how to get exact point location of a point cloud data

[ashbira...@gmail.com]

I want to pick the exact point location. i.e exact lat long value of particular point in a point cloud data on mouse click.

can someone help me out in partial code example.

[Omar Shehata]

You should just be able to use viewer.scene.pickPosition. 

So you'd use it like:

// Pick position of individual points 

var handler = new Cesium.ScreenSpaceEventHandler(viewer.canvas);

handler.setInputAction(function(event) {

    var position = viewer.scene.pickPosition(event.position);

    // position will be undefined if the user clicks something off the globe

    if (Cesium.defined(position)) {

        console.log(position);

    }

}, Cesium.ScreenSpaceEventType.LEFT_CLICK);

Here's a running Sandcastle example.

This will give you a cartesian position. You can then convert that to a long/lat.

[Abe]

Pickposition wont give you the exact location. Has anyone achieved this?

[Omar Shehata]

Can you elaborate on what you mean? Do you have a code example or a Sandcastle (https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/) where it's producing an incorrect result?

[Abe]

Hi Omar,

pickPosition gives you the position based on the depth buffer on the 2D screen. If you use that position to graph a sphere for example and zoom in, you can see the position computed using pickPosition is not the same as the position of the point that was clicked. I am looking to for a way to pick the exact position of a point clicked

[Omar Shehata]

Hey Abe,

Can you please show me how you're testing this? I tried this test you suggested, and I am seeing a sphere in exactly the location I clicked on. Here's a Sandcastle of this zoomed in on the grand canyon to make it easier to test. Below is an animation of clicking at a couple locations to show how pickPosition returns an accurate result.

Are you seeing different behavior in this code example?

[Omar Shehata]

Here's the animation on imgur if it's not playing in this forum post: https://imgur.com/qid4WTS

[satten...@datalive.com.au]

Yes we have the same issue. Our solution was to use a spatialite database to do a spatial query on the point cloud as this was able to bypass the screen-space inaccuracies and the fact that there's no GPU-CPU transfer of point cloud data. Obviously not ideal for many reasons, but it enabled us to show details for the exact location of a point.

Omar, it's the definition of "exact" that's the issue. The buffer will get *very* close, but when dealing with point clouds with an accuracy of 1/10mm, the buffer isn't good enough. Plus it sometimes gives wildly inaccurate values if you're on the very edge of a point (in the order of hundreds of metres inaccurate).

[Omar Shehata]

Stuart, thanks for the explanation and posting your current workaround. I see what you mean now. I posted some thoughts in this forum thread on what it would take to get 3D Tiles to save the points on the CPU and do this kind of spatial query:

https://groups.google.com/d/msg/cesium-dev/miFO7Fux1HY/FCk_en1pAwAJ

It looks like we might be prioritizing this soon. I'll definitely update these thread(s) when there's progress on it, but I'd also keep an eye on the GitHub/monthly releases.

[Abe]

Stuart, Omar, this is what makes a pickPosition based tool for point clouds the hardest  " Plus it sometimes gives wildly inaccurate values if you're on the very edge of a point (in the order of hundreds of metres inaccurate)." Any idea how to fix this? Thank you!

[Omar Shehata]

I appreciate the explanation Abe. I think the solution here would be to save the per-point x/y/z on the CPU and allow retrieving them on click, in the same way the "3D Tiles interactivity" demo on Sandcastle allows you to retrieve building height/specific metadata for each building on click. 

I've opened a feature request here along with some implementation notes from Sean:

https://github.com/AnalyticalGraphicsInc/cesium/issues/7953

Hopefully we'll get a chance to implement this soon.