The qpSum property of the RTCOutboundRtpStreamStats dictionary is a value generated by adding the Quantization Parameter (QP) values for every frame this sender has produced to date on the video track corresponding to this RTCOutboundRtpStreamStats object.

In general, the higher this number is, the more heavily compressed the video data is.

An unsigned 64-bit integer value which indicates the sum of the quantization parameter (QP) value for every frame sent so far on the track described by the RTCOutboundRtpStreamStats object. Since the value of QP is typically larger to indicate higher compression factors, the larger this sum is, the more heavily compressed the stream generally has been.

Quantization is the process of applying lossy compression to a range of values, resulting in a single quantum value. This value takes the place of the range of values, thereby reducing the number of different values that appear in the overall data set, making the data more compressible. The quantization process and the amount of compression can be controlled using one or more parameters.

It's important to keep in mind that the value of QP can change periodically—even every frame—so it's difficult to know for certain how substantial the compression is. The best you can do is make an estimate. You can use the value of RTCSentRtpStreamStats.framesEncoded to get the number of frames that have been encoded so far, and compute an average from there. See Calculating average quantization below for a function that does this.

Also, the exact meaning of the QP value depends on the codec being used. For example, for the VP8 codec, the QP value can be anywhere from 1 to 127 and is found in the frame header element "y_ac_qi", whose value is defined in RFC 6386, section 19.2. H.264 uses a QP which ranges from 0 to 51; in this case, it's an index used to derive a scaling matrix used during the quantization process. Additionally, QP is not likely to be the only parameter the codec uses to adjust the compression. See the individual codec specifications for details.

The calculateAverageQP() function shown below computes the average QP for the object that contains RTP stream statistics, returning 0 if the object doesn't describe an RTP stream.

function calculateAverageQP(stats) {
  let frameCount = 0;

  switch (stats.type) {
    case "inbound-rtp":
    case "remote-inbound-rtp":
      frameCount = stats.framesDecoded;
      break;
    case "outbound-rtp":
    case "remote-outbound-rtp":
      frameCount = stats.framesEncoded;
      break;
    default:
      return 0;
  }

  return status.qpSum / frameCount;
}