UV#

UV projection, unwrapping, layout and seams.

9 nodes.

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Contents#

• UV Auto Seams (PCG) — Finds UV seams automatically and writes them as an edge group for UV Flatten.
• UV Flatten (PCG) — Flattens surface pieces into UV space with a conformal (LSCM), spectral (SCP) or ABF solver.
• UV Fuse (PCG) — Welds vertices that are close together in UV space by averaging their UVs.
• UV Layout (PCG) — Finds UV islands and packs them into the 0..1 square (or UDIM tiles).
• UV Pelt (PCG) — Unwraps one chart by pinning its longest boundary loop to a frame and relaxing the interior with springs.
• UV Project (PCG) — Generates UVs by projection: planar, cylindrical, spherical, box, per-face, rows and columns, toroidal or plastic wrap.
• UV Smooth (PCG) — Relaxes UVs within UV space to smooth out distortion.
• UV Transform (PCG) — Translates, rotates and scales UVs in UV space.
• UV Unwrap (PCG) — Unwraps by assigning each face to its best projection plane, grouping connected faces into regions and laying the regions out.

Nodes#

UV Auto Seams (PCG)#

PCG_UVAutoSeams · category PCG/uv · Houdini: UV Autoseam (SOP)

Finds UV seams automatically and writes them as an edge group for UV Flatten.

Seams must satisfy two unrelated requirements: they should look good (cut along creases and separate high curvature so distortion is low) and they must make every piece a topological disk, otherwise LSCM has no solution. So candidate seams are chosen heuristically first and then completed with a topological algorithm.

With Make Disks on, each chart is guaranteed to be a disk, checked by the Euler characteristic V - E + F == 1 (V counts wedges, seam edges count twice). The completion uses a tree-cotree construction: cutting every edge outside a spanning tree of the dual graph always gives a disk, then dangling branches are pruned to return most edges. On a sphere this prunes to nothing, which is correct since a sphere needs one cut: the longest path found in the leftovers is protected from pruning, so one meridian-like cut remains.

Modes: angle cuts creases only; charts splits by normal cones (like Smart UV Project); both does both; existing_islands starts from edges where the existing vertex UVs are already discontinuous. Smaller Cone Angle gives more charts and lower distortion.

Include Edges and Island Attribute enforce cuts, Exclude Edges avoids them (Make Disks also routes around them; if they form a closed ring, one is still cut). Group limits cutting to edges touching the selected primitives.

Inputs

Outputs

Parameters

Compared with Houdini

Corresponds to UV Autoseam. Houdini's default Curvature Based splitting (granular regions that are then merged) is a different algorithm and is not reproduced; Enforce / Avoid cuts correspond to Include / Exclude Edges and Island Attribute.

UV Flatten (PCG)#

PCG_UVFlatten · category PCG/uv · Houdini: UV Flatten (SOP)

Flattens surface pieces into UV space with a conformal (LSCM), spectral (SCP) or ABF solver.

Projection only works for roughly planar, cylindrical or spherical shapes. For arbitrary shapes this node finds (u,v) so each triangle maps from 3D to 2D as a rotation and uniform scale with as little shear as possible. The conformal condition (Cauchy-Riemann) is linear, so the solution is one sparse least-squares problem rather than an iterative optimisation. The solver is a Jacobi-preconditioned CGNR written in numpy, started from the projection onto the best-fit plane, which is already near the answer for developable surfaces.

Seams do not cut the geometry. The unknowns are wedges: two corners belong to the same wedge when they share a point and the edge between them is not a seam. Each side of a seam is a separate wedge with its own (u,v). This is why UVs must be a vertex attribute here. Without a Seam Group the geometry's own boundary is used.

Closed pieces cannot be flattened (a sphere has no isometric flattening, Gauss's Theorema Egregium). The node raises an error telling you to select a ring of edges as seams, rather than producing overlapping UVs; UV AutoSeams can do this.

Methods: lscm is the original solver that pins two points; scp is spectral conformal, which pins nothing so the boundary is not pulled by pins; abf solves angles first and reconstructs (lowest area distortion, slowest).

