Mesh translates in world space incorrectly when pivot set

[JohnK]

The problem

Create a box

fig 1

Use

box.setPivotMatrix(BABYLON.Matrix.Translation(-0.5, 0.5, 0.5));

To set the pivot at the lower right front corner (shown by small grey sphere).

fig 2

Position the box at (2, 0, 0) and note that it is the pivot that is positioned at (2, 0, 0).

box.position = new BABYLON.Vector3(2, 0, 0);

fig 3

Delete box.position and replace with

box.translate(BABYLON.Axis.X, 2, BABYLON.Space.LOCAL);

As you would expect since the local and world axes are in alignment you get the same result.

fig 4

Now replace LOCAL with WORLD

box.translate(BABYLON.Axis.X, 2, BABYLON.Space.WORLD);

Since local and world axes are in alignment and translate is a vector displacement and the displacement is the same then you would expect the box to be at the same position. However it is not as is seen below.

fig 5

Probable Issue

Getting the position of the box for fig 2 it is at (0, 0, 0) ie the position of the pivot, the absolutePosition for the box in fig 2 is (-0.5, 0.5, 0.5) the position of the box's center in world space.

So a translation in either local or world space would give the boxes position as (2, 0, 0) and its absolute position as (1.5, 0.5, 0.5) which it is for fig 3 using position and for fig 4 for translate in local space.

However translation in world space gives a position of (1.5, 0.5, 0.5) and an absolutePosition of (1, 1, 1) = (1.5, 0.5, 0.5) + (-0.5, 0.5, 0.5) . It is out by the pivot vector.

Tracking down translate in Mesh/babylon.abstractMesh.ts

public translate(axis: Vector3, distance: number, space?: Space): AbstractMesh {
  var displacementVector = axis.scale(distance);
  if (!space || (space as any) === Space.LOCAL) {
     var tempV3 = this.getPositionExpressedInLocalSpace().add(displacementVector);
     this.setPositionWithLocalVector(tempV3);
  }
  else {
     this.setAbsolutePosition(this.getAbsolutePosition().add(displacementVector));
  }
  return this;
}

where for Space.WORLD there is a call to 'setAbsolutePosition' the code for which is

public setAbsolutePosition(absolutePosition: Vector3): AbstractMesh {
  if (!absolutePosition) {
     return;
  }
  var absolutePositionX;
  var absolutePositionY;
  var absolutePositionZ;
  if (absolutePosition.x === undefined) {
     if (arguments.length < 3) {
       return;
     }
     absolutePositionX = arguments[0];
     absolutePositionY = arguments[1];
     absolutePositionZ = arguments[2];
   }
   else {
     absolutePositionX = absolutePosition.x;
     absolutePositionY = absolutePosition.y;
     absolutePositionZ = absolutePosition.z;
   }
   if (this.parent) {
      var invertParentWorldMatrix = this.parent.getWorldMatrix().clone();
      invertParentWorldMatrix.invert();
      var worldPosition = new Vector3(absolutePositionX, absolutePositionY, absolutePositionZ);
      this.position = Vector3.TransformCoordinates(worldPosition, invertParentWorldMatrix);
   } else {
       this.position.x = absolutePositionX;
       this.position.y = absolutePositionY;
       this.position.z = absolutePositionZ;
   }
   return this;
}

which takes into account a parent but not a pivot. Also it sets the absolutePosition by setting only the position. However as is seen above when using a pivot, like for the parent case, position and absolute position are different by the pivot vector.

Possible Solution

Would be very nervous of suggesting changes to 'setAbsolutePosition' as I would not know what else it might affect.

So suggest the following change to translate - 'add(this.getPivotPoint())'  - but am not submitting a PR since I am not absolutely certain this is the correct way to go. Have tried it locally and it gives me what I expect but do not know whether it affects anything else and will wait until better BJS minds than mine check it out.

public translate(axis: Vector3, distance: number, space?: Space): AbstractMesh {
  var displacementVector = axis.scale(distance);
  if (!space || (space as any) === Space.LOCAL) {
     var tempV3 = this.getPositionExpressedInLocalSpace().add(displacementVector);
     this.setPositionWithLocalVector(tempV3);
  }
  else {
     this.setAbsolutePosition(this.getAbsolutePosition().add(displacementVector).add(this.getPivotPoint()));
  }
  return this;
}

 

[Sebavan]

Amazing details, thx. I completely agree it is an issue and I will try deeper your proposed solution today.

The fix should be in tonight.

[Sebavan]

I was not able to test deeply enough today, I ll check more tomorrow.

[Sebavan]

The issue looks deeper. We ll check with @Deltakosh next week as there is more issues and potential impacts than expected.

I close it from here and you can follow the open issue on Github:

https://github.com/BabylonJS/Babylon.js/issues/2896

[GameMonetize]

Will work on it this monday

[GameMonetize]

It is fixed.

 

I introduced a way to set a pivot matrix only (and not a premult only matrix):

https://www.babylonjs-playground.com/#KCMKTC (check line #28)