Bug 5953: Plotting: Patterned lines sometimes look solid [1]
Bug Number: 5953
Release notes (y/n): Yes
For Release Nums: 7.1

See document and screenshots:
http://cadswes2.colorado.edu/~philw/pub/5953/DensePattCurveFix.html
http://cadswes2.colorado.edu/~philw/pub/5953/DenseCurve2-Prob.png
http://cadswes2.colorado.edu/~philw/pub/5953/DenseCurve2-Fix.png
http://cadswes2.colorado.edu/~philw/pub/5953/FixDisabledDiag.png

Initial analysis:
http://cadswes2.colorado.edu/~philw/pub/2017/Qwt/DenseProb.html

RiverWare plot curves with a good number of data points (e.g. 500+) and a 
dotted or dashed line style sometimes appear as solid lines. This can occur 
with a thin line, but is more prevalent with a line width (thickness) of 
two or more pixels. The prevalence of this problem is also dependent on the 
curve style -- less so with 'Linear Interpolation' -- more with Spline, 
Points, or Step.

The fix involves a point filter (the Qwt "Weed Curve Fitter") which has the 
possibility of negatively impacting plot performance. We haven't actually 
seen a problem with the application of this point filter with long series 
(tested with 30,000-timestep series slots), but in case this does turn out 
to create a problem for particular users, we have provided a way to disable 
this filter through the use of a special environment variable. The filter is 
disabled if the following environment variable is defined and has a non-zero 
value (e.g. "1"):

  RIVERWARE_PLOT_DISABLE_WEED_FITTER

If the filter is disabled (using this environment variable), the following 
message is written to RiverWare diagnostics when the first non-empty plot 
is shown:

  The fix for dense dotted or dashed plot curves looking solid has been 
  disabled with the "RIVERWARE_PLOT_DISABLE_WEED_FITTER" environment 
  variable (non-zero value).

===============
Technical Notes
===============

The fix is effectively accomplished by removing points from the plotted 
curve which redundantly fall within a pixel or two of adjacent points -- 
at the current zoom level of the plot. (This doesn't remove point-symbols, 
if those are also shown for a curve). This is accomplished with the 
"QwtWeedingCurveFitter" class (part of the Qwt version 6.1.3 library used 
to implement RiverWare plots).

We have been warned that this filter could cause performance problems with 
long curves (having many points) if it is used in the canonical way 
(installed on the curve to be automatically applied to pixel coordinates 
each time the curve is drawn). The recommended alternative of applying it 
to application-unit points before handing the point series over to Qwt isn't 
really mathematically valid AND wouldn't reliably address the problem anyway.
In our own testing with long series (e.g. 30,000 timesteps) we haven't seen 
any additional slowness, beyond what is experienced with such large curves 
in general.

See the discussion in these thread on the QtCentre/Qwt forum:

  Dense Qwt curves with dotted or dashed line style generally look solid.
  http://www.qtcentre.org/threads/68094 [4-5-2017].

  QwtCurveFitter::fitCurve called only for "Lines" style QwtPlotCurves 
    -- not "Steps".
  http://www.qtcentre.org/threads/68109 [4-11-2017]

Each Qwt plot curve can be assigned a single "curve fitter" instance, of 
which there are two types: (a) QwtSplineCurveFitter, and (b) 
QwtWeedingCurveFitter. RiverWare had already been making use of the Spline 
Fitter -- this is how we provide the "Spline" curve style (as "Spline" is 
not a native Qwt curve style, as such). We had to sublcass QwtSplineCurveFitter
in order to apply both fitters for spline curves. For this fix, we also 
subclassed QwtWeedingCurveFitter so that we could control the application of 
the actual fitter computation with RiverWare application-level provisions. 

See these two classes in the Q3GUI/SlotCurve module, and this Qwt API 
reference webpage:

  class SlotCurve::SplineFitter : public QwtSplineCurveFitter;
  class SlotCurve::WeedFitter : public QwtWeedingCurveFitter;
  http://qwt.sourceforge.net/class_qwt_curve_fitter.html

Unfortunately, Qwt doesn't make use of its Curve Fitters for curves having 
the "Step" curve style. So, we needed to create our own "stepped" line data 
from slot data, and present that to Qwt with the "linear" (-interpolation) 
curve style. See this SlotCurve method:

  void SlotCurve::precomputeStepPoints (bool applyToo /*=false*/);

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