
/**
* A pool from which the frames are allocated by av_hwframe_get_buffer().
* This field may be set by the caller before calling av_hwframe_ctx_init().
* The buffers returned by calling av_buffer_pool_get() on this pool must
* have the properties described in the documentation in the corresponding hw
* type's header (hwcontext_*.h). The pool will be freed strictly before
* this struct's free() callback is invoked.
*
* This field may be NULL, then libavutil will attempt to allocate a pool
* internally. Note that certain device types enforce pools allocated at
* fixed size (frame count), which cannot be extended dynamically. In such a
* case, initial_pool_size must be set appropriately.
*/
AVBufferPool *pool;
/**
* Initial size of the frame pool. If a device type does not support
* dynamically resizing the pool, then this is also the maximum pool size.
*
* May be set by the caller before calling av_hwframe_ctx_init(). Must be
* set if pool is NULL and the device type does not support dynamic pools.
*/
int initial_pool_size;
/**
* The pixel format identifying the underlying HW surface type.
*
* Must be a hwaccel format, i.e. the corresponding descriptor must have the
* AV_PIX_FMT_FLAG_HWACCEL flag set.
*
* Must be set by the user before calling av_hwframe_ctx_init().
*/
enum AVPixelFormat format;
/**
* The pixel format identifying the actual data layout of the hardware
* frames.
*
* Must be set by the caller before calling av_hwframe_ctx_init().
*
* @note when the underlying API does not provide the exact data layout, but
* only the colorspace/bit depth, this field should be set to the fully
* planar version of that format (e.g. for 8-bit 420 YUV it should be
* AV_PIX_FMT_YUV420P, not AV_PIX_FMT_NV12 or anything else).
*/
enum AVPixelFormat sw_format;
/**
* The allocated dimensions of the frames in this pool.
*
* Must be set by the user before calling av_hwframe_ctx_init().
*/
int width, height;
} AVHWFramesContext;AVHWDeviceContext的结构体表示，硬件的抽象
c
/**
* This struct aggregates all the (hardware/vendor-specific) "high-level" state,
* i.e. state that is not tied to a concrete processing configuration.
* E.g., in an API that supports hardware-accelerated encoding and decoding,
* this struct will (if possible) wrap the state that is common to both encoding
* and decoding and from which specific instances of encoders or decoders can be
* derived.
*
* This struct is reference-counted with the AVBuffer mechanism. The
* av_hwdevice_ctx_alloc() constructor yields a reference, whose data field
* points to the actual AVHWDeviceContext. Further objects derived from
* AVHWDeviceContext (such as AVHWFramesContext, describing a frame pool with
* specific properties) will hold an internal reference to it. After all the
* references are released, the AVHWDeviceContext itself will be freed,
* optionally invoking a user-specified callback for uninitializing the hardware
* state.
*/
typedef struct AVHWDeviceContext {
/**
* A class for logging. Set by av_hwdevice_ctx_alloc().
*/
const AVClass *av_class;
/**
* Private data used internally by libavutil. Must not be accessed in any
* way by the caller.
*/
AVHWDeviceInternal *internal;
/**
* This field identifies the underlying API used for hardware access.
*
* This field is set when this struct is allocated and never changed
* afterwards.
*/
enum AVHWDeviceType type;
/**
* The format-specific data, allocated and freed by libavutil along with
* this context.
*
* Should be cast by the user to the format-specific context defined in the
* corresponding header (hwcontext_*.h) and filled as described in the
* documentation before calling av_hwdevice_ctx_init().
*
* After calling av_hwdevice_ctx_init() this struct should not be modified
* by the caller.
*/
void *hwctx;
/**
* This field may be set by the caller before calling av_hwdevice_ctx_init().
*
* If non-NULL, this callback will be called when the last reference to
* this context is unreferenced, immediately before it is freed.
*
* @note when other objects (e.g an AVHWFramesContext) are derived from this
* struct, this callback will be invoked after all such child objects
* are fully uninitialized and their respective destructors invoked.
*/
void (*free)(struct AVHWDeviceContext *ctx);
/**
* Arbitrary user data, to be used e.g. by the free() callback.
