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Fix #148436: Incorrect legacy hair offset with USD export

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The report in question uncovered two sources of issues. The primary one
being that there was an accidental double-transform from placing the
basis curves prim under the main Xform of the mesh too. This is solved
by considering the inverse of mesh's world transform when writing out
the curve points.

The second was exposed with the new, more correct, viewport drawing of
curves that showed that we were exporting the wrong curve type. This
would manifest as a disappearing curve segment at the beginning and end
of the curve. Fixed by explicitly writing out catmull-rom, pinned,
curves for the hair rather than using bsplines.

As a result the Storm-USD tests now match much closer to the native
Storm-Hydra variants.

Pull Request: https://projects.blender.org/blender/blender/pulls/148543
This commit is contained in:
Jesse Yurkovich 2025-10-22 22:26:28 +02:00 • committed by Jesse Yurkovich
parent 6cbbb54d8c
commit 67752cc66e
15 changed files with 52 additions and 38 deletions
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22
source/blender/io/usd/intern/usd_writer_hair.cc
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@ -9,6 +9,9 @@
#include "BKE_particle.h"
#include "BLI_math_matrix.hh"
#include "BLI_math_vector_types.hh"
#include "DNA_particle_types.h"
namespace blender::io::usd {
@ -27,14 +30,24 @@ void USDHairWriter::do_write(HierarchyContext &context)
pxr::UsdGeomBasisCurves curves = pxr::UsdGeomBasisCurves::Define(usd_export_context_.stage,
usd_export_context_.usd_path);
/* TODO(Sybren): deal with (psys->part->flag & PART_HAIR_BSPLINE) */
curves.CreateBasisAttr(pxr::VtValue(pxr::UsdGeomTokens->bspline));
curves.CreateTypeAttr(pxr::VtValue(pxr::UsdGeomTokens->cubic));
if (psys->part->flag & PART_HAIR_BSPLINE) {
curves.CreateBasisAttr(pxr::VtValue(pxr::UsdGeomTokens->bspline));
curves.CreateTypeAttr(pxr::VtValue(pxr::UsdGeomTokens->cubic));
}
else {
curves.CreateBasisAttr(pxr::VtValue(pxr::UsdGeomTokens->catmullRom));
curves.CreateTypeAttr(pxr::VtValue(pxr::UsdGeomTokens->linear));
curves.CreateWrapAttr(pxr::VtValue(pxr::UsdGeomTokens->pinned));
}
pxr::VtArray<pxr::GfVec3f> points;
pxr::VtIntArray curve_point_counts;
curve_point_counts.reserve(psys->totpart);
/* Reverse current transform since the Hair curves will be placed under the object's Xform and we
* don't want a double-transform to happen. */
const blender::float4x4 inv = blender::math::invert(context.object->object_to_world());
ParticleCacheKey *strand;
for (int strand_index = 0; strand_index < psys->totpart; ++strand_index) {
strand = cache[strand_index];
@ -43,7 +56,8 @@ void USDHairWriter::do_write(HierarchyContext &context)
curve_point_counts.push_back(point_count);
for (int point_index = 0; point_index < point_count; ++point_index, ++strand) {
points.push_back(pxr::GfVec3f(strand->co));
const float3 vert = blender::math::transform_point(inv, float3(strand->co));
points.push_back(pxr::GfVec3f(vert.x, vert.y, vert.z));
}
}

4
tests/files/render/hair/storm_usd_renders/hair_basemesh_intercept.png
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@ -1,3 +1,3 @@
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4
tests/files/render/hair/storm_usd_renders/hair_close_up.png
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tests/files/render/hair/storm_usd_renders/hair_info.png
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tests/files/render/hair/storm_usd_renders/hair_instancer_uv.png
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tests/files/render/hair/storm_usd_renders/hair_linear_close_up.png
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4
tests/files/render/hair/storm_usd_renders/hair_particle_random.png
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4
tests/files/render/hair/storm_usd_renders/hair_ribbon_close_up.png
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4
tests/files/render/hair/storm_usd_renders/principled_hair_absorptioncoefficient.png
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4
tests/files/render/hair/storm_usd_renders/principled_hair_directcoloring.png
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4
tests/files/render/hair/storm_usd_renders/principled_hair_melaninconcentration.png
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4
tests/files/render/mesh/storm_usd_renders/visibility_particles.png
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4
tests/files/render/motion_blur/storm_usd_renders/curve_motion_blur.png
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4
tests/python/bl_usd_export_test.py
View file

