c++ - 我应该释放内部分配的 MFT 输出缓冲区的返回 IMFSample 吗？

Question

媒体基础转换对象 (MFT) 可以实现输出缓冲区分配模型，其中缓冲区由 MFT 对象在内部分配。

如果是这种情况，内部分配的缓冲区将通过传递给 IMFTransform::ProcessOutput( ) 方法。

来自MFT_OUTPUT_DATA_BUFFER structure文档:

typedef struct _MFT_OUTPUT_DATA_BUFFER {
  DWORD         dwStreamID;
  IMFSample     *pSample;
  DWORD         dwStatus;
  IMFCollection *pEvents;
} MFT_OUTPUT_DATA_BUFFER;

pSample

Pointer to the IMFSample interface. Before calling ProcessOutput, set this member equal to a valid IMFSample pointer or NULL. See Remarks for more information.

来自IMFTransform::ProcessOutput文档:

输出缓冲区

The MFT returns output data for a stream through the pSample member of the MFT_OUTPUT_DATA_BUFFER structure. This structure member is a pointer to the IMFSample interface of a media sample. (See Media Samples.) The media sample is allocated either by the caller or by the MFT, depending on the MFT's allocation model. To find the allocation model, call IMFTransform::GetOutputStreamInfo and examine the dwFlags member of the MFT_OUTPUT_STREAM_INFO structure

...

If pSample is NULL and dwFlags does not contain the MFT_PROCESS_OUTPUT_DISCARD_WHEN_NO_BUFFER, the MFT provides a sample for the output data. The MFT sets pSample to point to the sample that it provides. The MFT can either allocate a new sample or re-use an input sample.

文档没有提到在这种情况下返回的 IMFSample 接口(interface)是否应该被释放。 似乎不是这样，因为文档非常明确任何通过同一结构返回的事件应由调用者释放。

来自MFT_OUTPUT_DATA_BUFFER structure文档:

pEvents

Before calling ProcessOutput, set this member to NULL. On output, the MFT might set this member to a valid IMFCollection interface pointer. The pointer represents a collecton that contains zero or more events. To get each event, call IMFCollection::GetElement and query the returned IUnknown pointer for the IMFMediaEvent interface. When the ProcessOutput method returns, the caller is responsible for releasing the IMFCollection pointer if the pointer is not NULL.

有人可以确认返回的 IMFSample 接口(interface)是否应该发布吗？

我认为如果我们不应该释放返回的接口(interface)，它应该被明确地记录下来，因为它违背了在我们使用完接口(interface)后释放接口(interface)的既定 COM 方式。

Answer 1

如果 MFT 使用非 NULL 值初始化指针(与调用者分配的缓冲区相反——在这种情况下，MFT 使用它但不需要 AddRef结构)。

下面的代码片段适用于所有三种模型:

• If the MFT_OUTPUT_STREAM_PROVIDES_SAMPLES flag is present, the MFT allocates the media sample.
• If the MFT_OUTPUT_STREAM_CAN_PROVIDE_SAMPLES flag is present, the caller can optionally provide a media sample. If pSample is NULL, the MFT will allocate the media sample.
• If neither of these two flags is present, the caller must allocate the media sample.

请注意，文档中没有提到调用方提供示例指针，而 MFT 将其替换为自己的指针的场景。

(虽然代码并不完美，只是引用计数的说明；如果有先验信息表明样本是调用者的，则无需执行附加/分离操作，当然)

CComPtr<IMFSample> pSample;
// pSample is NULL or not
MFT_OUTPUT_DATA_BUFFER Buffer;
Buffer.pSample = pSample.Detach();
// ...
const HRESULT nResult = pTransform->ProcessOutput(..., &Buffer, ...);
pSample.Attach(Buffer.pSample);
// pSample holds a valid properly ref'fed pointer
// No need to use Buffer.pSample below