Make axis labels more accurate by using qreals for label values

Also refactor axis formatter sub grid array to be one dimensional.
There is no need to know which segment each sub grid line belongs
to.

Change-Id: Ie9813088650fcc0ca844f3c358ea1abae9258367
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>

1 files changed, 39 insertions, 27 deletions

diff --git a/src/datavisualization/axis/qvalue3daxisformatter.cpp b/src/datavisualization/axis/qvalue3daxisformatter.cpp
index b1183023..7c7db398 100644
--- a/src/datavisualization/axis/qvalue3daxisformatter.cpp
+++ b/src/datavisualization/axis/qvalue3daxisformatter.cpp
@@ -104,11 +104,11 @@ QValue3DAxisFormatter *QValue3DAxisFormatter::createNewInstance() const
 }
 
 /*!
- * This method populates the label and grid line position arrays, as well as calculates
- * any values needed for mapping between value and position. It is allowed to access
+ * This method populates the label and grid line position arrays and the label value array, as well
+ * as calculates any values needed for mapping between value and position. It is allowed to access
  * the parent axis from inside this function.
  *
- * This method must be reimplemented in a subclass. The resetPositionArrays() method must be called
+ * This method must be reimplemented in a subclass. The resetArrays() method must be called
  * in the subclass implementation before the position arrays are recalculated. If the subclass
  * implementation changes any parent axis values, these changes must be done before
  * the resetPositionArrays() call.
@@ -116,7 +116,7 @@ QValue3DAxisFormatter *QValue3DAxisFormatter::createNewInstance() const
  * See gridPositions(), subGridPositions(), and labelPositions() documentation about the arrays
  * that need to be populated.
  *
- * \sa gridPositions(), subGridPositions(), labelPositions(), axis(), resetPositionArrays()
+ * \sa gridPositions(), subGridPositions(), labelPositions(), axis(), resetArrays()
  */
 void QValue3DAxisFormatter::recalculate()
 {
@@ -145,7 +145,7 @@ QString QValue3DAxisFormatter::labelForIndex(int index) const
  *
  * \sa recalculate(), labelForIndex(), QValue3DAxis::labelFormat
  */
-QString QValue3DAxisFormatter::stringForValue(float value, const QString &format) const
+QString QValue3DAxisFormatter::stringForValue(qreal value, const QString &format) const
 {
     return d_ptr->stringForValue(value, format);
 }
@@ -199,9 +199,9 @@ void QValue3DAxisFormatter::populateCopy(QValue3DAxisFormatter &copy) const
  *
  * \sa gridPositions(), subGridPositions(), labelPositions(), axis(), recalculate()
  */
-void QValue3DAxisFormatter::resetPositionArrays()
+void QValue3DAxisFormatter::resetArrays()
 {
-    d_ptr->resetPositionArrays();
+    d_ptr->resetArrays();
 }
 
 /*!
@@ -227,7 +227,7 @@ QValue3DAxis *QValue3DAxisFormatter::axis() const
 }
 
 /*!
- * \return a reference to the array of normalized grid positions.
+ * \return a reference to the array of normalized grid line positions.
  * The array size is equal to the segment count of the parent axis plus one.
  * The values should be between 0.0 (for minimum value) and 1.0 (for maximum value), inclusive.
  * The grid line at the index zero corresponds to the minimum value of the axis.
@@ -240,14 +240,14 @@ QVector<float> &QValue3DAxisFormatter::gridPositions() const
 }
 
 /*!
- * \return a reference to the array of normalized subgrid positions.
+ * \return a reference to the array of normalized subgrid line positions.
  * The array size is equal to segment count of the parent axis times sub-segment count of the parent
  * axis minus one.
  * The values should be between 0.0 (for minimum value) and 1.0 (for maximum value), inclusive.
  *
  * \sa QValue3DAxis::segmentCount, QValue3DAxis::subSegmentCount
  */
-QVector<QVector<float> > &QValue3DAxisFormatter::subGridPositions() const
+QVector<float> &QValue3DAxisFormatter::subGridPositions() const
 {
     return d_ptr->m_subGridPositions;
 }
@@ -265,6 +265,18 @@ QVector<float> &QValue3DAxisFormatter::labelPositions() const
     return d_ptr->m_labelPositions;
 }
 
