Question

如何使用ffmpeg解码具有硬件加速的文件？

我写过一个使用ffmpeg的工作视频播放器。我使用"av_hwaccel_next"检查了支持情况，发现mpeg2_dxva。

然而，当我加载mpeg2文件（像往常一样）时，我没有得到任何硬件加速。 AVCodecContext->hwaccel和AVCodecContext->hwaccelcontext均为空。

我是否必须在某处传递一些标志以启用hw-acceleration？

我无法找到任何关于此的信息，任何人都知道一个好的来源？

Answer 1

从阅读ffmpeg文档开始：https://trac.ffmpeg.org/wiki/HWAccelIntro并更好地回答How to use hardware acceleration with ffmpeg（对于linux检查页面https://wiki.archlinux.org/index.php/Hardware_video_acceleration）

当使用FFmpeg工具时，通过-hwaccel选项启用HW辅助解码，该选项启用特定解码器。每个解码器可能具有特定限制（例如，H.264解码器可能仅支持基线简档）。通过使用特定编码器（例如h264_nvenc）启用HW辅助编码。仅在几个过滤器中支持过滤硬件辅助处理。有一些硬件加速标准API，FFmpeg在某种程度上支持其中一些。

hwaccel激活由代码控制（在2013 https://github.com/FFmpeg/FFmpeg/commit/08303d774132775d49d4ba767092de5d426f089d之后重新格式化）

avctx->hwaccel = ff_find_hwaccel(avctx->codec->id, avctx->pix_fmt);

例如，在libavcodec / mpeg12dec.c https://github.com/FFmpeg/FFmpeg/blob/6c7254722ad43712db5686feee8bf75c74d8635b/libavcodec/mpeg12dec.c

中

avctx->pix_fmt = mpeg_get_pixelformat(avctx);
avctx->hwaccel = ff_find_hwaccel(avctx->codec->id, avctx->pix_fmt);

ff_find_hwaccel检查编解码器和pixelformat图像对以及所有可用的hwaccelerators。

AVHWAccel *ff_find_hwaccel(enum CodecID codec_id, enum PixelFormat pix_fmt)
{
    AVHWAccel *hwaccel=NULL;

    while((hwaccel= av_hwaccel_next(hwaccel))){
        if (   hwaccel->id      == codec_id
            && hwaccel->pix_fmt == pix_fmt)
            return hwaccel;
    }
    return NULL;
}

例如，dxva2（https://en.wikipedia.org/wiki/DirectX_Video_Acceleration）具有：

AVHWAccel mpeg2_dxva2_hwaccel = {
    .name           = "mpeg2_dxva2",
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = CODEC_ID_MPEG2VIDEO,
    .pix_fmt        = PIX_FMT_DXVA2_VLD,
    .capabilities   = 0,
    .start_frame    = start_frame,
    .decode_slice   = decode_slice,
    .end_frame      = end_frame,
    .priv_data_size = sizeof(struct dxva2_picture_context),
};

libavutil/pixfmt.h按像素格式列出所有支持的hw解码器/加速器https://ffmpeg.org/doxygen/3.2/pixfmt_8h.html

AV_PIX_FMT_XVMC_MPEG2_MC    - XVideo Motion Acceleration via common packet passing.
AV_PIX_FMT_XVMC_MPEG2_IDCT  - undocumented
AV_PIX_FMT_XVMC         - undocumented
AV_PIX_FMT_VDPAU_H264   - H.264 HW decoding with VDPAU, data[0] contains a vdpau_render_state struct which contains the bitstream of the slices as well as various fields extracted from headers.
AV_PIX_FMT_VDPAU_MPEG1  - MPEG-1 HW decoding with VDPAU, data[0] contains a vdpau_render_state struct which contains the bitstream of the slices as well as various fields extracted from headers.
AV_PIX_FMT_VDPAU_MPEG2  - MPEG-2 HW decoding with VDPAU, data[0] contains a vdpau_render_state struct which contains the bitstream of the slices as well as various fields extracted from headers.
AV_PIX_FMT_VDPAU_WMV3   - WMV3 HW decoding with VDPAU, data[0] contains a vdpau_render_state struct which contains the bitstream of the slices as well as various fields extracted from headers.
AV_PIX_FMT_VDPAU_VC1    - VC-1 HW decoding with VDPAU, data[0] contains a vdpau_render_state struct which contains the bitstream of the slices as well as various fields extracted from headers. 
AV_PIX_FMT_VAAPI_MOCO   - HW acceleration through VA API at motion compensation entry-point, Picture.data[3] contains a vaapi_render_state struct which contains macroblocks as well as various fields extracted from headers.
AV_PIX_FMT_VAAPI_IDCT   - HW acceleration through VA API at IDCT entry-point, Picture.data[3] contains a vaapi_render_state struct which contains fields extracted from headers.
AV_PIX_FMT_VAAPI_VLD    - HW decoding through VA API, Picture.data[3] contains a VASurfaceID. 
AV_PIX_FMT_VDPAU_MPEG4  - MPEG-4 HW decoding with VDPAU, data[0] contains a vdpau_render_state struct which contains the bitstream of the slices as well as various fields extracted from headers.
AV_PIX_FMT_DXVA2_VLD    - HW decoding through DXVA2, Picture.data[3] contains a LPDIRECT3DSURFACE9 pointer. 
AV_PIX_FMT_VDPAU        - HW acceleration through VDPAU, Picture.data[3] contains a VdpVideoSurface. 
AV_PIX_FMT_VDA          - HW acceleration through VDA, data[3] contains a CVPixelBufferRef. 
AV_PIX_FMT_QSV          - HW acceleration through QSV, data[3] contains a pointer to the mfxFrameSurface1 structure.
AV_PIX_FMT_MMAL         - HW acceleration though MMAL, data[3] contains a pointer to the MMAL_BUFFER_HEADER_T structure.
AV_PIX_FMT_D3D11VA_VLD  - HW decoding through Direct3D11, Picture.data[3] contains a ID3D11VideoDecoderOutputView pointer.
AV_PIX_FMT_CUDA         - HW acceleration through CUDA. data[i] contain CUdeviceptr pointers exactly as for system memory frames.

