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Material System Skill

Critical Rules

🚨 Connecting Function Call Inputs: Use Bare Names, Not Display Names

get_expression_pins(mat, expr_id) (MaterialNodeService) returns input pins with display labels including type suffixes like "TextureObject (T2d)", "TextureSize (V3)", "Use High Quality Normals (SB)". Connections are made through the engine MaterialTools.connect_expressions call, and UE's connect API matches against the bare input name only — never pass these display labels as to_input_name:

MT = "editor_toolset.toolsets.material.MaterialTools"
# ❌ WRONG — display label with type suffix; connection fails silently or gets ignored
call_tool(tool_name="connect_expressions", toolset_name=MT,
          arguments={"from_expression": src_id, "from_output_name": "",
                     "to_expression": fn_id, "to_input_name": "TextureObject (T2d)"})

# ✅ CORRECT — bare name
call_tool(tool_name="connect_expressions", toolset_name=MT,
          arguments={"from_expression": src_id, "from_output_name": "",
                     "to_expression": fn_id, "to_input_name": "TextureObject"})

The authoritative source for input names is get_function_info(function_path).inputs — those are the bare names (TextureObject, TextureSize, WorldPosition, etc.). For built-in expressions, use get_inputs_for_material_expression from unreal.MaterialEditingLibrary.

⚠️ Why this is easy to miss: get_expression_pins returns is_connected=True for both real and phantom connections, and our service used to silently produce phantom wires when given display names. The wires would appear in the graph export but the shader compiler would never see the texture flow through. Always verify with unreal.MaterialEditingLibrary.get_used_textures(mat) after wiring — if it returns 0 on a material that samples textures, your connections are phantom.

🚨 Verifying Wiring: Use get_material_diagnostics

MaterialNodeService.get_material_diagnostics(path) is the canonical way to verify a material is sampling textures and compiling cleanly. It returns the real compile errors, the textures the shader actually references, and node-type counts. Always run it after non-trivial graph rewiring:

diag = unreal.MaterialNodeService.get_material_diagnostics("/Game/Materials/M_Foo")
assert diag.success
print(f"compiled ok:                     {diag.is_compiled_ok}")
print(f"compile errors ({len(diag.compile_errors)}):")
for e in diag.compile_errors: print(f"  {e}")
print(f"referenced textures ({len(diag.referenced_texture_paths)}):")
for t in diag.referenced_texture_paths: print(f"  {t}")
print(f"expression count:                {diag.expression_count}")
print(f"texture sample count:            {diag.texture_sample_count}")
print(f"texture object parameter count:  {diag.texture_object_parameter_count}")
print(f"function call count:             {diag.function_call_count}")

Sample compile errors caught by this:

• "(Function WorldAlignedTexture) (Node TextureSample) Sampler type is Color, should be Masks for /Game/.../T_xevtfjz_2K_ORM" — ORM/data textures need SAMPLERTYPE_Masks or SAMPLERTYPE_LinearColor, not the default SAMPLERTYPE_Color.
• Type mismatch errors when wiring scalars to vector inputs without proper conversion.
• Missing required inputs on function calls.

Why prefer this over the older signals:

unreal.MaterialEditingLibrary.get_used_textures(mat) is unreliable for multi-branch graphs (BC + Normal + ORM, or anything with ComponentMask after a function-call output). It returns 0 even when the material is sampling textures correctly. Don't use it as proof of broken wiring.

Visual confirmation via the material editor preview is also useful but harder. Asset-editor capture now goes through the engine EditorAppToolset (toolset_name EditorToolset.EditorAppToolset via call_tool) — it opens, focuses, and captures the asset editor. (The old unreal.ScreenshotService and its capture_asset_editor are gone.) After the capture, attach_image(...) the returned file to inspect it.

