Fix Qt 6 performance regression when painting outside device

Painting wide lines and filling would be clipped to cliprect (by
default, the device rect) only if the bounding rect coordinates
exceeded QT_RASTER_COORD_LIMIT. In Qt 6, that limit was raised from
2^15 to 2^23, so a lot of time could be spent on rasterizing elements
that would anyway be outside the rendering area.

Fix by instead clipping whenever the path to be painted overshoots the
cliprect by a significant margin. At this point, the path is already
flattened to straight lines, so clipping is quick and precise. Testing
indicates that this solution improves performance a lot when large
portions of the elements to be painted fall outside the cliprect,
while not causing significant performance hits otherwise.

As a side effect, it is then no longer necessary to test the bounding
rect explicitly against QT_RASTER_COORD_LIMIT, since we already make
sure that the clip rect we check against is within that limit.

Fixes: QTBUG-110595
Pick-to: 6.5 6.4 6.2
Change-Id: Iaf1afbb481c2d7059405f334278796ad46f5bcb6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

3 files changed, 24 insertions, 17 deletions

diff --git a/src/gui/painting/qoutlinemapper.cpp b/src/gui/painting/qoutlinemapper.cpp
index 0dfb310ee9..93eac5cced 100644
--- a/src/gui/painting/qoutlinemapper.cpp
+++ b/src/gui/painting/qoutlinemapper.cpp
@@ -37,6 +37,24 @@ static const QRectF boundingRect(const QPointF *points, int pointCount)
     return QRectF(QPointF(minx, miny), QPointF(maxx, maxy));
 }
 
+void QOutlineMapper::setClipRect(QRect clipRect)
+{
+    auto limitCoords = [](QRect r) {
+        const QRect limitRect(QPoint(-QT_RASTER_COORD_LIMIT, -QT_RASTER_COORD_LIMIT),
+                              QPoint(QT_RASTER_COORD_LIMIT, QT_RASTER_COORD_LIMIT));
+        r &= limitRect;
+        r.setWidth(qMin(r.width(), QT_RASTER_COORD_LIMIT));
+        r.setHeight(qMin(r.height(), QT_RASTER_COORD_LIMIT));
+        return r;
+    };
+
+    if (clipRect != m_clip_rect) {
+        m_clip_rect = limitCoords(clipRect);
+        const int mw = 64; // margin width. No need to trigger clipping for slight overshooting
+        m_clip_trigger_rect = QRectF(limitCoords(m_clip_rect.adjusted(-mw, -mw, mw, mw)));
+    }
+}
+
 void QOutlineMapper::curveTo(const QPointF &cp1, const QPointF &cp2, const QPointF &ep) {
 #ifdef QT_DEBUG_CONVERT
     printf("QOutlineMapper::curveTo() (%f, %f)\n", ep.x(), ep.y());
@@ -200,16 +218,8 @@ void QOutlineMapper::endOutline()
            m_clip_rect.x(), m_clip_rect.y(), m_clip_rect.width(), m_clip_rect.height());
 #endif
 
-
-    // Check for out of dev bounds...
-    const bool do_clip = !m_in_clip_elements && ((controlPointRect.left() < -QT_RASTER_COORD_LIMIT
-                          || controlPointRect.right() > QT_RASTER_COORD_LIMIT
-                          || controlPointRect.top() < -QT_RASTER_COORD_LIMIT
-                          || controlPointRect.bottom() > QT_RASTER_COORD_LIMIT
-                          || controlPointRect.width() > QT_RASTER_COORD_LIMIT
-                          || controlPointRect.height() > QT_RASTER_COORD_LIMIT));
-
-    if (do_clip) {
+    // Avoid rasterizing outside cliprect: faster, and ensures coords < QT_RASTER_COORD_LIMIT
+    if (!m_in_clip_elements && !m_clip_trigger_rect.contains(controlPointRect)) {
         clipElements(elements, elementTypes(), m_elements.size());
     } else {
         convertElements(elements, elementTypes(), m_elements.size());
diff --git a/src/gui/painting/qoutlinemapper_p.h b/src/gui/painting/qoutlinemapper_p.h
index 1f4c3f6670..ff2fff6bac 100644
--- a/src/gui/painting/qoutlinemapper_p.h
+++ b/src/gui/painting/qoutlinemapper_p.h
@@ -79,6 +79,8 @@ public:
         m_curve_threshold = scale == 0 ? qreal(0.25) : (qreal(0.25) / scale);
     }
 
+    void setClipRect(QRect clipRect);
+
     void beginOutline(Qt::FillRule fillRule)
     {
 #ifdef QT_DEBUG_CONVERT
@@ -163,6 +165,7 @@ public:
     QDataBuffer<int> m_contours;
 
     QRect m_clip_rect;
+    QRectF m_clip_trigger_rect;
     QRectF controlPointRect; // only valid after endOutline()
 
     QT_FT_Outline m_outline;
diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 9cabef4a95..ac9af070f1 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -406,13 +406,7 @@ bool QRasterPaintEngine::begin(QPaintDevice *device)
 
     QRasterPaintEngineState *s = state();
     ensureOutlineMapper();
-    d->outlineMapper->m_clip_rect = d->deviceRect;
-
-    if (d->outlineMapper->m_clip_rect.width() > QT_RASTER_COORD_LIMIT)
-        d->outlineMapper->m_clip_rect.setWidth(QT_RASTER_COORD_LIMIT);
-    if (d->outlineMapper->m_clip_rect.height() > QT_RASTER_COORD_LIMIT)
-        d->outlineMapper->m_clip_rect.setHeight(QT_RASTER_COORD_LIMIT);
-
+    d->outlineMapper->setClipRect(d->deviceRect);
     d->rasterizer->setClipRect(d->deviceRect);
 
     s->penData.init(d->rasterBuffer.data(), this);