[vtk-developers] [GSoC 2014] Supporting a Visualization Grammar

[Marco Cecchetti]

Hi all,
after analyzing source code of several classes, such as vtkPlot and 
vtkChart, I succeeded in writing down a list of deliverables and a 
tentative time-line for tackling the implementation of a framework that 
supports a good subset of Vega and enhances charting capabilities in 
VTK. Any hint or critique is welcome.

Deliverables:

(1) A Vega grammar reader. A Vega grammar file is, indeed, in JSON 
format, so we could analyze the grammar starting by the tree 
representation of the parsed file. The actual parsing will be performed 
by exploiting the jsoncpp library.

(2) A Vega's Visualization would be mapped to a class vtkVegaChart 
similar to vtkChart, but with the ability to manage input data sets 
directly and sharing them between all plots added to the chart. All 
Vega's Visualization properties will be supported.

(3) All Vega's Data properties will be supported but only the JSON 
format for input data file sources. Usually Vega's data sets are 
represented by an array of objects. Each object has the same number and 
type of properties. That means we are able to transform a Vega's data 
set in an object of arrays: [{"x": 1, "y": 0}, {"x": 3, "y": 2}] -> 
{"x": [1, 3], "y": [0, 2]}. Hence, we can map Vega's data sets to 
vtkTable, where each column of the table represents a Vega's data set 
property.

(4) The following Vega's Data Manipulation Transforms will be supported: 
cross, facet, filter, formula, slice, sort, stats, unique. Vega's 
transforms will be mapped to new vtk data filters.

(5) The following Vega's Visual Encoding Transforms will be supported: 
pie, stack, link.

(6) The following Vega's Scale Types will be supported: ordinal, linear, 
log, sqrt, time. A vtkScale class would be implemented for managing 
scaling, for each scale type there will be a specific subclass of 
vtkScale. Part of this work is simplified by the fact that the 
implementation of a part of such scale types is already embedded in the 
vtkAxis class. The vtkScale class and its subclasses will provide 
support for more scale types and the ability to share the scaling 
business logic beetween several axis objects.

(7) The following Vega's Axes properties will be supported: type, scale, 
orient, title, titleOffset, values, ticks, tickPadding, tickSize, 
offset, grids. Most of these properties are already implemented by the 
vtkAxis class, the main goal here will be to split the presentation 
feautures from the computational part. Presentation will be supported by 
a vtkVegaAxis class that would demand computational task to a vtkScale 
object, that could be shared with other vtkVegaAxis objects.

(8) Vega's mark support:
      -- Top-Level Mark Properties: type, name, from, properties(enter, 
update, exit, hover), key.
      -- Value References: value, field, group (limited to "width" and 
"height"), scale, offset, band.
      -- Shared Visual Properties: x, x2, width, y, y2, height, opacity, 
fill, fillOpacity, stroke, strokeWidth, strokeOpacity.
      -- Specific Marks: rect, symbol, arc, area, line, image, text, group.

     Stroke and fill properties will be supported respectively by the 
already implemented vtkPen and vtkBrush.
     The following mapping from Vega's marks to vtkPlots is possible (in 
some case a simplified or an enhanced version could be needed):
     - rect     --> vtkPlotBar
     - symbol   --> vtkPlotPoints
     - arc      --> vtkPlotPie
     - line     --> vtkPlotLine (actually it can handle only polygonal 
lines)
     - area     --> vtkPlotArea (a new class is needed, but it will be 
almost the same than vtkPlotLine)
     Image and text marks can be implemented in a similar way of 
vtkPlotPoints, but with images or labels instead of symbols.
     Finally grouping layout would be handled by vtkVegaChart.

(9) Source code will be commented in order to be able to generate 
doxygen documentation.

(10) Test/sample programs of the supported Vega's mark will be provided.



Time-line

Week 1, 2
   Task 1 - Start implementation of a Vega grammar reader: Visualization 
properties, Top-Level Mark Properties, Value References by value and by 
field, Shared Visual Properties: x, x2, width, y, y2, height; rect mark 
type.

Week 3
   Task 2 - Support for "facet" and "stats" data transforms.
   Task 3 - Support for area and line marks.

Week 4, 5
   Task 4 - Implementation of vtkScale class with support for Scale 
types: linear, ordinal, log and sqrt.
   Task 5 - Implementation of vtkVegaAxis class with support for Axes 
properties: type, scale, orient, title, titleOffset, values, ticks, 
tickPadding, tickSize, offset, grids. vtkVegaAxis - vtkScale integration.

Week 6, 7
   Buffer - Spread from the GSoC start to the mid-term period

Week 8
   Task 6 - Support for more value reference types: scale, offset, band.
   Task 7 - Support for "stack" transform.

Week 9
   Task 8 - Support for "pie" transforms and "arc" marks.
   Task 9 - Support for "image" and "text" marks.

Week 10, 11
   Task 9 - Support for "filter", "slice", "sort" and "unique" data 
transforms.
   Task 10 - Support for "cross" and "formula" data transforms.
   Task 11 - Support for scale of type "date".

Week 12, 13
   Buffer - Spread from the mid-term period to the GSoC end.



Kind Regards,
-- Marco