FFmpeg之AVHWAccel

这也是ffmpeg解码器中比较重要的一个模块，很多人认识它应该是通过一条命令

ffmpeg -hwaccel cuda -hwaccel_output_format cuda -i input.mp4 -c:v h264_nvenc -b:v 5M output.mp4

命令地址：英伟达ffmpeg

大家可能觉得这就是nvcodec了，后来发现专门还有一个cuvid呢？瞬间不开心了，不知道这两个倒地时什么关系？如果你时看这个问题的，恭喜你来对了。
下面我们就说道说道。
ffmpeg是通过解码起家，所以它内部有很多自己写的软解码器，在这些软解码器的解码过程当中，比如说对于码流中的某些反量化，反变换等操作，把这些操作挪到一块硬件上，这块硬件就是加速设备。
我们可以看看ffmpeg的h264解码器，红框内部的都是h264解码器的加速插件，NVDEC只是其中之一。

再来看看代码

static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size)
{
    AVCodecContext *const avctx = h->avctx;
  ...
        switch (nal->type) {
        case H264_NAL_IDR_SLICE:
          ...
                if (h->avctx->hwaccel &&
                //这里就进入了加速解码的分支
                    (ret = h->avctx->hwaccel->start_frame(h->avctx, buf, buf_size)) < 0)
                    goto end;
            }
			...
            max_slice_ctx = avctx->hwaccel ? 1 : h->nb_slice_ctx;
            if (h->nb_slice_ctx_queued == max_slice_ctx) {
            //这里进入加速解码的分支中
                if (h->avctx->hwaccel) {
                    ret = avctx->hwaccel->decode_slice(avctx, nal->raw_data, nal->raw_size);
                    h->nb_slice_ctx_queued = 0;
                } else
                    ret = ff_h264_execute_decode_slices(h);
                if (ret < 0 && (h->avctx->err_recognition & AV_EF_EXPLODE))
                    goto end;
            }
            break;
            ...

这就是hwaccel加速解码的原理，它是在ffmpeg软解码的基础上将一些特定算法加载到硬件上去做。而cuvid是通过ffmpeg第三方插件库的方式去实现的。

可以看看AVHWAccel结构体定义，发现有几个关键的函数指针，这些指针就是你需要去实现的。
可能你又要问了，为什么英伟达解码器要实现两套？
这个问题留给你吧。或者在我博客里面找，一定有的。

/**
 * @defgroup lavc_hwaccel AVHWAccel
 *
 * @note  Nothing in this structure should be accessed by the user.  At some
 *        point in future it will not be externally visible at all.
 *
 * @{
 */
typedef struct AVHWAccel {
    /**
     * Name of the hardware accelerated codec.
     * The name is globally unique among encoders and among decoders (but an
     * encoder and a decoder can share the same name).
     */
    const char *name;

    /**
     * Type of codec implemented by the hardware accelerator.
     *
     * See AVMEDIA_TYPE_xxx
     */
    enum AVMediaType type;

    /**
     * Codec implemented by the hardware accelerator.
     *
     * See AV_CODEC_ID_xxx
     */
    enum AVCodecID id;

    /**
     * Supported pixel format.
     *
     * Only hardware accelerated formats are supported here.
     */
    enum AVPixelFormat pix_fmt;

    /**
     * Hardware accelerated codec capabilities.
     * see AV_HWACCEL_CODEC_CAP_*
     */
    int capabilities;

    /*****************************************************************
     * No fields below this line are part of the public API. They
     * may not be used outside of libavcodec and can be changed and
     * removed at will.
     * New public fields should be added right above.
     *****************************************************************
     */

    /**
     * Allocate a custom buffer
     */
    int (*alloc_frame)(AVCodecContext *avctx, AVFrame *frame);

    /**
     * Called at the beginning of each frame or field picture.
     *
     * Meaningful frame information (codec specific) is guaranteed to
     * be parsed at this point. This function is mandatory.
     *
     * Note that buf can be NULL along with buf_size set to 0.
     * Otherwise, this means the whole frame is available at this point.
     *
     * @param avctx the codec context
     * @param buf the frame data buffer base
     * @param buf_size the size of the frame in bytes
     * @return zero if successful, a negative value otherwise
     */
    int (*start_frame)(AVCodecContext *avctx, const uint8_t *buf, uint32_t buf_size);

    /**
     * Callback for parameter data (SPS/PPS/VPS etc).
     *
     * Useful for hardware decoders which keep persistent state about the
     * video parameters, and need to receive any changes to update that state.
     *
     * @param avctx the codec context
     * @param type the nal unit type
     * @param buf the nal unit data buffer
     * @param buf_size the size of the nal unit in bytes
     * @return zero if successful, a negative value otherwise
     */
    int (*decode_params)(AVCodecContext *avctx, int type, const uint8_t *buf, uint32_t buf_size);

    /**
     * Callback for each slice.
     *
     * Meaningful slice information (codec specific) is guaranteed to
     * be parsed at this point. This function is mandatory.
     * The only exception is XvMC, that works on MB level.
     *
     * @param avctx the codec context
     * @param buf the slice data buffer base
     * @param buf_size the size of the slice in bytes
     * @return zero if successful, a negative value otherwise
     */
    int (*decode_slice)(AVCodecContext *avctx, const uint8_t *buf, uint32_t buf_size);

    /**
     * Called at the end of each frame or field picture.
     *
     * The whole picture is parsed at this point and can now be sent
     * to the hardware accelerator. This function is mandatory.
     *
     * @param avctx the codec context
     * @return zero if successful, a negative value otherwise
     */
    int (*end_frame)(AVCodecContext *avctx);

    /**
     * Size of per-frame hardware accelerator private data.
     *
     * Private data is allocated with av_mallocz() before
     * AVCodecContext.get_buffer() and deallocated after
     * AVCodecContext.release_buffer().
     */
    int frame_priv_data_size;

    /**
     * Called for every Macroblock in a slice.
     *
     * XvMC uses it to replace the ff_mpv_reconstruct_mb().
     * Instead of decoding to raw picture, MB parameters are
     * stored in an array provided by the video driver.
     *
     * @param s the mpeg context
     */
    void (*decode_mb)(struct MpegEncContext *s);

    /**
     * Initialize the hwaccel private data.
     *
     * This will be called from ff_get_format(), after hwaccel and
     * hwaccel_context are set and the hwaccel private data in AVCodecInternal
     * is allocated.
     */
    int (*init)(AVCodecContext *avctx);

    /**
     * Uninitialize the hwaccel private data.
     *
     * This will be called from get_format() or avcodec_close(), after hwaccel
     * and hwaccel_context are already uninitialized.
     */
    int (*uninit)(AVCodecContext *avctx);

    /**
     * Size of the private data to allocate in
     * AVCodecInternal.hwaccel_priv_data.
     */
    int priv_data_size;

    /**
     * Internal hwaccel capabilities.
     */
    int caps_internal;

    /**
     * Fill the given hw_frames context with current codec parameters. Called
     * from get_format. Refer to avcodec_get_hw_frames_parameters() for
     * details.
     *
     * This CAN be called before AVHWAccel.init is called, and you must assume
     * that avctx->hwaccel_priv_data is invalid.
     */
    int (*frame_params)(AVCodecContext *avctx, AVBufferRef *hw_frames_ctx);
} AVHWAccel;