Alembic import: greatly improved curve/hair import performance

The U-resolution of the imported curves was kept at the default value
of 12, which is way too high for imported hair. We export hair at a
fairly high resolution already, so it's not needed to subdivide even
further when importing.

Of course this may have an impact on other curves that do require this
U-resolution to be higher. In that case the resolution can be
increased after importing.

I removed the default nu->orderu = num_verts, as that allowed every
point to influence the entire spline, which was more expensive for the
CPU, and unlikely to be needed. The orderu computations had off-by-one
errors in the curve importer, which are now also fixed. The correct
values are:

    - Linear: orderu = 2
    - Quadratic: orderu = 3
    - Cubic: orderu = 4

These values are also what is stored in the Alembic file for curves of
type kVariableOrder, according to the reference Maya exporter
maya/AbcExport/MayaNurbsCurveWriter.cpp, function
MayaNurbsCurveWriter::write(), in the Alembic source code.

The result is a frame rate increase of roughly 100x (tested with one
100-hair test on one machine, so take with grain of salt).

1 files changed, 13 insertions, 7 deletions

diff --git a/source/blender/alembic/intern/abc_curves.cc b/source/blender/alembic/intern/abc_curves.cc
index 0542255d84b..28e75db2862 100644
--- a/source/blender/alembic/intern/abc_curves.cc
+++ b/source/blender/alembic/intern/abc_curves.cc
@@ -205,6 +205,7 @@ void AbcCurveReader::readObjectData(Main *bmain, float time)
 
 	cu->flag |= CU_DEFORM_FILL | CU_3D;
 	cu->actvert = CU_ACT_NONE;
+	cu->resolu = 1;
 
 	m_object = BKE_object_add_only_object(bmain, OB_CURVE, m_object_name.c_str());
 	m_object->data = cu;
@@ -250,13 +251,18 @@ void read_curve_sample(Curve *cu, const ICurvesSchema &schema, const float time)
 		nu->pntsv = 1;
 		nu->flag |= CU_SMOOTH;
 
-		nu->orderu = num_verts;
-
-		if (smp.getType() == Alembic::AbcGeom::kCubic) {
-			nu->orderu = 3;
-		}
-		else if (orders && orders->size() > i) {
-			nu->orderu = static_cast<short>((*orders)[i] - 1);
+		switch (smp.getType()) {
+			case Alembic::AbcGeom::kCubic:
+				nu->orderu = 4;
+				break;
+			case Alembic::AbcGeom::kVariableOrder:
+				if (orders && orders->size() > i) {
+					nu->orderu = static_cast<short>((*orders)[i]);
+				}
+				break;
+			case Alembic::AbcGeom::kLinear:
+			default:
+				nu->orderu = 2;
 		}
 
 		if (periodicity == Alembic::AbcGeom::kNonPeriodic) {