Houdini UV Mapping Guide: UV Flatten, UV Unwrap and UV Layout

SideFX’s official Houdini documentation describes the UV Flatten, UV Unwrap and UV Layout nodes for different UV preparation needs. UV Unwrap creates initial regions, UV Flatten shapes the unwrap using seams and constraints, and UV Layout places the resulting islands in target areas. The UV Flatten and UV Layout documentation pages state that these nodes have been available since Houdini 17.0; the UV Unwrap page does not specify an initial version.

1. Choose the node that fits your workflow

Before starting UV preparation, clarify your goal: do you need to create an initial unwrap for a complex surface, control seams and shape on an existing unwrap, or fit islands into a particular texture area? The documented nodes address these needs separately.

UV Unwrap projects surfaces onto different planar directions to establish an initial layout. Connected polygons mapped to the same plane form regions, which are placed without overlapping. You can then move the resulting pieces with UV Edit or join them again with UV Fuse.

UV Flatten focuses on seams, which determine where pieces split, and constraints, which guide the unwrap. UV Layout can be used to pack existing UV islands. Depending on the model, one option is to create the unwrap with UV Unwrap or UV Flatten, then arrange it with UV Layout; this sequence is not mandatory for every model.

2. Work with seams and constraints in UV Flatten

1. Make the UV view visible in the viewport. You can switch to a multi-pane layout and set one pane to Set view › UV Viewport, or use Space + 5 while the pointer is over the relevant pane.

2. Select the geometry and launch UV Flatten from the Texture tab. You can change settings in the parameter editor or the operation toolbar above the viewport.

3. Define where the unwrap should split by setting seams. UV Flatten does not add seams automatically. If there are no seams, the node may fall back to projecting the geometry onto the UV plane; for anything beyond simple models, this can produce overlapping pieces or significant distortion. You can add seams manually with the Cut tool or use UV Auto Seam earlier in the node chain.

4. Use constraints such as pinning, alignment, straightening or rectangularization to shape the unwrap. A pin fixes a point at a UV coordinate; surrounding points may move to accommodate the new position. If constraints conflict, edges may not align exactly with the intended direction.

5. In interactive editing, press Repack to incorporate changes into the layout. This passes unwrap constraints added during interaction to the packing stage and repositions the pieces. For a procedural workflow where parameter changes are reflected in the output each time, turn off Enable manual layout.

3. Create initial regions with UV Unwrap

1. Select UV Unwrap from the Texture tab, mark the polygons to unwrap, and confirm the selection with Enter.

2. In the parameter editor, set the planes to use, the arrangement of regions, scaling behavior and spacing. UV Unwrap projects surface pieces onto planes while trying to minimize distortion, so the result depends on the chosen plane arrangement and the model’s geometry.

3. Choose a layout style to suit your needs. Strip arranges connected regions consecutively along the U direction. Square attempts to create a roughly square layout, starting with the larger regions.

4. Consider the differences between the scaling options. None does not change the scale; Uniform fits the layout to the unit square with uniform scaling; Stretch may scale horizontally and vertically by different amounts to fill the square. You can edit the unwrap with UV Edit and use UV Fuse where regions need to be joined. This node generates UV coordinates at the vertex level.

4. Pack islands and use UDIMs with UV Layout

UV Layout can place islands in the 0,0–1,1 unit square, in rectangles of different sizes and positions, or on UDIM targets. The documentation describes a typical UDIM grid as 10 cells wide and gives 1001 as the number of the bottom-left cell; some properties of this layout can be changed using node parameters.

1. Use UV Layout after the existing UV islands. The coordinate attribute is usually named uv. To process only specific polygons, use the Group field.

2. Set the scaling method and targets. Largest packable arranges islands at the largest size that fits the targets. When Fixed is selected, the specified scale is preserved; islands that do not fit may be placed outside the target bounds.

3. Evaluate rotation and spacing settings by checking the result. Trying more rotations can produce a better layout, but may increase computation time and reduce axis alignment. Island Padding defines the spacing between islands relative to Search resolution. To add spacing between islands and target boundaries as well, enable Apply Padding Between Island Target Boundary.

4. If packing only one polygon group, check whether unselected islands are being taken into account. With Avoid Packing Over Non-Group Island enabled, the node attempts to place selected islands around the others; when disabled, the other islands may be ignored, creating a risk of overlap.

What should you consider in an architectural visualization workflow?

In interior and architectural models, different surface groups, texture area usage and island arrangement can all matter. Houdini’s documentation states that UV Layout supports UDIM targets, rectangles and island scaling options, while UV Flatten offers interactive seam and constraint editing. These features can help you control UV preparation for each project, but the right unwrap depends on the model’s geometry and the desired texture layout.

UV Flatten’s interactive layout is designed for quick adjustments during work. If you need tighter packing or different target options, you can consider UV Layout as a separate stage. The available sources provide no information about hardware requirements, licensing terms or compatibility with other software, so no specific conclusions should be drawn on these topics.

Common mistakes

• Expecting an unwrap from UV Flatten without adding seams: complex models may produce overlapping or distorted pieces.

• Skipping the required Repack step after changing interactive constraints, then expecting to see the updated layout.

• Overlooking whether unselected islands are included in the packing calculation when using Group in UV Layout.

• Assuming Fixed scaling will fit every island into the target area; pieces that do not fit may remain outside it.