*/
void *user_opaque;
} AVHWDeviceContext;常用函数
这些函数其实挺重要的，因为都是用户函数，通过这些函数去从解码器硬件上获取到你要的数据，比如av_hwframe_transfer_data/av_hwframe_get_buffer，就是说你在ffmpeg中不要显示的调用你自己的cuMemD2H或者H2D的函数，ffmpeg已经给你预留好了接口了，用这些接口去获取更加简单。
c
/**
* Look up an AVHWDeviceType by name.
*
* @param name String name of the device type (case-insensitive).
* @return The type from enum AVHWDeviceType, or AV_HWDEVICE_TYPE_NONE if
* not found.
*/
enum AVHWDeviceType av_hwdevice_find_type_by_name(const char *name);
/** Get the string name of an AVHWDeviceType.
*
* @param type Type from enum AVHWDeviceType.
* @return Pointer to a static string containing the name, or NULL if the type
* is not valid.
*/
const char *av_hwdevice_get_type_name(enum AVHWDeviceType type);
/**
* Iterate over supported device types.
*
* @param type AV_HWDEVICE_TYPE_NONE initially, then the previous type
* returned by this function in subsequent iterations.
* @return The next usable device type from enum AVHWDeviceType, or
* AV_HWDEVICE_TYPE_NONE if there are no more.
*/
enum AVHWDeviceType av_hwdevice_iterate_types(enum AVHWDeviceType prev);
/**
* Allocate an AVHWDeviceContext for a given hardware type.
*
* @param type the type of the hardware device to allocate.
* @return a reference to the newly created AVHWDeviceContext on success or NULL
* on failure.
*/
AVBufferRef *av_hwdevice_ctx_alloc(enum AVHWDeviceType type);
/**
* Finalize the device context before use. This function must be called after
* the context is filled with all the required information and before it is
* used in any way.
*
* @param ref a reference to the AVHWDeviceContext
* @return 0 on success, a negative AVERROR code on failure
*/
int av_hwdevice_ctx_init(AVBufferRef *ref);
/**
* Open a device of the specified type and create an AVHWDeviceContext for it.
*
* This is a convenience function intended to cover the simple cases. Callers
* who need to fine-tune device creation/management should open the device
* manually and then wrap it in an AVHWDeviceContext using
* av_hwdevice_ctx_alloc()/av_hwdevice_ctx_init().
*
* The returned context is already initialized and ready for use, the caller
* should not call av_hwdevice_ctx_init() on it. The user_opaque/free fields of
* the created AVHWDeviceContext are set by this function and should not be
* touched by the caller.
*
* @param device_ctx On success, a reference to the newly-created device context
* will be written here. The reference is owned by the caller
* and must be released with av_buffer_unref() when no longer
* needed. On failure, NULL will be written to this pointer.
* @param type The type of the device to create.
* @param device A type-specific string identifying the device to open.
* @param opts A dictionary of additional (type-specific) options to use in
* opening the device. The dictionary remains owned by the caller.
* @param flags currently unused
*
* @return 0 on success, a negative AVERROR code on failure.
*/
int av_hwdevice_ctx_create(AVBufferRef **device_ctx, enum AVHWDeviceType type,
const char *device, AVDictionary *opts, int flags);
/**
* Create a new device of the specified type from an existing device.
*
* If the source device is a device of the target type or was originally
* derived from such a device (possibly through one or more intermediate
* devices of other types), then this will return a reference to the
* existing device of the same type as is requested.
*
* Otherwise, it will attempt to derive a new device from the given source
* device. If direct derivation to the new type is not implemented, it will
* attempt the same derivation from each ancestor of the source device in
* turn looking for an implemented derivation method.
*
* @param dst_ctx On success, a reference to the newly-created
* AVHWDeviceContext.
* @param type The type of the new device to create.
* @param src_ctx A reference to an existing AVHWDeviceContext which will be
* used to create the new device.
* @param flags Currently unused; should be set to zero.
* @return Zero on success, a negative AVERROR code on failure.
*/
int av_hwdevice_ctx_create_derived(AVBufferRef **dst_ctx,
enum AVHWDeviceType type,
AVBufferRef *src_ctx, int flags);
/**
* Create a new device of the specified type from an existing device.
*
* This function performs the same action as av_hwdevice_ctx_create_derived,
* however, it is able to set options for the new device to be derived.
*
* @param dst_ctx On success, a reference to the newly-created
* AVHWDeviceContext.
