refactor(LineGlyphRepresentation): offset cylinder center

With center offset, don't have to position glyph in center of segment,
just position at state origin.

Sources/Widgets/Representations/LineGlyphRepresentation/index.js

@@ -67,6 +67,8 @@ function vtkLineGlyphRepresentation(publicAPI, model) {
  source: publicAPI,
  glyph: vtkCylinderSource.newInstance({
  direction: [1, 0, 0],
+ center: [0.5, 0, 0],
+ capping: false,
  }),
  mapper: vtkGlyph3DMapper.newInstance({
  orientationArray: 'directions',
@@ -111,13 +113,21 @@ function vtkLineGlyphRepresentation(publicAPI, model) {
  3
  ).getData();
 
- const pos = []; // scratch
+ const tempVector = []; // scratch
  for (let point = 1; point < lines.length - 1; point++) {
+ const glyph = (point - 1) * 3; // start of glyph's 3 components in the arrays
+
+ // With cylinder glyph's offset center, position at state origins.
+ const origin = points[lines[point]];
+ [
+ glyphPositions[glyph],
+ glyphPositions[glyph + 1],
+ glyphPositions[glyph + 2],
+ ] = origin;
+
  // Orient glyphs to next point.
- const eye = points[lines[point]];
  const target = points[lines[point + 1]];
- const direction = vtkMath.subtract(target, eye, pos);
- const glyph = (point - 1) * 3;
+ const direction = vtkMath.subtract(target, origin, tempVector);
  [directions[glyph], directions[glyph + 1], directions[glyph + 2]] =
  direction;
 
@@ -126,16 +136,6 @@ function vtkLineGlyphRepresentation(publicAPI, model) {
  lengths[glyph] = distance;
  lengths[glyph + 1] = 1;
  lengths[glyph + 2] = 1;
-
- // Position glyph at center of line segment.
- vec3.normalize(pos, direction);
- vec3.scale(pos, pos, distance / 2);
- vec3.add(pos, eye, direction);
- [
- glyphPositions[glyph],
- glyphPositions[glyph + 1],
- glyphPositions[glyph + 2],
- ] = pos;
  }
 
  internalPolyData.getPoints().modified();