如何在Directshow中使用硬解码功能

    在Directshow中使用硬解码一般通过DXVA技术，这是微软针对Windows系统定义的硬件加速解码的一个标准接口，目前很多显卡厂商已经实现了。通过DXVA接口，我们以一种统一的接口形式访问各种显卡的硬件加速（解码）功能，还可以通过API接口设置参数来控制解码的一些流程。但这篇博文不是给大家讲怎么用DXVA API来解码视频，而是介绍另外一种简单的方法：通过LAV Video Decoder。这是一个Directshow解码器插件，是开源的LAV Filters里面的一个子插件。

   LAVVideo即支持软解码，也支持硬解码（封装了DXVA接口），并且Filter提供接口给外部设置解码器的一些参数。下载LAV Filters的二进制文件包，里面有个developer_info目录，进去找到LAVVideoSettings.h，这个头文件声明了LAVVideo Decode Filter实现的接口。这个类的声明如下：

// LAV Video configuration interface
[uuid("FA40D6E9-4D38-4761-ADD2-71A9EC5FD32F")]
interface ILAVVideoSettings : public IUnknown
{
  // Switch to Runtime Config mode. This will reset all settings to default, and no changes to the settings will be saved
  // You can use this to programmatically configure LAV Video without interfering with the users settings in the registry.
  // Subsequent calls to this function will reset all settings back to defaults, even if the mode does not change.
  //
  // Note that calling this function during playback is not supported and may exhibit undocumented behaviour. 
  // For smooth operations, it must be called before LAV Video is connected to other filters.
  STDMETHOD(SetRuntimeConfig)(BOOL bRuntimeConfig) = 0;

  // Configure which codecs are enabled
  // If vCodec is invalid (possibly a version difference), Get will return FALSE, and Set E_FAIL.
  STDMETHOD_(BOOL,GetFormatConfiguration)(LAVVideoCodec vCodec) = 0;
  STDMETHOD(SetFormatConfiguration)(LAVVideoCodec vCodec, BOOL bEnabled) = 0;

  // Set the number of threads to use for Multi-Threaded decoding (where available)
  //  0 = Auto Detect (based on number of CPU cores)
  //  1 = 1 Thread -- No Multi-Threading
  // >1 = Multi-Threading with the specified number of threads
  STDMETHOD(SetNumThreads)(DWORD dwNum) = 0;

  // Get the number of threads to use for Multi-Threaded decoding (where available)
  //  0 = Auto Detect (based on number of CPU cores)
  //  1 = 1 Thread -- No Multi-Threading
  // >1 = Multi-Threading with the specified number of threads
  STDMETHOD_(DWORD,GetNumThreads)() = 0;

  // Set whether the aspect ratio encoded in the stream should be forwarded to the renderer,
  // or the aspect ratio specified by the source filter should be kept.
  // 0 = AR from the source filter
  // 1 = AR from the Stream
  // 2 = AR from stream if source is not reliable
  STDMETHOD(SetStreamAR)(DWORD bStreamAR) = 0;

  // Get whether the aspect ratio encoded in the stream should be forwarded to the renderer,
  // or the aspect ratio specified by the source filter should be kept.
  // 0 = AR from the source filter
  // 1 = AR from the Stream
  // 2 = AR from stream if source is not reliable
  STDMETHOD_(DWORD,GetStreamAR)() = 0;

  // Configure which pixel formats are enabled for output
  // If pixFmt is invalid, Get will return FALSE and Set E_FAIL
  STDMETHOD_(BOOL,GetPixelFormat)(LAVOutPixFmts pixFmt) = 0;
  STDMETHOD(SetPixelFormat)(LAVOutPixFmts pixFmt, BOOL bEnabled) = 0;

  // Set the RGB output range for the YUV->RGB conversion
  // 0 = Auto (same as input), 1 = Limited (16-235), 2 = Full (0-255)
  STDMETHOD(SetRGBOutputRange)(DWORD dwRange) = 0;

  // Get the RGB output range for the YUV->RGB conversion
  // 0 = Auto (same as input), 1 = Limited (16-235), 2 = Full (0-255)
  STDMETHOD_(DWORD,GetRGBOutputRange)() = 0;

  // Set the deinterlacing field order of the hardware decoder
  STDMETHOD(SetDeintFieldOrder)(LAVDeintFieldOrder fieldOrder) = 0;

  // get the deinterlacing field order of the hardware decoder
  STDMETHOD_(LAVDeintFieldOrder, GetDeintFieldOrder)() = 0;

  // DEPRECATED, use SetDeinterlacingMode
  STDMETHOD(SetDeintAggressive)(BOOL bAggressive) = 0;

  // DEPRECATED, use GetDeinterlacingMode
  STDMETHOD_(BOOL, GetDeintAggressive)() = 0;

  // DEPRECATED, use SetDeinterlacingMode
  STDMETHOD(SetDeintForce)(BOOL bForce) = 0;

  // DEPRECATED, use GetDeinterlacingMode
  STDMETHOD_(BOOL, GetDeintForce)() = 0;

  // Check if the specified HWAccel is supported
  // Note: This will usually only check the availability of the required libraries (ie. for NVIDIA if a recent enough NVIDIA driver is installed)
  // and not check actual hardware support
  // Returns: 0 = Unsupported, 1 = Supported, 2 = Currently running
  STDMETHOD_(DWORD,CheckHWAccelSupport)(LAVHWAccel hwAccel) = 0;