@ -535,8 +535,8 @@ class USDExportTest(AbstractUSDTest):
hair_curves = UsdGeom.BasisCurves(hair_prim)
hair_samples = hair_curves.GetPointsAttr().GetTimeSamples()
self.assertEqual(hair_curves.GetTypeAttr().Get(), "cubic")
self.assertEqual(hair_curves.GetBasisAttr().Get(), "bspline")
self.assertEqual(hair_curves.GetTypeAttr().Get(), "linear")
self.assertEqual(hair_curves.GetBasisAttr().Get(), "catmullRom")
self.assertEqual(len(hair_samples), 10)
def check_primvar(self, prim, pv_name, pv_typeName, pv_interp, elements_len):

16
tests/python/bl_usd_import_test.py
View file

@ -904,8 +904,8 @@ class USDImportTest(AbstractUSDTest):
self.assertAlmostEqual(f.evaluate(10), 0.0, 2, "Unexpected value for rotation quaternion Z curve at frame 10")
def check_curve(self, blender_curve, usd_curve):
curve_type_map = {"linear": 1, "cubic-bezier": 2, "cubic-bspline": 3}
cyclic_map = {"nonperiodic": False, "periodic": True}
curve_type_map = {"linear-bezier": 1, "linear-catmullRom": 1, "cubic-bezier": 2, "cubic-bspline": 3}
cyclic_map = {"pinned": False, "nonperiodic": False, "periodic": True}
# Check correct spline count.
blender_spline_count = len(blender_curve.attributes["curve_type"].data)
@ -913,10 +913,7 @@ class USDImportTest(AbstractUSDTest):
self.assertEqual(blender_spline_count, usd_spline_count)
# Check correct type of curve. All splines should have the same type and periodicity.
usd_curve_type = usd_curve.GetTypeAttr().Get()
usd_curve_type_basis = usd_curve_type
if usd_curve_type != "linear":
usd_curve_type_basis = usd_curve_type + "-" + usd_curve.GetBasisAttr().Get()
usd_curve_type_basis = usd_curve.GetTypeAttr().Get() + "-" + usd_curve.GetBasisAttr().Get()
usd_cyclic = usd_curve.GetWrapAttr().Get()
expected_curve_type = curve_type_map[usd_curve_type_basis]
expected_cyclic = cyclic_map[usd_cyclic]
@ -935,7 +932,7 @@ class USDImportTest(AbstractUSDTest):
blender_positions = blender_curve.attributes["position"].data
point_count = 0
if usd_curve_type_basis == "linear":
if usd_curve_type_basis in ("linear-bezier", "linear-catmullRom"):
point_count = len(usd_positions)
self.assertEqual(len(blender_positions), point_count)
elif usd_curve_type_basis == "cubic-bezier":
@ -957,10 +954,13 @@ class USDImportTest(AbstractUSDTest):
if usd_curve_type_basis == "cubic-bspline":
return
if not usd_curve.GetWidthsAttr().IsAuthored():
return
usd_width_interpolation = usd_curve.GetWidthsInterpolation()
usd_radius = [w / 2 for w in usd_curve.GetWidthsAttr().Get()]
blender_radius = [r.value for r in blender_curve.attributes["radius"].data]
if usd_curve_type_basis == "linear":
if usd_curve_type_basis == "linear-bezier":
if usd_width_interpolation == "constant":
usd_radius = usd_radius * point_count