LineSegments2 : Fix raytracing when the mesh has a transformation applied.

[MixMasterMitch]

Description

The LineSegments2 mesh has support for raytracing in "world units" mode. However, when checking for intersection with each line segment, the segments do not have the world matrix transformation applied. Therefore, the raytracing does not behave correctly if the mesh has any transformation applied to it. This PR fixes this by applying the world matrix to each line segment before checking for an intersection with the ray.

The behavior before and after can be reproduced by applying a transformation in the Line2 ray tracing example (/examples/#webgl_lines_fat_raycasting). Here is an example patch that can be used:

diff --git a/examples/webgl_lines_fat_raycasting.html b/examples/webgl_lines_fat_raycasting.html
index f923378abd..3067935181 100644
--- a/examples/webgl_lines_fat_raycasting.html
+++ b/examples/webgl_lines_fat_raycasting.html
@@ -327,7 +327,8 @@
                                        'visualize threshold': matThresholdLine.visible,
                                        'width': matLine.linewidth,
                                        'alphaToCoverage': matLine.alphaToCoverage,
-                                       'threshold': raycaster.params.Line2.threshold
+                                       'threshold': raycaster.params.Line2.threshold,
+                                       'translation': raycaster.params.Line2.threshold
                                };

                                gui.add( param, 'line type', { 'LineGeometry': 0, 'LineSegmentsGeometry': 1 } ).onChange( function ( val ) {
@@ -371,6 +372,14 @@

                                } );

+                               gui.add( param, 'translation', 0, 10 ).onChange( function ( val ) {
+
+                                       segments.position.x = val;
+                                       segments.updateMatrix();
+                                       segments.updateMatrixWorld();
+
+                               } );
+
                        }

                </script>

[MixMasterMitch]

How do I get this merged? @gkjohnson I have seen you approve some similar PRs. Can you approve this one and merge it? Thanks in advance.

[gkjohnson]

cc @ariel-resonai @bergden-resonai I think this change looks right but I'd appreciate if you guys can confirm.

[bergden-resonai]

@gkjohnson Works for me, I think the change to the example is great and should be added to make it easier to find future issues with this

https://github.com/MixMasterMitch/three.js/pull/1/files

[gkjohnson]

This looks like a good change - at the moment it looks like raycasting will only work with world units when the mesh has no transform.

I have not tested with a non identity world transform so I'm trusting the contributor has validated this but you can see at least that the example behavior is still correct:

https://raw.githack.com/MixMasterMitch/three.js/line-segments-2-world-units-raycast-fix/examples/webgl_lines_fat_raycasting.html

[bergden-resonai]

@gkjohnson do you think adding the translation and/or rotation to the example is helpful?
Translation

Both