Whats is the best way of creating a line (QGraphicsLineItem) which starts at some point on the scene and continues to infinity at some angle.
The way I presently do this is by calculating were the line intersects the view and drawing the line segment.
Is there a better way?
Could I for example set the lines length to some massive number?
You could define its paint() and shape() functions so they always use all the space available and needed inside the scene, i.e. inside the visible part of qgraphicsview.
Guidelines:
Examine mapping functions for qgraphicsview, qgraphicsscene and
qgraphicsitem (mapToScene, mapToItem, mapToView or something like that)
Define your shape() and paint() functions as if your
line is exactly long all over the view (by using the mapping functions above)
So, how ever the user moves his view, repaint will examine the space used by qgraphicsview and draw exactly that long. The illusion is created that the line goes on-and-on.
Related
I have two QGraphicsItem in a scene and I want to draw a line between this two objects. However, these objects are movable and I don't know how to update a line after every movement ?
I just answer one of your other questions demonstrating just that: connecting 2 movable eclipses with a line.
There is a fully working example to get you going. See this other answer for more details.
Add a comment or update your question if something is still not clear. Otherwise please mark it as accepted.
In one application I implemented lines between objects by drawing actually three lines following way:
A----
:
:----B
To update the lines, I added 6 pointers to objects
QGraphicsLineItem *prvLineItems[3];
QGraphicsLineItem *nxtLineItems[3];
When objects and lines were created, I set pointers prvLineItems and nxtLineItems point to the created lines.
Then when location of the object changed, I moved corresponding lines also (in my case in the mouseReleaseEvent).
In your case you need just one pointer to the line to objects 1 and 2. When one of the objects changes location, change coordinates of the line also.
When using anti-aliasing rendering in Qt's QGraphicsScene, there is a behavior that makes drawings appear not as expected: overlapping lines become darker. I could not see any description of this behavior in the documentation, and I cannot find a way to disable it.
For example if I want to draw such a polygon:
Because of the number of points, it is impossible not to have overlapping lines - fine. But because anti-aliasing is activated, some borders appear 'thicker' than others.
Is there any way to avoid this and have anti-aliased lines that can overlap and yet at the same time be rendered without getting darker?
I know of course that I can redefine the paint() function and draw manually individual lines that do not overlap, but this is what I want to avoid. I am using Pyside and this would significantly slow down the application, due to the high frequency at which paint() is being called.
EDIT Fixed by defining the object shape using QPainterPath / QGraphicsPathItem instead of QPolygon / QGraphicsPolygonItem. In that case the moveTo function allows to avoid lines that overlap.
Another thing you could try is adding half a pixel to your coordinates (not dimensions). This fixed the anti-aliasing issue for me.
XCoord = int(XValue) + 0.5
YCoord = int(XValue) + 0.5
Also make sure that before that you have integer pixel values.
I draw few rectangles inside the QGraphicsView ; I use custom stipple pattern for these by creating a QBrush with my QPixmap. This gets displayed with the default zoom level as expected.
When I call view->scale(), the rectangles show up bigger or smaller as I expected. However Qt has scaled the individual bits of the stipple pattern which is not expected; I expected it to draw the larger or smaller rectangle again with the brush.
Eg.
If I had used a stipple pattern with one pixel dot and pixel space, after zooming in, I want to see a larger rectangle but I want the same stipple pattern with same pixel gaps. Is this achievable somehow? Thanks.
I ran into the same problem while developing an EDA tool companion in Qt.
After some trying, what I did (and seems to work for me) is to create a custom graphics item. On the paint method, I do:
QBrush newBrush = brush_with_pattern;
newBrush.setTransform(QTransform(painter->worldTransform().inverted()));
painter->setBrush(newBrush);
That is to apply the inverse transformation of the item to the brush (so it does not scale).
I think that the setDashOffset is only for the border of the shapes (not the fill).
You may use QPen::setDashOffset:
http://harmattan-dev.nokia.com/docs/library/html/qt4/qpen.html#setDashOffset
You'll need to set the offset based on the scenes zoom/scale level. You can grab a pointer to the scene in your item by calling scene(), don't forget to check for NULL though since it will be NULL when not added to the scene (although you shouldn't in theory get a paint() when not in a scene).
The other option is to use:
http://doc.qt.digia.com/qt/qpainter.html#scale
To undo the views scaling on your painter.
In case anyone is still looking on this, a related question here regarding scaling of standard fill patterns instead of pixmap fill patterns may help. Basically, it may not be possible to modify scaling of standard fill patterns (a few workaround ideas are listed), but, working with alpha values instead gives the desired effect if you are looking for varying colors, especially gray levels - and is much less convoluted.
I'm working on a Qt based application (actually in PyQt but I don't think that's relevant here), part of which involves plotting a potentially continuous stream of data onto a graph in real time.
