Using rocPyDecode

Using rocPyDecode#

To learn how to use the rocDecode SDK library and its different utilities, follow these instructions:

1. API overview#

All rocDecode APIs are exposed in the header files rocdecode.h and rocparser.h. You can find these files in the api folder in the rocDecode repository.

The samples use the RocVideoDecoder user class provided in roc_video_dec.h in the utils folder of the rocDecode repository.

A video parser (defined in rocparser.h) is needed to extract and decode headers from the bitstream in order to organize the data into a structured format for the hardware decoder. The parser is critical in video decoding, as it controls the decoding and display of a bitstream’s individual frames and fields.

The parser object in rocparser.h has three main APIs:

  • rocDecCreateVideoParser()

  • rocDecParseVideoData()

  • rocDecDestroyVideoParser()

2. Create a parser object#

The rocDecCreateVideoParser() API creates a video parser object for the codec that you specify. The API takes max_num_decode_surfaces, which determines the Decoded Picture Buffer (DPB) size for decoding. When creating a parser object, the application must register certain callback functions with the driver, which is called from the parser during decode.

  • pfn_sequence_callback is called when the parser encounters a new sequence header. The parser informs you of the minimum number of surfaces needed by the parser’s DPB to successfully decode the bitstream. In addition, the caller can set additional parameters, like max_display_delay, to control frame decoding and display.

  • The pfn_decode_picture callback function is triggered when a picture is set for decoding.

  • The pfn_display_picture callback function is triggered when a frame in display order is ready to be consumed by the caller.

  • The pfn_get_sei_msg callback function is triggered when your Supplementation Enhancement Information (SEI) message is parsed and sent back to the caller.

3. Parse video data#

Elementary stream video packets extracted from the de-multiplexer are fed into the parser using the rocDecParseVideoData() API.

During this call, the parser triggers the callbacks as it encounters a new sequence header, receives compressed frame/field data ready to be decoded, or when it’s ready to display a frame. If any of the callbacks return a failure, it is propagated back to the application so the decoding can be ended gracefully.

4. Query decode capabilities#

The rocDecGetDecoderCaps() API allows you to query the capabilities of the underlying hardware video decoder. Decoder capabilities usually include supported codecs, maximum resolution, and bit-depth.

The following pseudo-code illustrates the use of this API. The application handles the error appropriately for non-supported decoder capabilities.

RocdecDecodeCaps decode_caps;
memset(&decode_caps, 0, sizeof(decode_caps));
decode_caps.codec_type = p_video_format->codec;
decode_caps.chroma_format = p_video_format->chroma_format;
decode_caps.bit_depth_minus_8 = p_video_format->bit_depth_luma_minus8;

ROCDEC_API_CALL(rocDecGetDecoderCaps(&decode_caps));

if(!decode_caps.is_supported) {
    ROCDEC_THROW("Rocdec:: Codec not supported on this GPU: ", ROCDEC_NOT_SUPPORTED);
    return 0;
}

if ((p_video_format->coded_width > decode_caps.max_width) ||
    (p_video_format->coded_height > decode_caps.max_height)) {

    std::ostringstream errorString;
    errorString << std::endl
                << "Resolution          : " << p_video_format->coded_width << "x" << p_video_format->coded_height << std::endl
                << "Max Supported (wxh) : " << decode_caps.max_width << "x" << decode_caps.max_height << std::endl
                << "Resolution not supported on this GPU ";

    const std::string cErr = errorString.str();
    ROCDEC_THROW(cErr, ROCDEC_NOT_SUPPORTED);
    return 0;
}

5. Create a decoder#

rocDecCreateDecoder() creates an instance of the hardware video decoder object and provides you with a handle upon successful creation. Refer to the RocDecoderCreateInfo structure for information about the parameters passed for creating the decoder. For example, RocDecoderCreateInfo::codec_type represents the codec type of the video. The decoder handle returned by rocDecCreateDecoder() must be retained for the entire decode session because the handle is passed along with the other decoding APIs. In addition, you can inform display or crop dimensions along with this API.

6. Decode the frame#

After de-multiplexing and parsing, you can decode bitstream data containing a frame/field using hardware.