+/*!
+ * \return a reference to the array of values used to generate label strings.
+ * The array size is equal to the size of the label positions array and
+ * the indexes correspond to that array as well.
+ *
+ * \sa labelPositions()
+ */
+QVector<qreal> &QValue3DAxisFormatter::labelValues() const
+{
+    return d_ptr->m_labelValues;
+}
+
 // QValue3DAxisFormatterPrivate
 QValue3DAxisFormatterPrivate::QValue3DAxisFormatterPrivate(QValue3DAxisFormatter *q)
     : QObject(0),
@@ -298,24 +310,27 @@ void QValue3DAxisFormatterPrivate::recalculate()
 
 void QValue3DAxisFormatterPrivate::doRecalculate()
 {
-    resetPositionArrays();
+    resetArrays();
 
     int segmentCount = m_axis->segmentCount();
     int subGridCount = m_axis->subSegmentCount() - 1;
 
-    float segmentStep = 1.0f / float(segmentCount);
-    float subSegmentStep = 0;
+    // Use qreals for intermediate calculations for better accuracy on label values
+    qreal segmentStep = 1.0 / qreal(segmentCount);
+    qreal subSegmentStep = 0;
     if (subGridCount > 0)
-        subSegmentStep = segmentStep / float(subGridCount + 1);
+        subSegmentStep = segmentStep / qreal(subGridCount + 1);
 
     // Calculate positions
+    qreal rangeNormalizer = qreal(m_max - m_min);
     for (int i = 0; i < segmentCount; i++) {
-        float gridValue = segmentStep * i;
-        m_gridPositions[i] = gridValue;
-        m_labelPositions[i] = gridValue;
+        qreal gridValue = segmentStep * qreal(i);
+        m_gridPositions[i] = float(gridValue);
+        m_labelPositions[i] = float(gridValue);
+        m_labelValues[i] = gridValue * rangeNormalizer + qreal(m_min);
         if (m_subGridPositions.size()) {
             for (int j = 0; j < subGridCount; j++)
-                m_subGridPositions[i][j] = gridValue + subSegmentStep * (j + 1);
+                m_subGridPositions[i * subGridCount + j] = gridValue + subSegmentStep * (j + 1);
         }
     }
 
@@ -343,10 +358,10 @@ void QValue3DAxisFormatterPrivate::doPopulateCopy(QValue3DAxisFormatterPrivate &
 
 QString QValue3DAxisFormatterPrivate::labelForIndex(int index) const
 {
-    return q_ptr->stringForValue(q_ptr->valueAt(m_gridPositions.at(index)), m_axis->labelFormat());
+    return q_ptr->stringForValue(m_labelValues.at(index), m_axis->labelFormat());
 }
 
-QString QValue3DAxisFormatterPrivate::stringForValue(float value, const QString &format)
+QString QValue3DAxisFormatterPrivate::stringForValue(qreal value, const QString &format)
 {
     if (m_previousLabelFormat.compare(format)) {
         // Format string different than the previous one used, reparse it
@@ -382,24 +397,21 @@ void QValue3DAxisFormatterPrivate::setAxis(QValue3DAxis *axis)
     m_axis = axis;
 }
 
-void QValue3DAxisFormatterPrivate::resetPositionArrays()
+void QValue3DAxisFormatterPrivate::resetArrays()
 {
     m_gridPositions.clear();
     m_subGridPositions.clear();
     m_labelPositions.clear();
+    m_labelValues.clear();
 
     int segmentCount = m_axis->segmentCount();
     int subGridCount = m_axis->subSegmentCount() - 1;
 
     m_gridPositions.resize(segmentCount + 1);
-
-    if (subGridCount > 0) {
-        m_subGridPositions.resize(segmentCount);
-        for (int i = 0; i < segmentCount; i++)
-            m_subGridPositions[i].resize(subGridCount);
-    }
+    m_subGridPositions.resize(segmentCount * subGridCount);
 
     m_labelPositions.resize(segmentCount + 1);
+    m_labelValues.resize(segmentCount + 1);
 }
 
 void QValue3DAxisFormatterPrivate::markDirty(bool labelsChange)