* @param type The type of the new device to create.
* @param src_ctx A reference to an existing AVHWDeviceContext which will be
* used to create the new device.
* @param options Options for the new device to create, same format as in
* av_hwdevice_ctx_create.
* @param flags Currently unused; should be set to zero.
* @return Zero on success, a negative AVERROR code on failure.
*/
int av_hwdevice_ctx_create_derived_opts(AVBufferRef **dst_ctx,
enum AVHWDeviceType type,
AVBufferRef *src_ctx,
AVDictionary *options, int flags);
/**
* Allocate an AVHWFramesContext tied to a given device context.
*
* @param device_ctx a reference to a AVHWDeviceContext. This function will make
* a new reference for internal use, the one passed to the
* function remains owned by the caller.
* @return a reference to the newly created AVHWFramesContext on success or NULL
* on failure.
*/
AVBufferRef *av_hwframe_ctx_alloc(AVBufferRef *device_ctx);
/**
* Finalize the context before use. This function must be called after the
* context is filled with all the required information and before it is attached
* to any frames.
*
* @param ref a reference to the AVHWFramesContext
* @return 0 on success, a negative AVERROR code on failure
*/
int av_hwframe_ctx_init(AVBufferRef *ref);
/**
* Allocate a new frame attached to the given AVHWFramesContext.
*
* @param hwframe_ctx a reference to an AVHWFramesContext
* @param frame an empty (freshly allocated or unreffed) frame to be filled with
* newly allocated buffers.
* @param flags currently unused, should be set to zero
* @return 0 on success, a negative AVERROR code on failure
*/
int av_hwframe_get_buffer(AVBufferRef *hwframe_ctx, AVFrame *frame, int flags);
/**
* Copy data to or from a hw surface. At least one of dst/src must have an
* AVHWFramesContext attached.
*
* If src has an AVHWFramesContext attached, then the format of dst (if set)
* must use one of the formats returned by av_hwframe_transfer_get_formats(src,
* AV_HWFRAME_TRANSFER_DIRECTION_FROM).
* If dst has an AVHWFramesContext attached, then the format of src must use one
* of the formats returned by av_hwframe_transfer_get_formats(dst,
* AV_HWFRAME_TRANSFER_DIRECTION_TO)
*
* dst may be "clean" (i.e. with data/buf pointers unset), in which case the
* data buffers will be allocated by this function using av_frame_get_buffer().
* If dst->format is set, then this format will be used, otherwise (when
* dst->format is AV_PIX_FMT_NONE) the first acceptable format will be chosen.
*
* The two frames must have matching allocated dimensions (i.e. equal to
* AVHWFramesContext.width/height), since not all device types support
* transferring a sub-rectangle of the whole surface. The display dimensions
* (i.e. AVFrame.width/height) may be smaller than the allocated dimensions, but
* also have to be equal for both frames. When the display dimensions are
* smaller than the allocated dimensions, the content of the padding in the
* destination frame is unspecified.
*
* @param dst the destination frame. dst is not touched on failure.
* @param src the source frame.
* @param flags currently unused, should be set to zero
* @return 0 on success, a negative AVERROR error code on failure.
*/
int av_hwframe_transfer_data(AVFrame *dst, const AVFrame *src, int flags);
enum AVHWFrameTransferDirection {
/**
* Transfer the data from the queried hw frame.
*/
AV_HWFRAME_TRANSFER_DIRECTION_FROM,
/**
* Transfer the data to the queried hw frame.
*/
AV_HWFRAME_TRANSFER_DIRECTION_TO,
};
/**
* Get a list of possible source or target formats usable in
* av_hwframe_transfer_data().
*
* @param hwframe_ctx the frame context to obtain the information for
* @param dir the direction of the transfer
* @param formats the pointer to the output format list will be written here.
* The list is terminated with AV_PIX_FMT_NONE and must be freed
* by the caller when no longer needed using av_free().
* If this function returns successfully, the format list will
* have at least one item (not counting the terminator).
* On failure, the contents of this pointer are unspecified.
* @param flags currently unused, should be set to zero
* @return 0 on success, a negative AVERROR code on failure.
*/
int av_hwframe_transfer_get_formats(AVBufferRef *hwframe_ctx,
enum AVHWFrameTransferDirection dir,
enum AVPixelFormat **formats, int flags);