实际选择的像素格式是在ff_find_hwaccel之前调用的函数，static enum PixelFormat mpeg_get_pixelformat(AVCodecContext *avctx)为mpeg1 / 2的libavcodec/mpeg12dec.c。在早期版本的ffmpeg / libavcodec中，它检查avctx->xvmc_acceleration和/或avctx->codec->capabilities&CODEC_CAP_HWACCEL_VDPAU或调用avctx->get_format(avctx,ff_hwaccel_pixfmt_list_420);以在某些情况下启用hw解码。

在最近的版本（2017）中，它和附近的几个函数选择了hw编码器https://github.com/FFmpeg/FFmpeg/blob/aff8cf18cb0b1fa4f2e3d163c3da2f25aa6d1906/libavcodec/mpeg12dec.c#L1189。

基本上：应该启用硬件解码器及其api（过时的XVMC，VDPAU，VA API，MS DXVA或MS Direct3D11或videotoolbox）在你的ffmpeg和supported by your hardware及其驱动程序/库的构建中（许多是专有的，应该单独下载）。有时应该为ffmpeg或插件加载-hwaccel选项。在linux中，您可以使用vainfo和vdpauinfo命令来测试最流行的标准视频hw解码API的可用性和支持的配置文件。

输入文件（对于mpeg1 / 2）应该不是灰度，它应该有s->chroma_format少于2（4：2：0 Chroma subsampling，这通常用于ISO / IEC MPEG和ITU- T VCEG H.26x;但不适用于某些MPEG-4第2部分，而不适用于H.264 / MPEG-4 AVC的高4：4：4变体。

static const enum AVPixelFormat mpeg2_hwaccel_pixfmt_list_420[] = {
#if CONFIG_MPEG2_XVMC_HWACCEL
    AV_PIX_FMT_XVMC,
#endif
#if CONFIG_MPEG_VDPAU_DECODER && FF_API_VDPAU
    AV_PIX_FMT_VDPAU_MPEG2,
#endif
#if CONFIG_MPEG2_VDPAU_HWACCEL
    AV_PIX_FMT_VDPAU,
#endif
#if CONFIG_MPEG2_DXVA2_HWACCEL
    AV_PIX_FMT_DXVA2_VLD,
#endif
#if CONFIG_MPEG2_D3D11VA_HWACCEL
    AV_PIX_FMT_D3D11VA_VLD,
#endif
#if CONFIG_MPEG2_VAAPI_HWACCEL
    AV_PIX_FMT_VAAPI,
#endif
#if CONFIG_MPEG2_VIDEOTOOLBOX_HWACCEL
    AV_PIX_FMT_VIDEOTOOLBOX,
#endif
    AV_PIX_FMT_YUV420P,
    AV_PIX_FMT_NONE
};

static const enum AVPixelFormat mpeg12_pixfmt_list_422[] = {
    AV_PIX_FMT_YUV422P,
    AV_PIX_FMT_NONE
};

static const enum AVPixelFormat mpeg12_pixfmt_list_444[] = {
    AV_PIX_FMT_YUV444P,
    AV_PIX_FMT_NONE
};

#if FF_API_VDPAU
static inline int uses_vdpau(AVCodecContext *avctx) {
    return avctx->pix_fmt == AV_PIX_FMT_VDPAU_MPEG1 || avctx->pix_fmt == AV_PIX_FMT_VDPAU_MPEG2;
}
#endif

static enum AVPixelFormat mpeg_get_pixelformat(AVCodecContext *avctx)
{
    Mpeg1Context *s1  = avctx->priv_data;
    MpegEncContext *s = &s1->mpeg_enc_ctx;
    const enum AVPixelFormat *pix_fmts;

    if (CONFIG_GRAY && (avctx->flags & AV_CODEC_FLAG_GRAY))
        return AV_PIX_FMT_GRAY8;

    if (s->chroma_format < 2)
        pix_fmts = avctx->codec_id == AV_CODEC_ID_MPEG1VIDEO ?
                                mpeg1_hwaccel_pixfmt_list_420 :
                                mpeg2_hwaccel_pixfmt_list_420;
    else if (s->chroma_format == 2)
        pix_fmts = mpeg12_pixfmt_list_422;
    else
        pix_fmts = mpeg12_pixfmt_list_444;

    return ff_thread_get_format(avctx, pix_fmts);
}

static void setup_hwaccel_for_pixfmt(AVCodecContext *avctx)
{
    // until then pix_fmt may be changed right after codec init
    if (avctx->hwaccel
#if FF_API_VDPAU
        || uses_vdpau(avctx)
#endif
        )
        if (avctx->idct_algo == FF_IDCT_AUTO)
            avctx->idct_algo = FF_IDCT_SIMPLE;

    if (avctx->hwaccel && avctx->pix_fmt == AV_PIX_FMT_XVMC) {
        Mpeg1Context *s1 = avctx->priv_data;
        MpegEncContext *s = &s1->mpeg_enc_ctx;

        s->pack_pblocks = 1;
#if FF_API_XVMC
FF_DISABLE_DEPRECATION_WARNINGS
        avctx->xvmc_acceleration = 2;
FF_ENABLE_DEPRECATION_WARNINGS
#endif /* FF_API_XVMC */
    }
}

使用C ++中的ffmpeg hwaccel

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