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Audience: Developers and science teams

Purpose: Define Rich PNG standard


  1. Introduction
  2. Standard
  3. Examples
  4. Example Applications
  5. Unresolved
  6. To be done

1. Introduction

Png image files have the interesting feature that additional metadata can be buried inside them. For example, we identify images produced by Autoplot as having Software=Autoplot. We also embed metadata about the plot and axis information, so that future programs can better utilize the images. For example, the unix grep command can be used to look for strings within axis labels, and the thin client uses this metadata to interpret mouse gestures. We call this richPng, and the hope is that other image producers and consumers might adopt this standard as well.

2. Standard

Each png file block has a type, and one with type text (tEXt) is added with the keyword "plotInfo" and is equal to a JSON block with the following scheme:

{ 'size':[640,480], 'numberOfPlots':2, 'plots': [...] }

The plots node is an array of:

{ 'title':'the title', 'xaxis': { ... }, 'yaxis': { ... } }

with the yaxis, for example:

{ 'label':'B-field', 'min':0.1, 'max':100, 'type':'log', 'units':'nT', 'top':78, 'bottom':644 }

and the xaxis, for example:

{ 'label':'delay', 'min':0, 'max':10, 'type':'lin', 'units':'nT', 'left':78, 'right':644 }

You can also have UTC times encoded in ISO8601:

{ 'label':'Epoch', 'min':'2013-01-01T00:00Z', 'max':'2013-01-02T00:00Z', 'type':'lin', 'units':'UTC', 'left':78, 'right':644 }

3. Examples

For the image here (click to download),

there's the JSON block "plotInfo" within:

{ "size":[722,639],
  "plots": [
       "title":"AC/MFI  [PRELIMINARY VALUES - BROWSE USE ONLY] B-field magnitude", 
       "xaxis": { "label":"", "min":"2014-01-02T00:00:00.000Z", "max":"2014-01-03T00:00:00.000Z", "left":78, "right":644, "type":"lin", "units":"UTC" },
       "yaxis": { "label":"[PRELIM] <|B|> (nT)", "min":4.440892098500626E-16, "max":8.9, "top":52, "bottom":587, "type":"lin", "units":"nT" }


 unix> wget -N
 2015-12-10 05:53:29 (583 KB/s) - ‘product_20140102.png’ saved [34667/34667]
 unix> strings product_20140102.png | grep "AC/MFI" 
     "title":"AC/MFI  [PRELIMINARY VALUES - BROWSE USE ONLY] B-field magnitude", 

4. Example Applications

  • Autoplot Thin Client
  • grep for image labels
  • verify axis settings for a large set of png files.
  • Autoplot PNGWalk Tool allows digitizing
  • Autoplot ImageDataSource allows plots to be grabbed from images, rendering with correct axes to allow for overplots.

5. Unresolved

What happens with this is vaguely defined:

  • Inverted axes
  • colorbar labels
  • timerange node. timeranges are so ubiquitous in our field that it would be useful to allow the plot timerange to be easily found at the root JSON node.

6. To be done

  • make insert Rich PNG Metadata routines for Java, IDL, and Matlab
  • Figure out how to insert metadata into PDF as well.
  • define convention for decorating existing images, for example have ".richpng" file alongside the original.
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