das2.xtagPositionInDas2Streams

Purpose: Consider how to handle understanding of the x position in das2streams. Some readers serve the data with the beginning of the bin, and some use the center of the bin.

Audience: das2 Developers (This page will be moved once we have a better wiki).

Introduction

In a weekly meeting we were talking about how the x tags is represented in das2streams. Many of the das2streams tag the center of the bin, and the das2 code assumes this as well as most of the stream processors. However a good number of them tag the beginning of the bin, because this is how das1 streams worked.

das2 renders data (by default) using bilinear interpolation. Four points bracketing a region will be used to fill in the region. This works because the tags are at the center of the accumulation interval for the measurement, and the center best describes this location. Similarly, when a line plot draws a connecting line between to points, the same assumption is made.

The old das1 streams were rendered to the screen using the last point received at any given time, which is consistent with the old model that the tag was the beginning of the interval.

Codes that make the middle assumption

• binAverageSeconds assumes that the point is in the middle of the bin. X values are averaged together to make a new X value in the output.
• spectrogram renderer assumes that interpolation can be done between two points. The cadence is only calculated to detect breaks in the data.
• spectrogram renderer in nearest neighbor mode needs the cadence to figure out the beginning and ends of the bin.

Solutions

• das2streams should allow metadata to indicate the sampling time. For example, there's a xTagWidth width that is often used for the cadence, and we could add additional data like:
  • xTagPosition which would be one of "center" or "begin"
  • xBinRange which can be a DatumRange, for example "-30 to 30 seconds" "0 to 60 seconds" or "-7 to 7 percentIncrease"