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JS Custom HAS logic


We discuss here how to implement your custom DASH rate adaptation logic in JS.

A custom logic can be defined using a standalone script, or by attaching a JS object to the dashin filter in a JS session.

It is recommended to read the associated documentation.

Standalone script

The standalone mode works by specifying a JS file to the dashin filter using its -algo option:

gpac -i source.mpd --algo=mydash.js ...

The JS script is loaded with a global object called dashin with 4 callback functions:

  • period_reset: indicates start or end of a period (optional)
  • new_group: indicates setup of a new adaptation group (optional), i.e. DASH AdaptationSet or HLS Variant Stream. For HEVC tiling, each tile will be declared as a group, as well as the base tile track
  • rate_adaptation: performs rate adaptation for a group (mandatory). Return value is the new quality index, or -1 to keep as current, -2 to discard (debug, segments won't be fetched/decoded)
  • download_monitor: performs download monitoring, potentially resulting in download abort request (optional)

The following is a basic JS example performing dummy adaptation (always play lowest quality) :

import { Sys as sys } from 'gpaccore'

print("Hello DASH custom algo !");

let groups = [];

dashin.period_reset = function (reset_type)
    print("Period reset type " + reset_type);
    if (!reset_type)
        groups = [];

dashin.new_group = function (group)
    print("New group: " + JSON.stringify(group));
    //remember the group (adaptation set in dash)

dashin.rate_adaptation = function (group_idx, base_group_idx, force_lower_complexity, stats)
    print(`Getting called in custom algo ! group ${group_idx} base_group ${base_group_idx} force_lower_complexity ${force_lower_complexity}`);
    print('Stats: ' + JSON.stringify(stats));

    //always use lowest quality
    //you could check group.SRD to perform spatial adaptation
    return 0;

dashin.download_monitor = function (group_idx, stats)
    print("Download info " + JSON.stringify(stats));
    return -1;

Attaching algo from a JS session

The first step in your JS is to create a DASH object implementing the callbacks previously indicated:

//custom rate adaptation object
let dashalgo = {};
dashalgo.groups = [];

dashalgo.rate_adaptation = function (group, base_group, force_lower_complexity, stats)
    return 0;

dashalgo.new_group = function (group)

dashalgo.period_reset = function (type)
    if (!type)
        this.groups = [];

You will then need to setup a JS session monitoring filter creation process:

session.set_new_filter_fun( (f) => {
        print("new filter " +;

        //bind our custom rate adaptation logic
        if ( == "dashin") {
} ); 

And you're good to try new rate adaptation algorithm !