⚠️ Best-effort focus: when many asset editors are already open as tabs, the editor tab may not switch reliably. Close other asset editors first if the screenshot keeps catching the wrong tab:

ed = unreal.get_editor_subsystem(unreal.AssetEditorSubsystem)
for path in [other_open_paths]:
    ed.close_all_editors_for_asset(unreal.load_asset(path))

🚨 Tiling on Basic Shapes: Prefer a Child MI with Tiling Override

For Megascans M_MS_Srf and similar surface materials with a Tiling scalar parameter, the cleanest per-actor tiling fix is a child material instance with Tiling overridden — not a UV transform on the mesh. UV transforms apply to all faces uniformly and break orientation; the parent material's UV mapping is already correct per face.

# ✅ CORRECT pattern for "this disc shows brick too sparse / too dense"
# Material instances are created and overridden through the engine MaterialInstanceTools toolset.
MIT = "editor_toolset.toolsets.material_instance.MaterialInstanceTools"
call_tool(tool_name="create_material_instance", toolset_name=MIT,
          arguments={"parent_material": "/Game/Fab/Megascans/.../MI_xevtfjz",
                     "asset_name": "MI_xevtfjz_Disc", "folder_path": "/Game/Materials/"})
ip = "/Game/Materials/MI_xevtfjz_Disc"
call_tool(tool_name="set_scalar_parameter", toolset_name=MIT,
          arguments={"instance_path": ip, "parameter_name": "Tiling", "value": 4.0})
unreal.EditorAssetLibrary.save_asset(ip)
# assign ip to the actor

For triplanar / world-aligned (where the same material works on cube, sphere, cylinder with no UV concerns), build a master material wrapping WorldAlignedTexture / WorldAlignedNormal (functions live under /Engine/Functions/Engine_MaterialFunctions01/Texturing/). Use TextureObjectParameter (not TextureSampleParameter2D) to feed the TextureObject input on those functions — the function brings its own sampler. Feed TextureSize from a Constant3Vector or Multiply(Scalar, Constant3Vector(1,1,1)); a bare scalar will broadcast incorrectly to V3 in this input slot.

🚨 Inspect Before Modifying Existing Materials

Before adding, removing, or reconnecting nodes in an existing material, you MUST export and review the current graph first:

import unreal, json

path = "/Game/Materials/M_Existing"

# Step 1: Get high-level info (blend mode, shading model)
info = unreal.MaterialService.get_material_info(path)
print(f"Blend: {info.blend_mode}, Shading: {info.shading_model}")

# Step 2: Export the full node graph
graph = json.loads(unreal.MaterialNodeService.export_material_graph(path))
print(f"Expressions: {len(graph['expressions'])}")
print(f"Connections: {len(graph['connections'])}")
print(f"Output connections: {len(graph['output_connections'])}")

# Step 3: Review what already exists
for expr in graph['expressions']:
    name = expr.get('parameter_name') or expr.get('class')
    print(f"  [{expr['id']}] {expr['class']} - {name}")
for oc in graph['output_connections']:
    print(f"  Output '{oc['property']}' ← expr {oc['expression_id']}")

Why this matters:

• The material may already have nodes connected to the output you want to modify
• Blindly adding nodes creates duplicates and orphaned connections
• You need to know what IDs exist so you can reconnect or replace, not just append
• export_material_graph returns the ground truth — use it to plan your edits

Workflow for modifying existing materials:

1. Export the graph JSON and review expressions + connections
2. Plan your changes — identify which nodes to keep, replace, or add
3. Disconnect existing connections if needed before reconnecting
4. Add only the new nodes that don't already exist
5. Reconnect outputs to their correct sources
6. Compile and save

🚨 Return Types — create_parameter/create_function_call Return .id, NOT the Raw Object

Material/instance creation now goes through the engine toolsets (MaterialTools.create_material returns the Material object itself — not a result-with-.asset_path; MaterialInstanceTools creates the instance), so build the asset path yourself from folder_path + asset_name:

# Engine: create_material returns the Material object (no .asset_path) — reference by path you chose.
call_tool(tool_name="create_material",
          toolset_name="editor_toolset.toolsets.material.MaterialTools",
          arguments={"folder_path": "/Game/Materials/", "asset_name": "M_MyMat"})
path = "/Game/Materials/M_MyMat"

