Multithreading in QGraphicsItem painting

[karankumar1609]

Hello wysota,

your example is the pretty simple one.
But i am working it in a different way.

What your example does:
1. Create a View and Scene of a size
2. plot points(Items) on the scene
3. Scale the scene accordingly on whele event

but something i am working on is different
1. Create a view and scene of a size
2. create axis and plot area and set them in the layout (Layout for multiple graph stacked in a scene)
3. read all the data points and initialize to the plot item
4. On every update plot item draw each graph in it

The different is i can't use scale for zoomIn and ZoomOut purpose coz i have an item of fixed size., only the drawing will vary
means item will virtually zoom in and zoom out horizontally via drawing not via scale., coz if i scale my scene each graph and axis will scale instead of the graph item
and also i have to do something custom with the graph so i did my own painting.

well when i complete zoomOut your code nt remain smooth., anyway thanks for your code.
I think there is some issue with the pixmap painting or the calculation.
may be it should need optimization.

[wysota]

Then why are you using QGraphicsView at all?

[karankumar1609]

i cant scale my whole QGraphicsView to do that., just because if i scale my view i have to scale everything in the QGraphicsScene.

sorry for missunderstanding.,
but i said that i dont want to scale my view.
the problem with the scale is that i have to scale my whole scene .

[wysota]

Please answer my question.

[karankumar1609]

well graphicsView is the best API i have seen for drawing purpose.
It automatically update its child items which i dont want to handle.
QWidget need to update manually on each small changes.. that will effect the performance.

On Widget i have to mannually handle and draw each element.
like if i have to add a dynamic line element on graph i have to draw it and handle each and every moment.
In QGraphicsScene its easy.

[wysota]

well graphicsView is the best API i have seen for drawing purpose.
It automatically update its child items which i dont want to handle.

You only have one item currently.

QWidget need to update manually on each small changes.. that will effect the performance.

Based on what you have now, changing location of one point in the plot requires all 100k points to be redrawn. That sounds to me like "each small changes requires manual update of the whole plot".

On Widget i have to mannually handle and draw each element.

That's what you do now too. Both QWidget and QGraphicsView use QPainter API for painting. Each time a piece of your data changes, you redraw the whole canvas which is exactly the situation with widgets. With graphics view, modifying one point should only redraw that one point (and possibly others that intersect with it).

like if i have to add a dynamic line element on graph i have to draw it and handle each and every moment.

That's what you do now.

In QGraphicsScene its easy.

Yes, I know. Unfortunately you're using QGraphicsView like it were a plain widget.

[karankumar1609]

ya i did the same with QGraphicsItem like with QWidget.

Firstly i started my project with different idea, which suits with QGraphicsView.

nyway thanks for your help. i will do it in QWidget later.
well is QWidget is faster than QGraphicsView ?

[wysota]

If you use Graphics View incorrectly then QWidget is likely a bit faster since you don't have the overhead of setting up the environment which you are not using anyway. If you use Graphics View correctly then it will likely be much faster than QWidget, especially for use-cases such as yours where you have a lot of small items and a subset of them is changing in time.

[karankumar1609]

Thanks wysota,

I have got the problem.
Well the problem is in the two statements in painting.

std::vector<Plot_Aggregation_Element> plotAggregationElement = plotDataValue->plotData();
 
// Each vector contains the data of one day
std::vector<Date_Element> dateElement = plotAggregationElement.at(0).ID[0].ID_vector;

std::vector<Plot_Aggregation_Element> plotAggregationElement = plotDataValue->plotData();
 
// Each vector contains the data of one day
std::vector<Date_Element> dateElement = plotAggregationElement.at(0).ID[0].ID_vector;

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The above code takes 6-8 millisecond and if there is 8 graph then it takes 6x8 = 48ms.
Now i have to solve this ., and after that it will be faster than before (60 - 48 = 12ms , thats look great).

[wysota]

Don't copy all the data -- in the code you quoted you are possibly making two copies of a vector (x8, etc.).

[karankumar1609]

Hello wysota,

What is the better option to use data without copying it .

[wysota]

Return a const reference instead of a copy.