LineSegmentsGeometry: Calling "setPositions" twice does not update the lines correctly

Describe the bug

Calling LineSegmentsGeometry.setPositions a second time with more line segments still only renders the amount of line segments specified in the first call. Everything works as expected if you call “dispose” before calling “setPositions” again.

I’m not exactly sure why it’s happening. Perhaps it’s an issue related to instanced geometry rendering?

To Reproduce

Steps to reproduce the behavior:

1. Create a LineSegments2 object
2. Call line.geometry.setPositions with 1 line segment
3. See that it renders 1 line
4. 1 second later call line.geometry.setPositions with 2 line segments
5. See that it only renders the first of the two lines

Code

line = new LineSegments2();
line.geometry.setPositions( [ 0, -0.5, 0, 0, 0.5, 0 ] );

setTimeout( () => {

  // uncomment to display both lines
  // line.geometry.dispose();
  line.geometry.setPositions( [
    0, -0.5, 0, -0.5, 0.5, 0,
    0, 0, 0, 0.5, 0, 0,
  ] );

}, 1000 );

Live example

https://jsfiddle.net/rhjeL01n/

Expected behavior

The correct amount of lines are rendered after calling “setPositions”.

Screenshots

If applicable, add screenshots to help explain your problem (drag and drop the image).

Platform:

• Device: Desktop
• OS: Windows
• Browser: Chrome
• Three.js version: latest (r126)

[WestLangley]

@gkjohnson

1. Could you create a sufficiently-large buffer and then set InstancedBufferGeometry.instanceCount? That would be one way to resize.

2. Yes, _maxInstanceCount is a bit hacky. We knew that when it was added.

3. I agree it is hard to update the values in the buffers because the indexing is complex. I would welcome API improvements in that regard.

[Mugen87]

Is it the position of the project that setting or removing BufferAttributes on BufferGeometry between renders without calling dispose is not allowed?

You should modify the existing buffers. Not overwrite them (with new sizes). Nowhere in the examples is your pattern demonstrated.

If you need to change the size, dispose the geometry (since you can’t dispose buffer attributes) and create a new one. Feel free to clarify this in the documentation.