Groups¶

Groups let you select and operate on subsets of geometry. They're essential for targeted operations like "delete only the top faces" or "extrude just the windows."

What are Groups?¶

A group is a named selection of geometry components (points, edges, or faces).

Complete Mesh:  ●●●●●●●●●●  (10 vertices)
                ↓
Group "top":    ●●●○○○○○○○  (3 vertices selected)

Think of groups as filters that let operations affect only specific parts.

Why Use Groups?¶

Targeted Operations¶

Without groups:

Extrude → Affects entire mesh

With groups:

Group (top faces) → Extrude → Only top faces extrude

Named Selections¶

Save selections for reuse:

Create group "windows" → Use in multiple operations

Complex Workflows¶

Build sophisticated procedural models:

1. Select every other face → Group "pattern_a"
2. Select remaining faces → Group "pattern_b"
3. Color pattern_a red, pattern_b blue

Group Types¶

Point Groups¶

Select vertices:

Group all points where @P.y > 0.5

Use for: - Vertex-based deformations - Color assignments - Position-based selections

Edge Groups¶

Select edges:

Group edges forming boundary loops

Use for: - Beveling specific edges - Edge-based selections

Primitive (Face) Groups¶

Select triangles:

Group faces where @materialid == 2

Use for: - Material assignments - Face deletion - Per-face operations

Creating Groups¶

Use the Group node to create selections.

Method 1: Pattern-Based¶

Select by index patterns:

Example: Every Other Vertex

Group Node:
- Mode: Pattern
- Pattern: "0-100:2"  (0, 2, 4, 6, ... every 2nd)
- Group Name: "alternating"

Pattern Syntax:

0-10        → Indices 0 through 10
0,5,10      → Specific indices 0, 5, 10
0-100:2     → Every 2nd index (step of 2)
0-50,75-100 → Ranges 0-50 and 75-100

Method 2: Expression-Based¶

Select by attribute values:

Example: Upper Half

Group Node:
- Mode: Expression
- Expression: @P.y > 0.0
- Group Name: "upper_half"

Common Expressions:

@P.y > 0.5              → Top vertices
@P.x < 0 and @P.z < 0   → Quadrant selection
@Cd.r > 0.8             → Red vertices
@id % 2 == 0            → Even IDs
length(@P) < 2.0        → Within radius

Method 3: Manual¶

Future feature: Click to select in viewport

Using Groups¶

Deletion¶

Group Delete node removes selected geometry:

Sphere
  ↓
Group (Expression: @P.y < 0)  → Select bottom half
  ↓
Group Delete                   → Remove bottom
  ↓
Result: Hemisphere

Material Assignment¶

Assign materials to grouped faces:

Box
  ↓
Group (Expression: @P.y > 0.99)  → Top face
  ↓
Attribute Create:
  Name: materialid
  Value: 2                       → Material ID 2 for top

Conditional Extrusion¶

Extrude only selected faces:

Grid
  ↓
Group (Pattern: "0-100:3")  → Every 3rd face
  ↓
Extrude (if group exists, only affects grouped faces)

Group Operations¶

Combining Groups¶

Group Combine node merges or intersects groups:

Union (A or B):

Group A: "top_faces"
Group B: "side_faces"
  ↓
Group Combine: Union
  ↓
Result: "top_or_side" (all faces in either group)

Intersection (A and B):

Group A: "red_vertices"  (@Cd.r > 0.8)
Group B: "top_vertices"  (@P.y > 0.5)
  ↓
Group Combine: Intersection
  ↓
Result: "red_and_top" (only vertices in both groups)

Subtraction (A not B):

Group A: "all_faces"
Group B: "bottom_faces"
  ↓
Group Combine: Subtract
  ↓
Result: "not_bottom" (A minus B)

Inverting Groups¶

Select everything not in group:

Group "selected"
  ↓
Group Combine: Invert
  ↓
Result: "not_selected"

Common Patterns¶

Checkerboard Selection¶

Grid (10×10 cells)
  ↓
Group:
  Mode: Expression
  Expression: (floor(@P.x) + floor(@P.z)) % 2 == 0
  Group: "checkers"
  ↓
Group Delete → Removes every other cell

Ring Selection¶

Cylinder
  ↓
Group:
  Mode: Expression
  Expression: abs(@P.y - 0.5) < 0.1
  Group: "ring"
  ↓
Extrude → Only middle ring extrudes

Border Selection¶

Grid
  ↓
Group:
  Mode: Expression
  Expression: abs(@P.x) > 0.9 or abs(@P.z) > 0.9
  Group: "border"

Random Selection¶

Sphere
  ↓
Group:
  Mode: Expression
  Expression: rand(@id) > 0.5
  Group: "random_half"

Height Gradient¶

Cylinder
  ↓
Group:
  Mode: Expression
  Expression: @P.y > $threshold
  Group: "above_threshold"

(Change $threshold to select different heights)

Expression-Based Selection¶

Comparison Operators¶

@P.y > 0.5          → Greater than
@P.y < 0.5          → Less than
@P.y >= 0.5         → Greater or equal
@P.y <= 0.5         → Less or equal
@P.y == 0.5         → Equal (exactly)
@P.y != 0.5         → Not equal

Logical Operators¶

@P.y > 0 and @P.x > 0       → AND (both true)
@P.y > 0 or @P.x > 0        → OR (either true)
not (@P.y > 0)              → NOT (invert)

