Data and streaming

Streaming point replacement

Switch the feed on. The whole cloud is sent again each frame under the handle it was first given, so nothing is shown twice and nothing is left behind. Uploading again would also draw, and would leave a cloud resident for every frame it ran.

104 of 136

The call

plot.replacePoints(handle: PointsHandle, points: PointSet): void

Replace the points behind handle, keeping the handle.

What a live cloud should call each time its data changes. Uploading again also works and is what most people reach for first, but it issues a new handle every time and leaves the old cloud resident: a page feeding an instrument would leak a cloud a frame.

This sends the whole cloud, which is the right call when most of it changed or when its length moves every time. A feed rewriting part of a cloud that stays the same size wants writePoints, which costs the part rather than the cloud.

calls/streaming-point-replacement

const cloud = plot.uploadPoints(shell(count));
plot.showPoints(cloud);
plot.frameScene();
return {
  readout: `${count.toLocaleString("en-GB")} points`,
  stream(at) {
    // A cloud whose every point moved: the radius breathes.
    plot.replacePoints(cloud, shell(count, 10 + Math.sin(at * 3)));
  },
};

Also called

3 in the code above, in the order it makes them

plot.uploadPoints(points: PointSet): PointsHandle

Upload a point cloud and return a handle naming it.

plot.showPoints(handle: PointsHandle): void

Draw the point cloud handle names.

plot.frameScene(): void

Point the camera at the middle of everything uploaded, far enough back to see it.

Call after uploading. A scene measured in grid cells or in millimetres is nowhere near where a camera starts, and without this the first thing a caller sees is an empty canvas.