With Normalize on all islands share one scale so the largest fills 0..1 and the others keep their relative size. Islands with two or more pins are not rescaled. The optional distortion attribute stores the UV area over 3D area per primitive divided by the global median; 1 means typical texel density. A conformal map preserves angles, not area, so it will not be 1 everywhere; use it to find parts that are stretched too much.

Inputs

Outputs

Parameters

Solver

Pack

Output

Compared with Houdini

Corresponds to UV Flatten. Houdini 3.0 offers only SCP and ABF; LSCM is kept as the plugin's original behaviour and default. Pins stay at their existing UVs; per-pin coordinate entry is not available (use UV Transform first).

UV Fuse (PCG)#

PCG_UVFuse · category PCG/uv · Houdini: UV Fuse (SOP)

Welds vertices that are close together in UV space by averaging their UVs.

Vertices whose UVs are within Distance of each other are merged to their average. Shared Points only welds vertices that sit on the same geometric point, which cleans up small gaps left by a projection. Distance ignores whether the vertices share a point, so it can stitch the touching edges of two islands together.

Inputs

Outputs

Parameters

UV Layout (PCG)#

PCG_UVLayout · category PCG/uv · Houdini: UV Layout (SOP)

Finds UV islands and packs them into the 0..1 square (or UDIM tiles).

Islands are found from UV connectivity, not geometric connectivity: two faces sharing an edge are not connected in UV when the UVs on the two sides of that edge differ, which is the definition of a seam. Splitting by geometry is a common mistake and shows up as a six-way box projection that still looks like a single blob after packing.

Packing: shelf fills shelves, wrapped in a binary search for the largest scale that fits (the fit test is cheap, so the search lands on the maximum texel density). raster rasterizes islands into masks and interlocks them by shape, trying several rotations, so triangular and L-shaped islands do not each reserve a bounding box; it typically raises coverage on reduced rocks from about 40% to over 60%. Pack Resolution is the occupancy grid size: larger is tighter and slower.

Rotate to Fit uses rotating calipers to find the minimum-area bounding rectangle orientation (testing only the convex hull edges is enough); a 35 degree strip dropped from 0.76 to 0.47 occupancy. Scale Mode uniform uses one scale for all islands (consistent texel density, what baking wants); individual normalizes each island by its longer side first (sizes evened out); fixed applies Fixed Scale as is and does not search for the maximum, islands that do not fit are placed outside the target and added to the Nonpacked group. Pack Into UDIM Tiles packs across several tiles starting at Starting UDIM with Columns per row (with Fixed, tiles open as needed).

Inputs

Outputs

Parameters

Islands to Pack

Packing

Targets

Output

Compared with Houdini

Corresponds to UV Layout. Pack Into supports the unit square and UDIM tiles only; packing into rectangles and islands from a second input are not implemented.

UV Pelt (PCG)#

PCG_UVPelt · category PCG/uv · Houdini: UV Pelt (SOP)

Unwraps one chart by pinning its longest boundary loop to a frame and relaxing the interior with springs.

Unlike UV Flatten, which solves a conformal energy with a free boundary, Pelt works like stretching an animal skin. The surface is cut along the seams into a topological disk, the longest boundary loop of the chart containing the Hint Primitive is attached by arc length to a frame (a circle by default; if a closed face is connected to the second input, its P.x and P.y are used as U and V), and the interior is pulled flat by springs.

The Pelting Method sets spring weights: spring_relaxation uses adjacent triangle area over squared edge length; tutte_barycentric uses unit weights, which cannot flip faces with a convex frame; discrete_harmonic uses cotangent weights. The boundary is not hard pinned: each boundary point is attached to its frame position by a spring of strength Boundary Springs, and the resulting linear system is solved with conjugate gradients. Unknowns are wedges, so seams do not cut the geometry.

Only the chart that contains the hint primitive is unwrapped; the UVs of all other faces are left as they are (0 if there were none).

Inputs

Outputs

Parameters

Pelt

Frame

Compared with Houdini

Corresponds to UV Pelt 2.0. The exact spring weight formulas are not documented by Houdini; the area / length^2 form here is this plugin's choice.