Use the rocDecDecodeFrame() API to submit a new frame for hardware decoding. Underneath the driver, the Video Acceleration API (VA-API) is used to submit compressed picture data to the driver. The parser extracts all the necessary information from the bitstream and fills the RocdecPicParams structure that’s appropriate for the codec. The high-level RocVideoDecoder class connects the parser and decoder used for all sample applications.

The rocDecDecodeFrame() call takes the decoder handle and the pointer to the RocdecPicParams structure and initiates the video decoding using VA-API.

7. Query the decoding status#

After submitting a frame for decoding, you can call rocDecGetDecodeStatus() to query the decoding status for a given frame. A structure pointer, RocdecDecodeStatus*, is filled and returned.

The API inputs are:

  • decoder_handle: A RocDecoder handler, rocDecDecoderHandle.

  • pic_idx: An int value for the picIdx for which you want a status in order to index of the picture.

  • decode_status: A pointer to RocdecDecodeStatus as a return value.

The API returns one of the following statuses:

  • Invalid (0): Decode status is not valid.

  • In Progress (1): Decoding is in progress.

  • Success (2): Decoding was successful and no errors were returned.

  • Error (8): The frame was corrupted, but the error was not concealed.

  • Error Concealed (9): The frame was corrupted and the error was concealed.

  • Displaying (10): Decode is complete, display in progress.

8. Prepare the decoded frame for further processing#

The decoded frames can be used for further postprocessing using rocDecGetVideoFrame(). The successful completion of rocDecGetVideoFrame() indicates that the decoding process is complete and the device memory pointer is inter-opped into the ROCm HIP address space in order to further process the decoded frame in device memory. The caller gets the necessary information on the output surface, such as YUV format, dimensions, and pitch from this call. In the high-level RocVideoDecoder class, we provide four different surface type modes for the mapped surface, as specified in OutputSurfaceMemoryType.

typedef enum OutputSurfaceMemoryType_enum {
    OUT_SURFACE_MEM_DEV_INTERNAL = 0,      /**<  Internal interopped decoded surface memory **/
    OUT_SURFACE_MEM_DEV_COPIED = 1,        /**<  decoded output will be copied to a separate device memory **/
    OUT_SURFACE_MEM_HOST_COPIED = 2        /**<  decoded output will be copied to a separate host memory **/
    OUT_SURFACE_MEM_NOT_MAPPED = 3         /**<  decoded output is not available (interop won't be used): useful for decode only performance app*/
} OutputSurfaceMemoryType;

If the mapped surface type is OUT_SURFACE_MEM_DEV_INTERNAL, the direct pointer to the decoded surface is provided. You must call ReleaseFrame() (RocVideoDecoder class). If the requested surface type is OUT_SURFACE_MEM_DEV_COPIED or OUT_SURFACE_MEM_HOST_COPIED, the internal decoded frame is copied to another buffer, either in device memory or host memory. After that, it’s immediately unmapped for re-use by the RocVideoDecoder class.

Refer to the RocVideoDecoder class and samples for details on how to use these APIs.

9. Reconfigure the decoder#

You can call rocDecReconfigureDecoder() to reuse a single decoder for multiple clips or when the video resolution changes during the decode. The API currently supports resolution changes, resize parameter changes, and target area parameter changes for the same codec without destroying an ongoing decoder instance. This can improve performance and reduce overall latency.

The API inputs are:

  • decoder_handle: A RocDecoder handler, rocDecDecoderHandle.

  • reconfig_params: You must specify the parameters for the changes in RocdecReconfigureDecoderInfo. The width and height used for reconfiguration cannot exceed the values set for max_width and max_height, defined in RocDecoderCreateInfo. If you need to change these values, you have to destroy and recreate the session.

Note

You must call rocDecReconfigureDecoder() during RocdecParserParams::pfn_sequence_callback.

10. Destroy the decoder#

You must call the rocDecDestroyDecoder() to destroy the session and free up resources.

The API input is:

  • decoder_handle: A RocDecoder handler, rocDecDecoderHandle.

The API returns a RocdecDecodeStatus value.

11. Destroy the parser#

You must call rocDecDestroyVideoParser() to destroy the parser object and free up all allocated resources at the end of video decoding.