B-Frame Video Encoding

Starting from API version 20, B-frame video encoding is supported.

B-frames, short for Bidirectional Predictive Frames, are one of the primary frame types defined in video encoding standards. The main difference between B-frames and P-frames is that while P-frames support only forward prediction, B-frames support bidirectional prediction.

The prediction process in B-frame encoding utilizes information from both preceding and succeeding frames, which can significantly reduce temporal redundancy in the signal and achieve higher compression efficiency.

Enabling B-frames has the following impacts:

• Enabling B-frames increases the distance between reference frames for P-frame encoding, which may affect the compression ratio and visual quality of P-frames. Generally, in scenarios with complex temporal dynamics, the combined impact of P-frames and B-frames should be considered.
• Enabling B-frames also increases the end-to-end latency for both encoding and decoding.

When to Use

• Video recording
• Editing and exporting
• Video transcoding
• Non-low-latency live streaming

Constraints

• Supported platforms: This feature is platform-dependent. You can query the support using the OH_AVCapability_IsFeatureSupported API.
• Supported API versions: API version 20 and later.
• Supported encoders: This feature is encoder-dependent. You can query the support using the OH_AVCodec_GetCapabilityByCategory API.
• Supported bit rate control modes: VBR, CBR, SQR, and CQ.
• B-frame encoding is not supported alongside temporally scalable video encoding.
• B-frame encoding is not supported alongside long-term reference frames.

Available APIs

B-frame encoding must be configured at the initialization phase and takes effect throughout the process. Dynamic modification is not supported.

The configuration parameters are as follows:

Parameter	Description	Format
OH_MD_KEY_VIDEO_ENCODER_ENABLE_B_FRAME	Enabled status of B-frame video recording. B-frame encoding can be enabled only when it is supported.	int
OH_MD_KEY_VIDEO_ENCODER_MAX_B_FRAMES	Maximum number of B-frames supported by video encoding. It is used only for query.	int

How to Develop

For details about basic encoding functionality, see Video Encoding. The following are specific instructions for B-frame encoding.

1. When creating an encoder instance, check whether the video encoder supports B-frame encoding.

   // 1.1 Obtain the video encoder capability instance. The following uses H.265 as an example.
   OH_AVCapability *cap = OH_AVCodec_GetCapabilityByCategory(OH_AVCODEC_MIMETYPE_VIDEO_HEVC, true, HARDWARE);
   if (cap != nullptr) {
       OH_LOG_INFO("Get codec Capability success!");
       // 1.2 Check whether B-frame encoding is supported.
       bool isSupported = OH_AVCapability_IsFeatureSupported(cap, VIDEO_ENCODER_B_FRAME);
       int32_t supportedMaxBFrameCount = 0;
       if (isSupported) {
           OH_LOG_INFO("Feature VIDEO_ENCODER_B_FRAME is Supported!");
           // 1.3 Query the maximum number of B-frames that can be supported.
           OH_AVFormat *property = OH_AVCapability_GetFeatureProperties(cap, VIDEO_ENCODER_B_FRAME);
           OH_AVFormat_GetIntValue(property, OH_MD_KEY_VIDEO_ENCODER_MAX_B_FRAMES, &supportedMaxBFrameCount);
           OH_LOG_INFO("Get supportedMaxBFrameCount,value is %{public}d!", supportedMaxBFrameCount);
           // 1.4 Destroy the temporary AVFormat.
           OH_AVFormat_Destroy(property);
       } else {
           OH_LOG_ERROR("Feature VIDEO_ENCODER_B_FRAME is not Supported!");
       }
   } else {
       OH_LOG_ERROR("codec Capability is null!");
   }
   

   If B-frame encoding is supported, you can enable it.

2. Configure B-frame encoding during the configuration phase.

   // 2.1 Create a temporary AVFormat used for configuration.
   OH_AVFormat *format = OH_AVFormat_Create();
   // 2.2 Fill in the key-value pair of the parameter used to enable the feature.
   OH_AVFormat_SetIntValue(format, OH_MD_KEY_VIDEO_ENCODER_ENABLE_B_FRAME, 1);
   // 2.3 Set parameters.
   int32_t ret = OH_VideoEncoder_Configure(videoEnc, format);
   if (ret != AV_ERR_OK) {
       // Handle exceptions.
   }
   // 2.4 Destroy the temporary AVFormat after the configuration is complete.
   OH_AVFormat_Destroy(format);
   

   NOTE

   • Enabling B-frame encoding is weakly checked during the configure phase. If the parameter configuration is incorrect or the encoder does not support the feature, AV_ERR_OK is returned, with a warning message printed. Check whether the feature is truly enabled based on the encoded stream and runtime logs.
   • When B-frame encoding is enabled, the actual B-frame structure and number of B-frames in the encoded stream depend on the platform's capabilities.