  // Set which HW Accel method is used
  // See LAVHWAccel for options.
  STDMETHOD(SetHWAccel)(LAVHWAccel hwAccel) = 0;

  // Get which HW Accel method is active
  STDMETHOD_(LAVHWAccel, GetHWAccel)() = 0;

  // Set which codecs should use HW Acceleration
  STDMETHOD(SetHWAccelCodec)(LAVVideoHWCodec hwAccelCodec, BOOL bEnabled) = 0;

  // Get which codecs should use HW Acceleration
  STDMETHOD_(BOOL, GetHWAccelCodec)(LAVVideoHWCodec hwAccelCodec) = 0;

  // Set the deinterlacing mode used by the hardware decoder
  STDMETHOD(SetHWAccelDeintMode)(LAVHWDeintModes deintMode) = 0;

  // Get the deinterlacing mode used by the hardware decoder
  STDMETHOD_(LAVHWDeintModes, GetHWAccelDeintMode)() = 0;

  // Set the deinterlacing output for the hardware decoder
  STDMETHOD(SetHWAccelDeintOutput)(LAVDeintOutput deintOutput) = 0;

  // Get the deinterlacing output for the hardware decoder
  STDMETHOD_(LAVDeintOutput, GetHWAccelDeintOutput)() = 0;

  // Set whether the hardware decoder should force high-quality deinterlacing
  // Note: this option is not supported on all decoder implementations and/or all operating systems
  STDMETHOD(SetHWAccelDeintHQ)(BOOL bHQ) = 0;

  // Get whether the hardware decoder should force high-quality deinterlacing
  // Note: this option is not supported on all decoder implementations and/or all operating systems
  STDMETHOD_(BOOL, GetHWAccelDeintHQ)() = 0;

  // Set the software deinterlacing mode used
  STDMETHOD(SetSWDeintMode)(LAVSWDeintModes deintMode) = 0;

  // Get the software deinterlacing mode used
  STDMETHOD_(LAVSWDeintModes, GetSWDeintMode)() = 0;

  // Set the software deinterlacing output
  STDMETHOD(SetSWDeintOutput)(LAVDeintOutput deintOutput) = 0;

  // Get the software deinterlacing output
  STDMETHOD_(LAVDeintOutput, GetSWDeintOutput)() = 0;

  // DEPRECATED, use SetDeinterlacingMode
  STDMETHOD(SetDeintTreatAsProgressive)(BOOL bEnabled) = 0;

  // DEPRECATED, use GetDeinterlacingMode
  STDMETHOD_(BOOL, GetDeintTreatAsProgressive)() = 0;

  // Set the dithering mode used
  STDMETHOD(SetDitherMode)(LAVDitherMode ditherMode) = 0;

  // Get the dithering mode used
  STDMETHOD_(LAVDitherMode, GetDitherMode)() = 0;

  // Set if the MS WMV9 DMO Decoder should be used for VC-1/WMV3
  STDMETHOD(SetUseMSWMV9Decoder)(BOOL bEnabled) = 0;

  // Get if the MS WMV9 DMO Decoder should be used for VC-1/WMV3
  STDMETHOD_(BOOL, GetUseMSWMV9Decoder)() = 0;

  // Set if DVD Video support is enabled
  STDMETHOD(SetDVDVideoSupport)(BOOL bEnabled) = 0;

  // Get if DVD Video support is enabled
  STDMETHOD_(BOOL,GetDVDVideoSupport)() = 0;

  // Set the HW Accel Resolution Flags
  // flags: bitmask of LAVHWResFlag flags
  STDMETHOD(SetHWAccelResolutionFlags)(DWORD dwResFlags) = 0;

  // Get the HW Accel Resolution Flags
  // flags: bitmask of LAVHWResFlag flags
  STDMETHOD_(DWORD, GetHWAccelResolutionFlags)() = 0;

  // Toggle Tray Icon
  STDMETHOD(SetTrayIcon)(BOOL bEnabled) = 0;

  // Get Tray Icon
  STDMETHOD_(BOOL,GetTrayIcon)() = 0;

  // Set the Deint Mode
  STDMETHOD(SetDeinterlacingMode)(LAVDeintMode deintMode) = 0;

  // Get the Deint Mode
  STDMETHOD_(LAVDeintMode,GetDeinterlacingMode)() = 0;
};

我们主要关注SetHWAccel方法：  STDMETHOD(SetHWAccel)(LAVHWAccel hwAccel) = 0; 传入参数的类型LAVHWAccel的定义如下：

// Type of hardware accelerations
typedef enum LAVHWAccel {
  HWAccel_None,
  HWAccel_CUDA,
  HWAccel_QuickSync,
  HWAccel_DXVA2,
  HWAccel_DXVA2CopyBack = HWAccel_DXVA2,
  HWAccel_DXVA2Native

};

我们一般选HWAccel_DXVA2，通过这种方式解码后的数据放在显存，直接显示效率很高。

后面的工作就简单了。构建一个播放文件的Filter Graph，将LAVVideo Decoder与Video Renderer连接，记住Renderer必须是EVR，否则硬解将不能够生效或者解码时的CPU比较高。运行Filter Graph，然后打开任务管理器查看CPU，会发现播放所占用的CPU很低（在我的酷睿i3 CPU的机器上小于5%）。