# Engine: create the instance via MaterialInstanceTools, then reference the path you chose.
call_tool(tool_name="create_material_instance",
          toolset_name="editor_toolset.toolsets.material_instance.MaterialInstanceTools",
          arguments={"parent_material": "/Game/Materials/M_Base",
                     "asset_name": "MI_Red", "folder_path": "/Game/Materials/"})
instance_path = "/Game/Materials/MI_Red"

The surviving VibeUE graph-authoring calls still return result objects — extract the field:

# create_parameter / create_function_call → MaterialExpressionInfo
expr = unreal.MaterialNodeService.create_parameter(path, "Vector", "BaseColor", "Surface", "1,0,0,1", -400, 0)
node_id = expr.id  # ← use .id, NOT expr itself

⚠️ Passing a MaterialExpressionInfo where a string is expected gives: TypeError: Nativize: Cannot nativize 'MaterialExpressionInfo' as 'String'

⚠️ Two Services

• MaterialService - Create materials, instances, manage properties
• MaterialNodeService - Build material graphs, expressions, parameters

⚠️ Compile After Graph Changes

expr = unreal.MaterialNodeService.create_parameter(path, "Vector", "BaseColor", ...)
# Engine: connect_to_output lives on MaterialTools; property is the MP_* name.
call_tool(tool_name="connect_to_output",
          toolset_name="editor_toolset.toolsets.material.MaterialTools",
          arguments={"expression": expr.id, "output_name": "", "material_property": "MP_BaseColor"})  # use expr.id
unreal.MaterialService.compile_material(path)  # REQUIRED
unreal.EditorAssetLibrary.save_asset(path)

⚠️ Parameter Types

Type	Use	Example Value
Scalar	Single float	0.5
Vector	Color/vector	(R=1.0,G=0.0,B=0.0,A=1.0)
Texture	Texture parameter	/Game/T_Diffuse.T_Diffuse
TextureObject	Texture object (no sampler)	/Game/T_Diffuse.T_Diffuse
StaticSwitch	Static bool switch	true or false

⚠️ Material Output Names

• BaseColor, Metallic, Specular, Roughness
• EmissiveColor, Normal, Opacity, OpacityMask
• WorldPositionOffset, AmbientOcclusion

⚠️ Discovering Node Types & Categories

To answer "what nodes / categories are available", use the MaterialNodeService discovery calls — do not hunt through discover_python_module or a material-editor palette API (there isn't a Python one):

# All material-expression categories (Math, Color, Constants, Coordinates, Parameters, ...)
cats = unreal.MaterialNodeService.get_categories()

# Search node types by name/category (returns MaterialExpressionTypeInfo:
#   class_name, display_name, category, description, is_parameter)
types = unreal.MaterialNodeService.discover_types(category="", search_term="Add", max_results=100)

To create a node, pass the bare class name minus the MaterialExpression prefix ("Add", "Multiply", "Constant3Vector", "OneMinus", "LinearInterpolate"/"Lerp") as expression_class to the engine MaterialTools.add_expression call (single node), or to the surviving MaterialNodeService.batch_create_expressions (many nodes in one transaction).

⚠️ Check Node Existence

# WRONG - crashes if not found
add_node = next((n for n in nodes if "Add" in n.display_name))

# CORRECT
add_node = next((n for n in nodes if "Add" in n.display_name), None)
if add_node:
    pins = unreal.MaterialNodeService.get_expression_pins(mat_path, add_node.id)

⚠️ Property Names

Use discover_python_class() first (batch: class_name='unreal.A, unreal.B'):

• MaterialExpressionTypeInfo uses display_name, NOT name
• MaterialOutputConnectionInfo uses connected_expression_id, NOT expression_id

Complete field lists (don't guess):