I've implemented this by creating a class derived from QWidget which buffers incoming data, and plots the graph every 30ms (by default). In __init__(), a QPixmap is created, and on every tick of a QTimer, (1) the graph is shifted to the left by the number of pixels that the new data will take up, (2) a rectangle painted in the space, (3) the points plotted, and (4) update() called on the widget, as follows (cut down):
# Amount of pixels to scroll
scroll=penw*len(points)
# The first point is not plotted now, so don't shift the graph for it
if (self.firstPoint()):
scroll-=1
p=QtGui.QPainter(pm)
# Brush setup would be here...
pm.scroll(0-scroll, 0, scroll, 0, pm.width()-scroll, pm.height())
p.drawRect(pm.width()-scroll, 0, scroll, pm.height())
# pen setup etc happens here...
offset=scroll
for point in points:
yValNew = self.graphHeight - (self.scalePoint(point))
# Skip first point
if (not(self.firstPoint())):
p.drawLine(pm.width()-offset-penw, self.yVal, pm.width()-offset, yValNew)
self.yVal = yValNew
offset-=penw
self.update()
Finally, the paintEvent simply draws the pixmap onto the widget:
p = QtGui.QPainter(self)
p.drawPixmap(0, 0, self.graphPixmap)
As far as I can see, this should work correctly, however, when data is received very fast (i.e. the entire graph is being plotted on each tick), and the widget is larger than a certain size (approx 700px), everything to the left of the 700px area lags considerably. This is perhaps best demonstrated in this video: http://dl.dropbox.com/u/1362366/keep/Graph_bug.swf.html (the video is a bit laggy due to the low frame rate, but the effect is visible)
Any ideas what could be causing this or things I could try?
Thanks.
I'm not 100% sure if this is the problem or not, but I thought I might make at least some contribution.
self.update() is an asynchronous call, which will cause a paint event at some point later when the main event loop is reached again. So it makes me wonder if your drawing is having problems because of the sync issue between when you are modifying your pixmap vs when its actually getting used in the paintEvent. Almost seems like what you would need for this exact code to work is a lock in your paintEvent, but thats pretty naughty sounding.
For a quick test, you might try forcing the event loop to flush right after your call to update:
self.update()
QtGui.QApplication.processEvents()
Not sure that will fix it, but its worth a try.
This actually might be a proper situation to be using repaint() and causing a direct paint event, since you are doing an "animation" using a controlled framerate: self.repaint()
I noticed a similar question to yours, by someone trying to graph a heart monitor in real time: http://qt-project.org/forums/viewthread/10677
Maybe you could try restructuring your code similar to that. Instead of splitting the painting into two stages, he is using a QLabel as the display widget, setting the pixmap into the QLabel, and painting the entire graph immediately instead of relying on calls to the widget.update()
i am not really newbie in Qt, but there are a few things i don't know...
I am programming in Python, but feel free to post your answers in ANY language.
So, i have a few QGraphicsItem (s), positioned inside a QGraphicsScene, the scene is viewed with a normal QGraphicsView. Everything is normal.
My scene is very large, 10,000 x 10,000 pixels, all graphic items are scattered around.
For example :
# Creating Scene object.
scene = QtGui.QGraphicsScene()
scene.setSceneRect(0, 0, 10000, 10000)
# Creating View object.
view = QtGui.QGraphicsView()
view.setScene(scene)
# Adding some objects to scene.
# scene.addItem(...)
# ...
# The list of items.
items = scene.items()
# This is how i center on item.
view.centerOn(some_item)
view.fitInView(some_item, Qt.KeepAspectRatio)
My question is, how can i center the view on every item, using something similar to centerOn, but smoothly ?
Currently, centerOn goes FAST on next item, i want to move it slooowly, maybe using QPropertyAnimation with easing curve ?
I tried to move the view to the next item using view.translate(1, 1) in a big cicle, but the movement is too fast, just like centerOn.
I tried to put some waiting with time.sleep(0.01) between the translating, but the windows blocks untill the cicle exists... so it's bad.
Thank you very much !
I once used a QTimeLine (with EaseInOutCurve), connected it to a slot, and then used that value to translate the view rect, like this:
const QRectF r = ...calculate rect translated by timeline value and trajectory...
view->setSceneRect( r );
view->fitInView( r, Qt::KeepAspectRatio );
For the trajectory I used a QLineF with start and end position for the smooth scrolling.
Then one can use the value emitted by timeline nicely with QLineF::pointAt().
In my case I needed to set both SceneRect and fitInView to make it behave like I wanted.
I solved my problem by placing a high value on setSceneRect. Then I centralize the scene on an object or position.
example:
this->ui->graphicsView->setSceneRect (0,0,100000000, 100000000);
this->ui->graphicsView->centerOn(500,1030);
With the setSceneRect size GraphicsView does not work right.