Merge "media: clarify MediaFormat.KEY_FRAME_RATE for video encoders" into tm-dev

    /**

     * A key describing the frame rate of a video format in frames/sec.

     * <p>

     * The associated value is normally an integer when the value is used by the platform,

     * but video codecs also accept float configuration values.

     * Specifically, {@link MediaExtractor#getTrackFormat MediaExtractor} provides an integer

     * value corresponding to the frame rate information of the track if specified and non-zero.

     * Otherwise, this key is not present. {@link MediaCodec#configure MediaCodec} accepts both

     * float and integer values. This represents the desired operating frame rate if the

     * float and integer values.

     * <p>

     * This represents the desired operating frame rate if the

     * {@link #KEY_OPERATING_RATE} is not present and {@link #KEY_PRIORITY} is {@code 0}

     * (realtime). For video encoders this value corresponds to the intended frame rate,

     * although encoders are expected

     * to support variable frame rate based on {@link MediaCodec.BufferInfo#presentationTimeUs

     * buffer timestamp}. This key is not used in the {@code MediaCodec}

     * (realtime). Otherwise, this is just informational.

     * <p>

     * For video encoders this value corresponds to the intended frame rate (the rate at which

     * the application intends to send frames to the encoder, as calculated by the buffer

     * timestamps, and not from the actual real-time rate that the frames are sent to

     * the encoder). Encoders use this hint for rate control, specifically for the initial

     * frames, as encoders are expected to support variable frame rate (for rate control) based

     * on the actual {@link MediaCodec.BufferInfo#presentationTimeUs buffer timestamps} of

     * subsequent frames.

     * <p>

     * This key is not used in the {@code MediaCodec}

     * {@link MediaCodec#getInputFormat input}/{@link MediaCodec#getOutputFormat output} formats,

     * nor by {@link MediaMuxer#addTrack MediaMuxer}.

     */