RTCRtpStreamStats: qpSum property
=================================

 The **`qpSum`** property of the
 `RTCRtpStreamStats` dictionary is a value generated by adding the
 **Quantization Parameter** (**QP**) values for every frame
 sent or received to date on the video track corresponding to this
 `RTCRtpStreamStats` object.

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

Value
-----

 An unsigned 64-bit integer value which indicates the sum of the quantization parameter
 (QP) value for every frame sent or received so far on the track described by the
 `RTCRtpStreamStats` 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.

**Note:** This value is only available for video media.

Usage notes
-----------

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, for example, use the value of
 `RTCReceivedRtpStreamStats.framesDecoded` if receiving the media or
 `RTCSentRtpStreamStats.framesEncoded` if sending it to get the number of
 frames handled 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.

Examples
--------

### Calculating average quantization

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


```js
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;
}
```

Specifications
--------------

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