Understanding the vpx_codec_control Function

This article provides a comprehensive overview of the vpx_codec_control function within the libvpx API, the software video codec SDK for VP8 and VP9. You will learn about its primary purpose, how it functions within an application, its syntax, and common use cases for both video encoding and decoding.

What is vpx_codec_control?

The vpx_codec_control function is a generic, highly flexible interface used to get or set specific configuration parameters and state information for an active codec instance. Because different video codecs (like VP8 and VP9) and different modes (encoding vs. decoding) have unique settings that cannot be easily represented in a single, static configuration structure, libvpx uses this control function to pass custom commands to the codec engine.

Essentially, it acts as a mailbox where developers can send specific control IDs and arguments to fine-tune the behavior of the encoder or decoder in real-time.

Syntax and Parameters

The function signature in the libvpx API is defined as follows:

vpx_codec_err_t vpx_codec_control(vpx_codec_ctx_t *ctx, int id, ...);

• ctx: A pointer to the initialized codec context (vpx_codec_ctx_t). This identifies the specific encoder or decoder instance you want to control.
• id: An integer representing the control ID. These IDs are defined as enums in the libvpx headers (such as vp8cx.h for the encoder and vp8dx.h for the decoder) and usually start with prefixes like VP8E_, VP9E_, VP8D_, or VP9D_.
• ...: A variable argument list. Depending on the control ID used, this will typically be a single value (such as an int) or a pointer to a variable/struct where the codec will write or read data.

Return Values

The function returns a status of type vpx_codec_err_t.

• VPX_CODEC_OK: The control command was processed successfully.
• VPX_CODEC_INVALID_PARAM: An invalid parameter or argument was passed for the specified control ID.
• VPX_CODEC_ERROR: A generic error occurred while processing the control command.

Common Use Cases

vpx_codec_control is used extensively during the lifecycle of a video stream to adjust settings on the fly without having to destroy and reinitialize the codec context.

1. Adjusting Encoder CPU Usage and Speed

One of the most common uses in real-time encoding is setting the CPU usage vs. quality tradeoff.

// Set the CPU usage/speed parameter for VP9 encoder (higher numbers mean faster/lower quality)
vpx_codec_control(&codec, VP9E_SET_CPUUSED, 5);

2. Configuring Real-time Bitrate and Quality

Developers can dynamically adjust the target bitrate or set quantization limits (CQ level) depending on network conditions.

// Adjust the constrained quality level dynamically
vpx_codec_control(&codec, VP8E_SET_CQ_LEVEL, 10);

3. Keyframe Management

You can force the encoder to make the next frame a keyframe (spatial/temporal anchor).

// Force the next frame to be an intra/keyframe
vpx_codec_control(&codec, VP8E_SET_ENABLEAUTOALTREF, 0);

4. Retrieving Decoder Reference Frames

On the decoding side, vpx_codec_control can be used to extract specific reference frame buffers for post-processing or diagnostics.

vpx_ref_frame_t ref;
ref.frame_type = VP8_LAST_FRAME;
// Retrieve the last decoded reference frame
vpx_codec_control(&codec, VP8_GET_REFERENCE, &ref);