UV Project (PCG)#

PCG_UVProject · category PCG/uv · Houdini: UV Project (SOP), UV Texture (SOP)

Generates UVs by projection: planar, cylindrical, spherical, box, per-face, rows and columns, toroidal or plastic wrap.

UVs are written as a vertex attribute by default, not point. A point can have several UV positions (either side of a seam, island borders), which a point attribute cannot store. Cylindrical and spherical projection always create a seam; as point UVs, the faces along it would interpolate u from 0.98 back to 0.02 and smear the texture.

Fix Seams checks each face's u range and, if it spans more than half a period, adds 1 to the smaller half, so a seam face gets u of (0.98, 0.99, 1.01, 1.02) which is continuous. Values outside 0..1 are fine because textures repeat. This is only possible with vertex attributes.

Modes: planar, cylindrical, spherical project along Axis. box is six-sided and picks the axis by face normal (point normals would tear faces apart in UV space), giving six islands for UV Layout. face gives each face its own 0..1 square (for brick or card per-piece textures). rows_columns assigns row and column UVs to a structured quad grid by BFS on the dual graph, independent of primitive order. toroidal wraps u around the main axis and v around the tube (for tori); plastic_wrap wraps from the positive axis end down, pinching only at the back and with no seam.

By default the projection is centered on the bounding box and fitted to 0..1, then rotated about (0.5, 0.5), scaled and translated. With Projector Transform on, Houdini semantics apply: a unit-size projector (square, cylinder, sphere or torus) is placed in the scene with the given transform, UVs come from the parameters on that projector and are remapped by U / V Range, without centering or fitting. Box, Face and Rows and Columns are not affected by the projector.

Inputs

Outputs

Parameters

Transform

Projector

Compared with Houdini

Combines UV Texture and UV Project. The Projector section follows UV Project's Transformation page. Plastic Wrap uses an azimuthal equidistant mapping built from Houdini's description, as its exact formula is not published.

UV Smooth (PCG)#

PCG_UVSmooth · category PCG/uv · Houdini: UV Smooth (SOP)

Relaxes UVs within UV space to smooth out distortion.

Corners at the same point with the same UV form one wedge, so the two sides of a seam move independently and are not welded together. Edge weights are cotangent (computed on the 3D triangles, with linear precision: planar regions with linear UVs stay unchanged and only distortion that disagrees with the 3D shape is removed) or uniform.

relax runs a number of iterations, each moving every free wedge toward the weighted average of its neighbours by Strength. harmonic solves directly to convergence, equivalent to infinitely many relax steps; it needs fixed wedges (Pin Boundary or a group), otherwise an island would collapse to a point. Pin Boundary fixes the boundary of each UV island including seams; points outside the group are also fixed.

Inputs

Outputs

Parameters

Compared with Houdini

Corresponds to UV Smooth.

UV Transform (PCG)#

PCG_UVTransform · category PCG/uv · Houdini: UV Transform (SOP)

Translates, rotates and scales UVs in UV space.

Each UV is scaled and rotated about the pivot and then translated. With Per Island on, every island is transformed about its own center instead of the Pivot. This is what you want to rotate scattered islands by 90 degrees each, rather than orbiting them around the center of the atlas.

Inputs

Outputs

Parameters

Compared with Houdini

Corresponds to UV Transform.

UV Unwrap (PCG)#

PCG_UVUnwrap · category PCG/uv · Houdini: UV Unwrap (SOP)

Unwraps by assigning each face to its best projection plane, grouping connected faces into regions and laying the regions out.

Not the same as UV Flatten: nothing is solved. Each face picks the plane with the smallest projection distortion, i.e. the largest absolute dot product between face normal and plane normal. By default the planes are the three axis planes, rotated together by Rotate; if polygons are connected to the second input, they are used as planes instead. Connected faces projected onto the same plane form one region, and regions are placed side by side: strip in a row, square filled into a square from largest to smallest. Scale then sizes the result.

Faces on the back of a plane form their own region with u negated, so that the UV handedness matches the face winding (not mirrored). Faces outside Group keep their UVs.

Inputs

Outputs

Parameters

Compared with Houdini

Corresponds to UV Unwrap. Houdini's menu for default plane orientations is not reproduced; only the axis coordinate planes are available.