FFmpeg之HWContextType

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);