Struct	Fields
MaterialExpressionInfo (from list_expressions / get_expression_info)	id, class_name, display_name, category, description, pos_x, pos_y, is_parameter, parameter_name, input_names, output_names — the class is class_name, not expression_class
MaterialNodePropertyInfo (from expression property lists)	name, value, property_type, is_editable — it's name here, not property_name
MaterialSummary (from MaterialService.summarize)	material_name, material_path, blend_mode, shading_model, material_domain, two_sided, expression_count, parameter_count, parameter_names, key_properties, editable_properties — it's material_name/material_path/parameter_count, not name/path/num_parameters
MaterialDetailedInfo (from MaterialService.get_material_info)	material_name, material_path, blend_mode, shading_model, material_domain, two_sided, expression_count, texture_sample_count, is_material_instance, parent_material, parameters — it's two_sided, not is_two_sided
MaterialPropertyInfo_Custom (from MaterialService.list_properties / get_property_info)	property_name, display_name, property_type, current_value, allowed_values, category, is_editable, is_advanced — it's property_name/current_value, not name/value (differs from MaterialNodePropertyInfo above)

🚨 Setting Material Properties: set_property Takes Display OR Internal Name

MaterialService.set_property(path, name, value) / get_property accept either the editor display name ("Two Sided", "Blend Mode", "Opacity Mask Clip Value") or the internal property name ("TwoSided", "BlendMode", "OpacityMaskClipValue") — matching is case-insensitive and ignores spaces. Both of these work:

unreal.MaterialService.set_property(path, "Two Sided", "true")   # display name
unreal.MaterialService.set_property(path, "BlendMode", "BLEND_Masked")  # internal name
unreal.MaterialService.set_property(path, "OpacityMaskClipValue", "0.33")
unreal.MaterialService.compile_material(path)   # enum/blend changes need a recompile
unreal.MaterialService.save_material(path)

⚠️ set_property returns a bool (True/False), it does not raise on an unknown property — a False means the name didn't resolve or the value didn't parse. Don't assume success: check the return, or verify with get_property / summarize afterward. Enum values are the BLEND_* / MSM_* / MD_* identifiers (see below), not the friendly labels.

Discover legal enum values, don't guess: list_properties / get_property_info populate allowed_values for enum properties — including the classic TEnumAsByte ones (BlendMode, ShadingModel, MaterialDomain). Read them instead of probing unreal for the enum class:

for p in unreal.MaterialService.list_properties(path):
    if p.property_name == "BlendMode":
        print(p.allowed_values)
        # ['BLEND_Opaque','BLEND_Masked','BLEND_Translucent','BLEND_Additive', ...]

Common material property internal names: TwoSided, BlendMode, ShadingModel, MaterialDomain, OpacityMaskClipValue, bUsedWithStaticLighting, DitheredLODTransition.

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Task Index

Task	Workflow	Sample script
Create a material (+ params)	workflows.md → Create a basic material	scripts/create_material.txt
Create a material instance	workflows.md → Create a material instance	scripts/create_instance.txt
Add texture / math / function / HLSL nodes	workflows.md	—
Build a graph with batch ops	workflows.md → Batch create/connect/set	scripts/material_graph_batch.txt
Inspect/export an existing graph	reference.md → Export JSON schema	scripts/export_graph.txt
Recreate a material from export	reference.md → Recreate from export	—
Verify wiring / compile	Critical Rules → get_material_diagnostics	—

Sub-docs

• workflows.md — create material/instance, texture/math/function/HLSL/collection/switch nodes, set properties, and batch create/connect/set.
• reference.md — parameter types, output names, enum formats, the export_material_graph JSON schema, recreate-from-export steps, and Material Function authoring.

Verification

After non-trivial wiring run MaterialNodeService.get_material_diagnostics(path) and confirm is_compiled_ok and the expected referenced_texture_paths — do not rely on get_used_textures (unreliable for multi-branch graphs). compile_material + save_asset to persist.

Related skills

• landscape-materials — LandscapeLayerBlend nodes.
• landscape-auto-material — production landscape materials (functions, RVT, instances).