Fix gradient artifacts on some edge cases

When static optimization is in use with mesh objects and range
gradient, and the object Y-value resolves into a texture coordinate
that is exactly on the texel boundary, the rendered fragments
of the object are not all same colors on some graphics cards,
despite all vertices having the same UV value.
Fixed by adjusting the Y-value slightly if it is close to the
boundary.

Task-number: QTRD-3370
Change-Id: Ie028602cbd9a00bb0e17049eb8f40feb8b18a6bf
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>

2 files changed, 27 insertions, 1 deletions

diff --git a/src/datavisualization/utils/scatterobjectbufferhelper.cpp b/src/datavisualization/utils/scatterobjectbufferhelper.cpp
index dae60b96..6135c225 100644
--- a/src/datavisualization/utils/scatterobjectbufferhelper.cpp
+++ b/src/datavisualization/utils/scatterobjectbufferhelper.cpp
@@ -20,6 +20,7 @@
 #include "objecthelper_p.h"
 #include <QtGui/QVector2D>
 #include <QtGui/QMatrix4x4>
+#include <QtCore/qmath.h>
 
 QT_BEGIN_NAMESPACE_DATAVISUALIZATION
 
@@ -232,6 +233,8 @@ uint ScatterObjectBufferHelper::createRangeGradientUVs(ScatterSeriesRenderCache
     const ScatterRenderItemArray &renderArray = cache->renderArray();
     const bool updateAll = (cache->updateIndices().size() == 0);
     const int updateSize = updateAll ? renderArray.size() : cache->updateIndices().size();
+    const float yAdjustment = 0.1f / gradientTextureHeight;
+    const float flippedYAdjustment = 0.9f / gradientTextureHeight;
 
     QVector2D uv;
     uv.setX(0.0f);
@@ -243,7 +246,17 @@ uint ScatterObjectBufferHelper::createRangeGradientUVs(ScatterSeriesRenderCache
             continue;
 
         float y = ((item.translation().y() + m_scaleY) * 0.5f) / m_scaleY;
+
+        // Avoid values near gradient texel boundary, as this causes artifacts
+        // with some graphics cards.
+        const float floorY = float(qFloor(y * gradientTextureHeight));
+        const float diff = y - floorY;
+        if (diff < yAdjustment)
+            y += yAdjustment;
+        else if (diff > flippedYAdjustment)
+            y -= yAdjustment;
         uv.setY(y);
+
         int offset = pos * uvsCount;
         for (int j = 0; j < uvsCount; j++)
             buffered_uvs[j + offset] = uv;
diff --git a/tests/scattertest/scatterchart.cpp b/tests/scattertest/scatterchart.cpp
index c7b93c32..e5a7acc3 100644
--- a/tests/scattertest/scatterchart.cpp
+++ b/tests/scattertest/scatterchart.cpp
@@ -723,8 +723,21 @@ void ScatterDataModifier::changeBunch()
     if (m_targetSeries->dataProxy()->array()->size()) {
         int amount = qMin(m_targetSeries->dataProxy()->array()->size(), 100);
         QScatterDataArray items(amount);
-        for (int i = 0; i < items.size(); i++)
+        for (int i = 0; i < items.size(); i++) {
             items[i].setPosition(randVector());
+            // Change the Y-values of first few items to exact gradient boundaries
+            if (i == 0)
+                items[i].setY(0.65f);
+            else  if (i == 1)
+                items[i].setY(0.1f);
+            else  if (i == 2)
+                items[i].setY(-0.45f);
+            else  if (i == 3)
+                items[i].setY(-1.0f);
+            else  if (i == 4)
+                items[i].setY(1.2f);
+        }
+
         m_targetSeries->dataProxy()->setItems(0, items);
         qDebug() << m_loopCounter << "Changed bunch, array size:" << m_targetSeries->dataProxy()->array()->size();
     }