Introduce new "snapToPixelGrid" flag to GL2 engine for drawText

When we're rendering text, the glyphs need to be aligned to the
pixel grid otherwise we get strange artifacts. Normally text is
drawn at integer coordinates, however it is still possible to
have a transform which translates by a non-integer offset. This
patch adds a flag to the engine which can be used to snap any
translate to the pixel grid.

Task-number: QTBUG-7094
Reviewed-By: Kim

2 files changed, 44 insertions, 5 deletions

diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index 0f8e945..a41d439 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -589,19 +589,28 @@ void QGL2PaintEngineExPrivate::updateMatrix()
 
     const GLfloat wfactor = 2.0f / width;
     const GLfloat hfactor = -2.0f / height;
+    GLfloat dx = transform.dx();
+    GLfloat dy = transform.dy();
+
+    // Non-integer translates can have strange effects for some rendering operations such as
+    // anti-aliased text rendering. In such cases, we snap the translate to the pixel grid.
+    if (snapToPixelGrid && transform.type() == QTransform::TxTranslate) {
+        // 0.50 needs to rounded down to 0.0 for consistency with raster engine:
+        dx = ceilf(dx - 0.5f);
+        dy = ceilf(dy - 0.5f);
+    }
 
     if (addOffset) {
-        pmvMatrix[2][0] = (wfactor * (transform.dx() + 0.49f)) - transform.m33();
-        pmvMatrix[2][1] = (hfactor * (transform.dy() + 0.49f)) + transform.m33();
-    } else {
-        pmvMatrix[2][0] = (wfactor * transform.dx()) - transform.m33();
-        pmvMatrix[2][1] = (hfactor * transform.dy()) + transform.m33();
+        dx += 0.49f;
+        dy += 0.49f;
     }
 
     pmvMatrix[0][0] = (wfactor * transform.m11())  - transform.m13();
     pmvMatrix[1][0] = (wfactor * transform.m21())  - transform.m23();
+    pmvMatrix[2][0] = (wfactor * dx) - transform.m33();
     pmvMatrix[0][1] = (hfactor * transform.m12())  + transform.m13();
     pmvMatrix[1][1] = (hfactor * transform.m22())  + transform.m23();
+    pmvMatrix[2][1] = (hfactor * dy) + transform.m33();
     pmvMatrix[0][2] = transform.m13();
     pmvMatrix[1][2] = transform.m23();
     pmvMatrix[2][2] = transform.m33();
@@ -699,6 +708,11 @@ void QGL2PaintEngineExPrivate::drawTexture(const QGLRect& dest, const QGLRect& s
         matrixDirty = true;
     }
 
+    if (snapToPixelGrid) {
+        snapToPixelGrid = false;
+        matrixDirty = true;
+    }
+
     if (prepareForDraw(opaque))
         shaderManager->currentProgram()->setUniformValue(location(QGLEngineShaderManager::ImageTexture), QT_IMAGE_TEXTURE_UNIT);
 
@@ -856,6 +870,11 @@ void QGL2PaintEngineExPrivate::fill(const QVectorPath& path)
         matrixDirty = true;
     }
 
+    if (snapToPixelGrid) {
+        snapToPixelGrid = false;
+        matrixDirty = true;
+    }
+
     // Might need to call updateMatrix to re-calculate inverseScale
     if (matrixDirty)
         updateMatrix();
@@ -1249,6 +1268,11 @@ void QGL2PaintEngineExPrivate::stroke(const QVectorPath &path, const QPen &pen)
         matrixDirty = true;
     }
 
+    if (snapToPixelGrid) {
+        snapToPixelGrid = false;
+        matrixDirty = true;
+    }
+
     const Qt::PenStyle penStyle = qpen_style(pen);
     const QBrush &penBrush = qpen_brush(pen);
     const bool opaque = penBrush.isOpaque() && s->opacity > 0.99;
@@ -1519,6 +1543,10 @@ void QGL2PaintEngineExPrivate::drawCachedGlyphs(const QPointF &p, QFontEngineGly
         addOffset = false;
         matrixDirty = true;
     }
+    if (!snapToPixelGrid) {
+        snapToPixelGrid = true;
+        matrixDirty = true;
+    }
 
     QBrush pensBrush = q->state()->pen.brush();
     setBrush(pensBrush);
@@ -1637,6 +1665,11 @@ void QGL2PaintEngineExPrivate::drawPixmaps(const QDrawPixmaps::Data *drawingData
         matrixDirty = true;
     }
 
+    if (snapToPixelGrid) {
+        snapToPixelGrid = false;
+        matrixDirty = true;
+    }
+
     bool allOpaque = true;
 
     for (int i = 0; i < dataCount; ++i) {
@@ -1918,6 +1951,10 @@ void QGL2PaintEngineExPrivate::writeClip(const QVectorPath &path, uint value)
         addOffset = false;
         matrixDirty = true;
     }
+    if (snapToPixelGrid) {
+        snapToPixelGrid = false;
+        matrixDirty = true;
+    }
 
     if (matrixDirty)
         updateMatrix();
diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h b/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h
index 9a5c447..7d5cb52 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2_p.h
@@ -167,6 +167,7 @@ public:
             width(0), height(0),
             ctx(0),
             useSystemClip(true),
+            snapToPixelGrid(false),
             addOffset(false),
             inverseScale(1)
     { }
@@ -260,6 +261,7 @@ public:
     GLfloat staticVertexCoordinateArray[8];
     GLfloat staticTextureCoordinateArray[8];
 
+    bool snapToPixelGrid;
     bool addOffset; // When enabled, adds a 0.49,0.49 offset to matrix in updateMatrix
     GLfloat pmvMatrix[3][3];
     GLfloat inverseScale;