Delay with map gestures about mapbox-gl-qml HOT 3 CLOSED

I agree, there is a delay which, while short, is sometimes observable by the user. And it should be reduced as much as possible.

The delay is essentially caused by the fact that QMapboxGL "lives" in a thread (or threads) that is different from the GUI thread. So, wherever we want to get something out of Mapbox GL, we need to send signals between threads, one for request from GUI to OpenGL rendering and one back to the GUI.

In case of the bubbles with the WhoGo example:

• On click, MapboxMapGestureArea requests queryCoordinateForPixel from the map object.

• queryCoordinateForPixel is a signal that is sent over queued connection to QSGMapboxGLTextureNode and activates OpenGL rendering update to wakeup the thread where QSGMapboxGLTextureNode is.

• QSGMapboxGLTextureNode calculates the result and emits replyCoordinateForPixel back to GUI thread via queued connection.

• replyCoordinateForPixel is received by MapboxMapGestureArea and forwarded to the main code via emitted clickedGeo. The latter signal should be direct (aka fast) since it is in the same thread.

• clickedGeo is followed by WhoGo maps and, on receiving it, there is a loop that runs through all POIs to check if any of them was clicked.

As you could see, there is some processing involved and some of the delays could be induced due to
connections between threads. I am not sure whether I can significantly reduce for occasional click. But for POIs that we know beforehand, maybe its possible to figure out something.

We do have location tracking facility. However, I doubt that it maybe too expensive to keep an eye on all POIs at each frame refresh.

I'll think what I can do to reduce the delay.

from mapbox-gl-qml.

Sounds complicated. I figured it was due to the separation, but was hoping there was maybe something simple like a polling frequency. Have you checked how QtLocation's Mapbox GL plugin handles this?

We do have location tracking facility. However, I doubt that it maybe too expensive to keep an eye on all POIs at each frame refresh.

It could be done so that each frame refresh would just mark the map as changed and a timer would run e.g. every 500 ms and update the tracking cache if the map is changed. Then if the user waits that 500 ms between panning and tapping, which is likely to be the usual case, it would work fast, otherwise as currently. But, of course, that would likely require app-specific code, so it's probably more a workaround than a fix.

from mapbox-gl-qml.

Not much I can do about it; closing

from mapbox-gl-qml.