Add a new optimization property to QGraphicsScene

The minimumRenderSize is a qreal value that is used as a lower bound to
determine what items are visible when a scene is rendered. If an item's
view-transformed width or height are less than minimumRenderSize then
this item is considered to insignificantly affect the final result and
is not drawn. If the item clips its children to its shape they are
automatically not drawn. This greatly reduces the drawing overhead
for scenes with many items rendered in a zoomed out view.

[ChangeLog][QtWidgets][QGraphicsScene] Added the minimumRenderSize
property which can be used to speed up rendering by not painting
items, smaller than a give size.

Change-Id: Ie208234707dffb4d2fc620fc5d1514e0c144d9a8
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>

4 files changed, 81 insertions, 9 deletions

diff --git a/src/widgets/graphicsview/qgraphicsscene.cpp b/src/widgets/graphicsview/qgraphicsscene.cpp
index bccdb1fbaa..a7584ef198 100644
--- a/src/widgets/graphicsview/qgraphicsscene.cpp
+++ b/src/widgets/graphicsview/qgraphicsscene.cpp
@@ -299,6 +299,7 @@ QGraphicsScenePrivate::QGraphicsScenePrivate()
       painterStateProtection(true),
       sortCacheEnabled(false),
       allItemsIgnoreTouchEvents(true),
+      minimumRenderSize(0.0),
       selectionChanging(0),
       rectAdjust(2),
       focusItem(0),
@@ -4718,17 +4719,37 @@ void QGraphicsScenePrivate::drawSubtreeRecursive(QGraphicsItem *item, QPainter *
 
     const bool itemClipsChildrenToShape = (item->d_ptr->flags & QGraphicsItem::ItemClipsChildrenToShape);
     bool drawItem = itemHasContents && !itemIsFullyTransparent;
-    if (drawItem) {
+    if (drawItem || minimumRenderSize > 0.0) {
         const QRectF brect = adjustedItemEffectiveBoundingRect(item);
         ENSURE_TRANSFORM_PTR
-        QRect viewBoundingRect = translateOnlyTransform ? brect.translated(transformPtr->dx(), transformPtr->dy()).toAlignedRect()
-                                                        : transformPtr->mapRect(brect).toAlignedRect();
-        viewBoundingRect.adjust(-int(rectAdjust), -int(rectAdjust), rectAdjust, rectAdjust);
-        if (widget)
-            item->d_ptr->paintedViewBoundingRects.insert(widget, viewBoundingRect);
-        drawItem = exposedRegion ? exposedRegion->intersects(viewBoundingRect)
-                                 : !viewBoundingRect.normalized().isEmpty();
-        if (!drawItem) {
+        QRectF preciseViewBoundingRect = translateOnlyTransform ? brect.translated(transformPtr->dx(), transformPtr->dy())
+                                                                : transformPtr->mapRect(brect);
+
+        bool itemIsTooSmallToRender = false;
+        if (minimumRenderSize > 0.0
+            && (preciseViewBoundingRect.width() < minimumRenderSize
+                || preciseViewBoundingRect.height() < minimumRenderSize)) {
+           itemIsTooSmallToRender = true;
+           drawItem = false;
+        }
+
+        bool itemIsOutsideVisibleRect = false;
+        if (drawItem) {
+            QRect viewBoundingRect = preciseViewBoundingRect.toAlignedRect();
+            viewBoundingRect.adjust(-int(rectAdjust), -int(rectAdjust), rectAdjust, rectAdjust);
+            if (widget)
+                item->d_ptr->paintedViewBoundingRects.insert(widget, viewBoundingRect);
+            drawItem = exposedRegion ? exposedRegion->intersects(viewBoundingRect)
+                                     : !viewBoundingRect.normalized().isEmpty();
+            itemIsOutsideVisibleRect = !drawItem;
+        }
+
+        if (itemIsTooSmallToRender || itemIsOutsideVisibleRect) {
+            // We cannot simply use !drawItem here. If we did it is possible
+            // to enter the outter if statement with drawItem == false and minimumRenderSize > 0
+            // and finally end up inside this inner if, even though none of the above two
+            // conditions are met. In that case we should not return from this function
+            // but call draw() instead.
             if (!itemHasChildren)
                 return;
             if (itemClipsChildrenToShape) {
@@ -5729,6 +5750,49 @@ bool QGraphicsScene::sendEvent(QGraphicsItem *item, QEvent *event)
     return d->sendEvent(item, event);
 }
 
