Add support for the 'reference' attribute on the vector gm

The 'reference' attribute has been available to read/write on
the vector graphics method, but until now has not been implemented.
Now, if it is set, the default behavior of choosing a sensible value
for the arrow length based on the screen space availabe, text size,
etc., will be overridden.  Instead the arrow length will simply be
chosen to match the reference attribute.  Hence, this feature should
be used with care, as it could result in a vector legend with either
very large or very small reference arrow.

tests/test_vcs_vectors_scale.py

@@ -6,7 +6,7 @@
 
 
 class TestVCSVectorScale(basevcstest.VCSBaseTest):
-    def coreTest(self, scalingType, scale=None, scalerange=None):
+    def coreTest(self, scalingType, scale=None, scalerange=None, ref=None):
         u = cdms2.createAxis(np.array([[1, 1, 0, 1, 1, 1, 0, 1],
                                        [1, 1, 0, 1, 1, 1, 0, 1],
                                        [1, 1, 0, 1, 1, 1, 0, 1],
@@ -32,11 +32,15 @@ def coreTest(self, scalingType, scale=None, scalerange=None):
             gv.scalerange = scalerange
         if scale is not None:
             gv.scale = scale
+        if ref is not None:
+            gv.reference = ref
         self.x.plot(u, v, gv, bg=self.bg, ratio=1)
         label = scalingType
         if (scalingType == "constant"):
             label += "_" + str(scale)
         outFilename = 'test_vcs_vectors_scale_%s.png' % label
+        if ref is not None:
+            outFilename = 'test_vcs_vectors_scale_%s_ref_%s.png' % (label, ref)
         self.checkImage(outFilename)
         self.x.clear()
 
@@ -50,4 +54,8 @@ def testVCSVectorScalingOptions(self):
         # test clamping
         self.coreTest("constant", scale=0.1)
         self.coreTest("constant", scale=2)
+        # test reference
+        self.coreTest("constant", scale=0.5, ref=1)
+        self.coreTest("constant", scale=0.1, ref=5)
+        self.coreTest("constant", scale=2, ref=0.5)
     testVCSVectorScalingOptions.vectors = 1

vcs/utils.py

@@ -2838,7 +2838,8 @@ def _createLegendString(value, unit):
 def drawVectorLegend(canvas, templateLegend,
                      linecolor, linetype, linewidth,
                      unitString, maxNormInVp=1., maxNorm=1.,
-                     minNormInVp=0., minNorm=0., bg=False, render=True):
+                     minNormInVp=0., minNorm=0., bg=False, render=True,
+                     reference=1e20):
     """Draws a legend with vector line/text inside a template legend box
     Auto adjust text size to make it fit inside the box
 
@@ -2895,14 +2896,29 @@ def drawVectorLegend(canvas, templateLegend,
     :param render: Boolean value indicating whether or not to render the new
         lines.
     :type render: `bool`_
+
+    : param reference: Desired length of reference vector in plot legend.  The
+        default is to choose a reasonable size for the max vector in the legend
+        based on the amount of space available.  This behavior can be
+        overridden by providing the "reference" parameter, and then the size of
+        the arrow will be computed to match.  Be aware this may cause the arrow
+        to be very large (not fitting nicely within the legend) or very small,
+        even invisible.
+    : type reference: `float`_
     """
 
+    print('drawVectorLegend, maxNormInVp = {0}, maxNorm = {1}, reference = {2}'.format(maxNormInVp, maxNorm, reference))
+
+    useReferenceValue = not numpy.allclose(reference, 1e20)
+
     # Figure out space length
     text = vcs.createtext(To_source=templateLegend.textorientation,
                           Tt_source=templateLegend.texttable)
     text.x = .5
     text.y = .5
     maxLegendString = _createLegendString(maxNorm, unitString)
+    if useReferenceValue:
+        maxLegendString = _createLegendString(reference, unitString)
     text.string = maxLegendString
     maxExt = canvas.gettextextent(text)[0]
 
