Series from Reactive Scripts in PlotJuggler

Understanding the problem they solve

To use an analogy from databases, we may say that PlotJuggler works with column-oriented series.

Given a message called "position" with theses fields:

The following timeseries will be created when multiple messages are received at time t:

Field Value
X ...
Y ...
Z ...
position/x t1; x1 t2; x2 t3; x3 ...
position/y t1; y1 t2; y2 t3; y3 ...
position/x t1; x1 t2; x2 t3; x3 ...
position/z t1; z1 t2; z2 t3; z3 ...

But what if a single message contain an array of values?

Considered this hypothetical "Trajectory2D" message, that contains an array of X/Y positions.

 

In this case, we may talk about row-oriented series.

 

Any single message should create an entire ScatterXY series, using:

  • pos/x on the horizontal axis and
  • pos/y on the vertical one. 
Field Value
node.0/pos/x ...
node.0/pos/y ...
node.1/pos/x ...
node.1/pos/y ...
node.2/pos/x ...
node.2/pos/y ...
node.3/pos/x ...
node.3/pos/y ...
... ....

Why is this more difficult for PlotJuggler?

  • PlotJuggler can not make assumptions about the way the X/Y pairs should be extracted. Any message might be packed in a different way.

 

  • What should we do when a new message is received or when the time-tracker is moved? Overwrite the series or append to it?

 

  • What if you want to apply a specific operation such as "capture only a range" or "downsample and take one every N points"?

Solution: the Reactive Scripts Editor

"Reactive scrips" are Lua script which are executed every time:

  • New data is received.
  • The time-tracker slider is moved.

 

Inside these scripts, you can:

  1. Access any timeseries stored in memory.
  2. Create or update existing series.
  3. Reuse helper functions written by you earlier, to make your code more terse and readable.

How to access existing timeseries

-- Get the handle to access a timeseries called "trajectory/node.0/position/x"
series_x = TimeseriesView.find( "trajectory/node.0/position/x" )

-- Very often, you may want to do something like this
node_num = 0
str = string.format("trajectory/node.%d/position/x", node_num)
series_x = TimeseriesView.find( str )

-- If the series doesn't exist, nil is retuned
if series_x == nil then
  print("nope")
end

-- Get the number of elements
n = series_x:size()

-- Access time/value pairs at index i, where the first index is 0
t, x = series_x:at(i)

-- More often, it is useful to find a value by timestamp, instead of index
x = series:atTime(t)

How to create a new ScatterXY series

-- Create a new series or clear its points, if the series exists already
new_series = ScatterXY.new("trajectory_XY")

-- Add data
new_series:push_back(x, y)

-- Clear previous data
new_series:clear()

-- Count number of elements
n = new_series:size()

-- Access x/y  value pairs at index i, where the first index is 0
x, y = new_series:at(i)

Time to write your first script :)

Time to write your first script :)

--- Create a new series or overwite the previous one.
--- Do this once, outside the function
new_series = ScatterXY.new("trajectory_XY")
  
function( tracker_time )
  index = 0
  while(true) do

    str = string.format("trajectory/node.%d/pos/x", index) -- X
    series_x = TimeseriesView.find( str )
    str = string.format("trajectory/node.%d/pos/y", index) -- Y
    series_y = TimeseriesView.find( str )
    
    -- stop the loop if any of those series can't be found
    if series_x == nil or series_y == nil then break end	 
    
    --- Append points to new_series
    x = series_x:atTime(tracker_time)
    y = series_y:atTime(tracker_time)
    new_series:push_back(x,y)

    index = index + 1
  end -- end loop
end

To save you time, PlotJuggler allows you to create a "library" of reusable helper functions, pre-loaded by all your scripts

PlotJuggler 04: reactive scripts

By Davide Faconti

PlotJuggler 04: reactive scripts

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