+/*!
+    \property QGraphicsScene::minimumRenderSize
+    \since 5.4
+    \brief the minimal view-transformed size an item must have to be drawn
+
+    When the scene is rendered, any item whose width or height, transformed
+    to the target view, is smaller that minimumRenderSize(), will not be
+    rendered. If an item is not rendered and it clips its children items
+    they will also not be rendered. Set this value to speed up rendering
+    of scenes with many objects rendered on a zoomed out view.
+
+    The default value is 0. If unset, or if set to 0 or a negative value,
+    all items will always be rendered.
+
+    For example, setting this property can be especially useful if a scene
+    is rendered by multiple views, one of which serves as an overview which
+    always displays all items. In scenes with many items, such a view will
+    use a high scaling factor so that all items can be shown. Due to the
+    scaling, smaller items will only make an insignificant contribution to
+    the final rendered scene. To avoid drawing these items and reduce the
+    time necessary to render the scene, you can call setMinimumRenderSize()
+    with a non-negative value.
+
+    \note Items that are not drawn as a result of being too small, are still
+    returned by methods such as items() and itemAt(), and participate in
+    collision detection and interactions. It is recommended that you set
+    minimumRenderSize() to a value less than or equal to 1 in order to
+    avoid large unrendered items that are interactive.
+
+    \sa QStyleOptionGraphicsItem::levelOfDetailFromTransform()
+*/
+qreal QGraphicsScene::minimumRenderSize() const
+{
+    Q_D(const QGraphicsScene);
+    return d->minimumRenderSize;
+}
+void QGraphicsScene::setMinimumRenderSize(qreal minSize)
+{
+    Q_D(QGraphicsScene);
+    d->minimumRenderSize = minSize;
+    update();
+}
+
 void QGraphicsScenePrivate::addView(QGraphicsView *view)
 {
     views << view;
diff --git a/src/widgets/graphicsview/qgraphicsscene.h b/src/widgets/graphicsview/qgraphicsscene.h
index cde0eda125..ba47d45d63 100644
--- a/src/widgets/graphicsview/qgraphicsscene.h
+++ b/src/widgets/graphicsview/qgraphicsscene.h
@@ -107,6 +107,7 @@ class Q_WIDGETS_EXPORT QGraphicsScene : public QObject
     Q_PROPERTY(QFont font READ font WRITE setFont)
     Q_PROPERTY(bool sortCacheEnabled READ isSortCacheEnabled WRITE setSortCacheEnabled)
     Q_PROPERTY(bool stickyFocus READ stickyFocus WRITE setStickyFocus)
+    Q_PROPERTY(qreal minimumRenderSize READ minimumRenderSize WRITE setMinimumRenderSize)
 
 public:
     enum ItemIndexMethod {
@@ -249,6 +250,9 @@ public:
 
     bool sendEvent(QGraphicsItem *item, QEvent *event);
 
+    qreal minimumRenderSize() const;
+    void setMinimumRenderSize(qreal minSize);
+
 public Q_SLOTS:
     void update(const QRectF &rect = QRectF());
     void invalidate(const QRectF &rect = QRectF(), SceneLayers layers = AllLayers);
diff --git a/src/widgets/graphicsview/qgraphicsscene_p.h b/src/widgets/graphicsview/qgraphicsscene_p.h
index 9e5bcec488..9f50dcfea7 100644
--- a/src/widgets/graphicsview/qgraphicsscene_p.h
+++ b/src/widgets/graphicsview/qgraphicsscene_p.h
@@ -117,6 +117,8 @@ public:
     quint32 allItemsIgnoreTouchEvents : 1;
     quint32 padding : 15;
 
+    qreal minimumRenderSize;
+
     QRectF growingItemsBoundingRect;
 
     void _q_emitUpdated();
diff --git a/src/widgets/styles/qstyleoption.cpp b/src/widgets/styles/qstyleoption.cpp
index 5913b2f261..f642a05d13 100644
--- a/src/widgets/styles/qstyleoption.cpp
+++ b/src/widgets/styles/qstyleoption.cpp
@@ -3702,6 +3702,8 @@ QStyleOptionGraphicsItem::QStyleOptionGraphicsItem(int version)
     of the painter used to draw the item. By default, if no
     transformations are applied, its value is 1. If zoomed out 1:2, the level
     of detail will be 0.5, and if zoomed in 2:1, its value is 2.
+
+    \sa QGraphicsScene::minimumRenderSize()
 */
 qreal QStyleOptionGraphicsItem::levelOfDetailFromTransform(const QTransform &worldTransform)
 {