Mesh picking - Barycentric coordinates not normalized

[smfloery]

Dear all,

the "face_coords" in the event.pick_info are not normalized (their sum does not equal 1). If one wants to use those coordinates to calculate the coordinates of a picked point on a triangle mesh in world coordinates, this leads to erroneous calculations. While it is not a big problem, as it's enough to divide by their sum, I somehow automatically assumed them to be normalized and it did cost me some time to realize that this might be the problem!

Is this an intended behavior? If so, then I would suggest adding this information somewhere in the documentation. Or is it already mentioned and I didn't find it?

Sebastian

[almarklein]

Hi Sebastian,

You are quite right. The picking info is stored in a limited amount of memory, leaving just 6 bits for each of the coords, so the deviation from 1 is due to rounding errors.

I agree that it would be good to normalize the values so that they add up to one again. That code is here:

pygfx/pygfx/objects/_more.py

Lines 152 to 177 in f4687bc

	def _wgpu_get_pick_info(self, pick_value):
	values = unpack_bitfield(
	pick_value, wobject_id=20, index=26, coord1=6, coord2=6, coord3=6
	)
	face_index = values["index"]
	face_coord = [
	values["coord1"] / 63,
	values["coord2"] / 63,
	values["coord3"] / 63,
	]
	if (
	self.geometry.indices.data is not None
	and self.geometry.indices.data.shape[-1] == 4
	):
	triangle_index = face_index % 2
	face_index = face_index // 2
	if triangle_index == 0:
	# The sub indices are 0, 1, 2, so we just add the zero for index 3.
	face_coord.append(0.0)
	else:
	# The sub indices are 0, 2, 3. The index 3 uses the
	# face_coord slot of index 1, (see meshshader.py), so
	# we put that at the end and put a zero in its place.
	face_coord = face_coord[0], 0.0, face_coord[2], face_coord[1]
	return {"face_index": face_index, "face_coord": tuple(face_coord)}

Would you perhaps be interested in contributing a PR for that?