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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

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。

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

/*** 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

/*** 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。

/*** 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);