[Interest] Synchronous painting on QGraphicsScene

[Christian Gagneraud]

On 8 October 2017 at 06:27, Patrick Stinson <patrickkidd at gmail.com> wrote:
> Hmm. Actually, I think that the elastic node example is implemented in the
> same way as my app. Because of the delayed response of the “elasticity” in
> the algorithm it is not possible to demonstrate a solution to my problem
> even if it solved it.
>
> Like the elastic node example, I am implementing QGraphicsItem::itemChange
> for the node listening for QGraphicsItem::ItemPositionHasChanged, and then
> updating the edges’s geometry just after calling ::prepareGeometryChange.
> This queues an update for the next paint buffer, which is what causes my
> problem.

If you can notice the time lag b/w the 2 paint events, it means that
something is slow. Slow enough to cause a 20+ ms delay.
So maybe your problem is elsewhere. Do you have this problem even
without the opengl view?

If i'm not wrong, although the paint() on the edge item is not called
within your mouse event of the node. the edge should be repainted
right after the mouse event has finished, which should be a very short
time period.

But maybe you're talking about "paint buffer" in the OpenGL pipeline?

> What I am wondering is if it is possible to implement painting in a
> synchronous way, where all paint events which result from a single mouse
> event are done in the same buffer frame.

Or maybe your python code is too slow to make the 2 paint event fits
in a single buffer frame timeline.

Chris

>
> Make sense?
>
>
> On Oct 7, 2017, at 1:01 AM, Christian Gagneraud <chgans at gmail.com> wrote:
>
> On 7 October 2017 at 16:08, Patrick Stinson <patrickkidd at gmail.com> wrote:
>
> Hi there!
>
> I have a graph-ish diagram app where dragging some objects (like nodes)
> necessitates dragging other objects (like connecting edges). However, if I
> update an edge from the mouse event or ItemChanged(position) event then the
> edge isn’t painted until the next paint event. At times, this is quite
> obvious for example when nudging a node with an arrow key and the node is
> going in one direction and edge goes in the other direction for one frame
> because it is one buffer behind.
>
> Any thoughts on synchronous painting?
>
>
> Have you check the "Elastic node"  example? It does exactly what
> you're describing. And last time i tried it, it worked very well, no
> painting artifact.
> How is your "update"  done exactly?
>
> Chris
>
>
> Thank you
> -Patrick
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