[Question] API for chekcing if point is in view or not given projection

[Nicholas Geneva]

Hi!

I'm working with D3 geo and was doing some manual plotting of points on a certain projection. I can use the projection object to convert a lat/long coordinate to a pixel location on a canvas. This works fine however, this projection is rotating, so this point may be on the front of the globe or backside.

My question is if there's some built in API I can call to tell me if a given point is in view of the current canvas / context? How is this done? I'm assuming this process typically occurs somewhere when calling the geoPath to plot, but I need to plot these points manually. Thank you!

// Spin globe
for(var i = 0; i < 360; i+=1){
      let projection = d3.geoOrthographic()
        .scale(250)
        .center([0, 0])
        .rotate([i,-30])
      // Get point
     point1 = projection(point);
     // Is the point on front/backside of the globe?
     if(point is infront of globe){
        // Draw point on canvas
    }
}

[Mike Bostock]

Try this: https://observablehq.com/@d3/testing-projection-visibility

[Nicholas Geneva]

Hi @mbostock
Great, this is very helpful. Thanks!

Follow up: Are there any other example(s) I can look at to learn more about these projection streams and modify them? I'm not able to fully understand them just from the API docs and am trying to get a little better understanding of what is going on here under the hood.

Thanks!

[Philippe Rivière]

Not sure where there are examples in the traditional sense, but one place where we use them in a relatively readable way is in Plot's projection source, starting on line 77: the stream is computed as
{stream: (s) => projection.stream(transform.stream(clip(s)))}

which you can read as: "project, then apply the (linear) transform (scale + translate), then clip"