@@ -2912,27 +2928,30 @@ def drawVectorLegend(canvas, templateLegend,
 
     # space between line and label - one character long
     spaceLength = (maxExt[1] - maxExt[0]) / len(maxLegendString)
-    # line vector - min 2 and max 15 characters long
-    minMaxNormLineLength = 2 * spaceLength
-    maxMaxNormLineLength = 15 * spaceLength
-    # clamp lineLegth between 2 and 15 spaceLength
     maxLineLength = maxNormInVp
+    if useReferenceValue:
+        maxLineLength = (maxNormInVp * reference) / maxNorm
+    else:
+        # line vector - min 2 and max 15 characters long
+        minMaxNormLineLength = 2 * spaceLength
+        maxMaxNormLineLength = 15 * spaceLength
+        # clamp lineLegth between 2 and 15 spaceLength
+        ratio = 1.0
+        if (maxLineLength < minMaxNormLineLength):
+            while (maxLineLength < minMaxNormLineLength):
+                maxLineLength *= 2
+                ratio *= 2
+        elif (maxLineLength > maxMaxNormLineLength):
+            while (maxLineLength > maxMaxNormLineLength):
+                maxLineLength /= 2
+                ratio /= 2
+
+        # update maxLegendString with the clamped value
+        if (ratio != 1):
+            maxLegendString = _createLegendString(maxNorm * ratio, unitString)
+            text.string = maxLegendString
+            maxExt = canvas.gettextextent(text)[0]
     minLineLength = minNormInVp
-    ratio = 1.0
-    if (maxLineLength < minMaxNormLineLength):
-        while (maxLineLength < minMaxNormLineLength):
-            maxLineLength *= 2
-            ratio *= 2
-    elif (maxLineLength > maxMaxNormLineLength):
-        while (maxLineLength > maxMaxNormLineLength):
-            maxLineLength /= 2
-            ratio /= 2
-
-    # update maxLegendString with the clamped value
-    if (ratio != 1):
-        maxLegendString = _createLegendString(maxNorm * ratio, unitString)
-        text.string = maxLegendString
-        maxExt = canvas.gettextextent(text)[0]
 
     maxLegendLength = maxExt[1] - maxExt[0]
     maxheight = maxExt[3] - maxExt[2]

vcs/vcsvtk/vectorpipeline.py

@@ -245,7 +245,7 @@ def _plotInternal(self):
             minNormInVp *= worldToViewportXScale
         vcs.utils.drawVectorLegend(
             self._context().canvas, self._template.legend, lcolor, lstyle, lwidth,
-            unitString, maxNormInVp, maxNorm, minNormInVp, minNorm)
+            unitString, maxNormInVp, maxNorm, minNormInVp, minNorm, reference=self._gm.reference)
 
         kwargs['xaxisconvert'] = self._gm.xaxisconvert
         kwargs['yaxisconvert'] = self._gm.yaxisconvert

vcs/vector.py

@@ -308,7 +308,10 @@ class Gv(vcs.bestMatch):
 
             .. code-block:: python
 
-                # Can be an integer or float
+                # Can be an integer or float.  Setting the reference attribute
+                # overrides the default behavior of picking a reasonable size
+                # for the vector legend arrow.  This may result in a very large
+                # or very small arrow, depending on the value of vc.reference.
                 vc.reference=4
     """
     __slots__ = [
@@ -772,7 +775,7 @@ def list(self):
         print("datawc_calendar = ", self.datawc_calendar)
         print("xaxisconvert = ", self.xaxisconvert)
         print("yaxisconvert = ", self.yaxisconvert)
-        print("line = ", self.line)
+        print("linetype = ", self.linetype)
         print("linecolor = ", self.linecolor)
         print("linewidth = ", self.linewidth)
         print("scale = ", self.scale)