QGraphicsScene/QGraphicsView window limitations

[mistertoony]

i am an undergraduate student working with Qt. i've used QPolygonF to create polygons that have been attached to a scene as pathitems. using the following code for attaching points:

polygonC << QPointF(x,y);

x values range from 0 to 5000 and y values range from 0 to 150.

after successfully plotting the data, i have noticed that the majority of the data has not been plotted. examine the screenshot below.

[jacek]

What is the size of your QGraphicsScene?

[mistertoony]

i initialized QGraphicsScene without parameters, for example, QGraphicsScene myScene.

[jacek]

What does polygonC.boundingRect() return?

[mistertoony]

whats the best way to display the information returned from polygonC.boundingRect()? through QMessageBox?

EDIT: i was unable to figure out how to display the information returned from .boundingRect(), however i set:

QGraphicsScene *scene = new QGraphicsScene(0,0,50000,0);

i noticed that the all four curves are cut off. is there a limitation on QPolygonF?

[jacek]

whats the best way to display the information returned from polygonC.boundingRect()? through QMessageBox?

qDebug() should be the quickest one.

[mistertoony]

jacek, polygons are cut off. im wondering if there is a maximum number of points a polygon may have.

[Bitto]

You might have run into a bug that was recently fixed; try applying this patch and see if it helps...

--- qgraphicsview.cpp  2007/02/05 13:30:58.000000000
+++ qgraphicsview.cpp  2007/02/12 16:26:20.000000000
 
@@ -338,6 +338,14 @@
     int height = maxSize.height();
     QRectF viewRect = matrix.mapRect(q->sceneRect());
 
+    // Adjust the maximum width and height of the viewport based on the width
+    // of visible scrollbars.
+    int scrollBarExtent = q->style()->pixelMetric(QStyle::PM_ScrollBarExtent, 0, q);
+    if (viewRect.width() >= maxSize.width())
+        height -= scrollBarExtent;
+    if (viewRect.height() >= maxSize.height())
+        width -= scrollBarExtent;
+
     // Setting the ranges of these scroll bars can/will cause the values to
     // change, and scrollContentsBy() will be called correspondingly. This
     // will reset the last center point.

--- qgraphicsview.cpp  2007/02/05 13:30:58.000000000
+++ qgraphicsview.cpp  2007/02/12 16:26:20.000000000

@@ -338,6 +338,14 @@
     int height = maxSize.height();
     QRectF viewRect = matrix.mapRect(q->sceneRect());

+    // Adjust the maximum width and height of the viewport based on the width
+    // of visible scrollbars.
+    int scrollBarExtent = q->style()->pixelMetric(QStyle::PM_ScrollBarExtent, 0, q);
+    if (viewRect.width() >= maxSize.width())
+        height -= scrollBarExtent;
+    if (viewRect.height() >= maxSize.height())
+        width -= scrollBarExtent;
+
     // Setting the ranges of these scroll bars can/will cause the values to
     // change, and scrollContentsBy() will be called correspondingly. This
     // will reset the last center point.

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[rajesh]

First you initialize QGraphicsScene without parameters, for example,
QGraphicsScene myScene.

later you calculate rect area(width & height) after drawing all items, then set following

scene->setSceneRect(0, 0, width,height);

(width = width of longest QGrapicsItem)


This will set scroll bar automatically. No need to set yourself.

[mistertoony]

First you initialize QGraphicsScene without parameters, for example,
QGraphicsScene myScene.

later you calculate rect area(width & height) after drawing all items, then set following

scene->setSceneRect(0, 0, width,height);

(width = width of longest QGrapicsItem)


This will set scroll bar automatically. No need to set yourself.

how do i calculate the rect area? i believe the problem is that polygons are unfinished, i manually extended the width of the scene and the polygons appear cut off.

[mistertoony]

You might have run into a bug that was recently fixed; try applying this patch and see if it helps...

--- qgraphicsview.cpp  2007/02/05 13:30:58.000000000
+++ qgraphicsview.cpp  2007/02/12 16:26:20.000000000
 
@@ -338,6 +338,14 @@
     int height = maxSize.height();
     QRectF viewRect = matrix.mapRect(q->sceneRect());
 
+    // Adjust the maximum width and height of the viewport based on the width
+    // of visible scrollbars.
+    int scrollBarExtent = q->style()->pixelMetric(QStyle::PM_ScrollBarExtent, 0, q);
+    if (viewRect.width() >= maxSize.width())
+        height -= scrollBarExtent;
+    if (viewRect.height() >= maxSize.height())
+        width -= scrollBarExtent;
+
     // Setting the ranges of these scroll bars can/will cause the values to
     // change, and scrollContentsBy() will be called correspondingly. This
     // will reset the last center point.

--- qgraphicsview.cpp  2007/02/05 13:30:58.000000000
+++ qgraphicsview.cpp  2007/02/12 16:26:20.000000000

@@ -338,6 +338,14 @@
     int height = maxSize.height();
     QRectF viewRect = matrix.mapRect(q->sceneRect());

+    // Adjust the maximum width and height of the viewport based on the width
+    // of visible scrollbars.
+    int scrollBarExtent = q->style()->pixelMetric(QStyle::PM_ScrollBarExtent, 0, q);
+    if (viewRect.width() >= maxSize.width())
+        height -= scrollBarExtent;
+    if (viewRect.height() >= maxSize.height())
+        width -= scrollBarExtent;
+
     // Setting the ranges of these scroll bars can/will cause the values to
     // change, and scrollContentsBy() will be called correspondingly. This
     // will reset the last center point.

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i can't find this cpp file on my system.

[Bitto]

The patch is against Qt's source code; you can download the sources, apply the patch, and compile/install the library, or wait for an official upgrade (this patch is part of 4.3).