Complex Conditions¶

(@P.y > 0.5 and @P.y < 0.8) or @Cd.r > 0.9
→ Select if (Y between 0.5 and 0.8) OR (very red)

Math Functions¶

length(@P) < 2.0                → Within radius 2
abs(@P.x - @P.z) < 0.1          → Near diagonal
sin(@P.x * 10) > 0              → Wavy pattern
floor(@P.x) % 2 == 0            → Stripe pattern

Workflows¶

Delete Bottom Half¶

1. Sphere
2. Group:
   Expression: @P.y < 0
   Group: "bottom"
3. Group Delete

Color Stripes¶

1. Grid
2. Group:
   Expression: floor(@P.x * 5) % 2 == 0
   Group: "stripes"
3. Attribute Create:
   Name: Cd
   Value: @stripes ? [1,0,0] : [0,0,1]

(Red and blue stripes)

Extrude Windows¶

1. Box
2. Group:
   Expression: @P.z > 0.99  (front face)
   Group: "front"
3. Subdivide (to create face grid)
4. Group:
   Pattern: "0-100:3"  (every 3rd face)
   Group: "windows"
5. Extrude (inward for window recesses)

Scatter on Selection¶

1. Grid
2. Group:
   Expression: rand(@id) > 0.7
   Group: "scatter_points"
3. Scatter (uses group as distribution)

Group Attributes¶

Groups are stored as point/face attributes:

Group "selected" creates:
@selected = 1  (in group)
@selected = 0  (not in group)

Access in expressions:

@selected == 1  → Check if in group
@mygroup        → Direct group check

Best Practices¶

1. Name Groups Clearly¶

✅ "top_faces", "window_locations", "border_verts"
❌ "group1", "sel", "temp"

2. Document Complex Expressions¶

Expression: (@P.y > $base_height) and (rand(@id) > $density)
Comment: "Select random points above base height"

3. Test on Simple Geometry¶

✅ Test expression on Sphere → Apply to complex model
❌ Write complex expression on 10k poly mesh (hard to debug)

4. Use Graph Parameters¶

Expression: @P.y > $select_threshold

Now $select_threshold is adjustable in UI!

5. Visualize Groups¶

Use colors to verify selections:

Group (create selection)
  ↓
Attribute Create:
  Name: Cd
  Value: @groupname ? [1,0,0] : [0,1,0]

Red = in group, Green = not in group

Troubleshooting¶

Group Not Selecting Anything¶

Check: 1. Expression syntax correct? 2. Attribute exists? (e.g., @P always exists) 3. Value ranges? (is @P.y ever > 0.5?) 4. Group type matches (point vs face)?

Debug:

Add Attribute Create: @debug = @P.y
View values in viewport

Wrong Geometry Selected¶

Fix: 1. Simplify expression 2. Test components: @P.y > 0 first, add more conditions later 3. Visualize with colors

Group Not Affecting Operation¶

Check: 1. Does node support groups? 2. Group name matches? 3. Group was created upstream?

Advanced Techniques¶

Multi-Level Selection¶

1. Group A: "upper_half" (@P.y > 0)
2. Within A, Group B: "upper_corners" (@P.x > 0.5)
3. Result: Corners of upper half

Animated Selection¶

With graph parameters:

Expression: @P.y < $cutoff_height

Animate $cutoff_height → Moving selection plane

Geometric Patterns¶

Radial: length([@P.x, @P.z]) < $radius
Grid: (floor(@P.x) + floor(@P.z)) % 2 == 0
Spiral: atan2(@P.z, @P.x) + length([@P.x,@P.z]) < $param

Neighbor-Based¶

Future: Select based on neighboring vertices

Performance Tips¶

Optimize Expressions¶

✅ @P.y > 0  (fast: single comparison)
❌ length(@P - [complex_calc]) > threshold  (slow: many ops)

Cache Groups¶

If using same selection multiple times:

✅ Create group once → Use many times
❌ Recreate selection in every node

Pattern vs Expression¶

Pattern mode: Fast for index-based selections
Expression mode: Flexible but slower for complex math

Examples¶

Delete Every Other Face¶

Grid
  ↓
Group:
  Mode: Pattern
  Pattern: "0-1000:2"
  ↓
Group Delete

Color by Distance¶

Sphere
  ↓
Group:
  Expression: length(@P) < 0.8
  Group: "inner"
  ↓
Attribute Create:
  Cd: @inner ? [1,0,0] : [0,0,1]

Mask with Noise¶

Grid
  ↓
Attribute Create:
  Name: noise_val
  Value: rand(@id)
  ↓
Group:
  Expression: @noise_val > $threshold

Next Steps¶

Use Groups in Workflows - Procedural Patterns
See Group Node - Group Node Reference
Learn Attributes - Attributes
Advanced Selection - Expression Syntax

Key Takeaways¶

✅ Groups = Named selections of geometry subsets ✅ Types = Point, Edge, Face (primitive) groups ✅ Creation = Pattern (indices) or Expression (attributes) ✅ Operations = Combine, Intersect, Subtract, Invert ✅ Uses = Targeted deletion, coloring, extrusion, etc. ✅ Stored as = Attributes (@groupname)

Remember: Groups are the key to selective operations - enabling complex procedural control!