Styling and Colors (Presentation)

NOTE

Target Audience: Intermediate to Advanced Implementers
Prerequisites: Read the Data Model Map first.

In STEP, visual representation (colors, layers, transparency) is decoupled from the geometry. This allows for rich visual data without altering the underlying mathematical definitions.

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🚦 Version Compatibility

Not all Application Protocols (APs) support visual styling. Choosing the right version is critical.

Feature	AP203 (Ed.2)	AP214	AP242
Basic Colors (RGB)	✅ (Yes)	✅ (Yes)	✅ (Yes)
Layers	✅ (Yes)	✅ (Yes)	✅ (Yes)
Transparency	❌ (No)	⚠ (Partial)	✅ (Yes)

• AP203: The oldest standard. Edition 1 does not store colors or layers. Edition 2 added support for basic styling and layers to match AP214.
• AP214: The most common standard for "colored" STEP files. Highly compatible with most CAD systems.
• AP242: The modern standard. Supports everything in AP214 plus advanced transparency and PMI styling.

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🗺️ Presentation Hierarchy

Styles are assigned to geometry via a chain of entities. The most common path is for surface colors.

Key Entities

Entity	Role
STYLED_ITEM	The entry point. Links a specific geometry item to a style.
PRESENTATION_STYLE_ASSIGNMENT	A container for multiple styles (Surface, Curve, Fill, etc.).
SURFACE_STYLE_USAGE	Specifies which side of the surface the style applies to (.BOTH., .POSITIVE., or .NEGATIVE.).
SURFACE_STYLE_RENDERING	Defines visual properties like color and transparency.
COLOUR_RGB	Defines color in RGB space using values from 0.0 to 1.0.

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🎯 Assignment Levels & Compatibility

Where you attach a STYLED_ITEM significantly impacts how CAD systems interpret the file.

1. Face Level (Best Practice)

Attaching colors directly to ADVANCED_FACE entities.

• Pros: Highest compatibility. Supported by almost all STEP-compliant software.
• Cons: Increases file size if many faces share the same color.

2. Shell/Solid Level

Attaching colors to CLOSED_SHELL or MANIFOLD_SOLID_BREP.

• Pros: Smaller file size. Sets a "default" color for the whole body.
• Cons: Some older parsers or lightweight viewers ignore styles attached at this level.

3. Part (Product) Level

Styling is sometimes attached at the SHAPE_REPRESENTATION level.

• Note: This is often treated as a "fallback" or "layer color."

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🌓 Overriding Logic

If a color is defined for a Solid and a different color is defined for one of its Faces, which one wins?

STEP uses OVER_RIDING_STYLED_ITEM to handle this.

• The OVER_RIDING_STYLED_ITEM explicitly references another STYLED_ITEM (the one being overridden).
• This is commonly used in assemblies where a part has a default color, but a specific instance in the assembly needs to look different.

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🌫️ Transparency and Opacity

Transparency is defined within the SURFACE_STYLE_RENDERING block.

#240 = SURFACE_STYLE_RENDERING(#250,.MATTE.,#260);
#250 = COLOUR_RGB('Glass',0.8,0.9,1.0);
#260 = SURFACE_STYLE_TRANSPARENT(0.5);  /* 50% Transparent */

• 0.0: Fully Opaque (Solid)
• 1.0: Fully Transparent (Invisible)
• Note: Not all viewers support transparency. It is primarily an AP214 and AP242 feature.

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🖋️ Curve and Edge Styling

While surfaces use SURFACE_STYLE, edges (wireframes) use CURVE_STYLE.

• CURVE_STYLE: Can define both color and line width.
• Many CAD systems export edges as "Black" by default, even if the faces are colored.

/* Example: Styling an Edge (Blue, 0.5mm width) */
#100 = EDGE_CURVE('Edge_A',#110,#120,#130,.T.);
#200 = STYLED_ITEM('',(#210),#100);
#210 = PRESENTATION_STYLE_ASSIGNMENT((#220));
#220 = CURVE_STYLE('',$,#230,#240);
#230 = COLOUR_RGB('',0.0,0.0,1.0);  /* Blue */
#240 = POSITIVE_LENGTH_MEASURE(0.5); /* 0.5mm Width */

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🗂️ Layer Management

Layers in STEP are managed by PRESENTATION_LAYER_ASSIGNMENT. Unlike colors, layers are "grouping" entities that reference geometry items.

#300 = PRESENTATION_LAYER_ASSIGNMENT('Mechanical_Parts','Visible parts',(#100,#200));

• Name: The layer name (e.g., 'CONSTRUCTION', 'SKETCH', '0').
• Assigned Items: A list of geometry entities belonging to this layer.
• Note: An item can belong to multiple layers, though most CAD systems only use one.

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💡 Implementation Tips

1. Normalization: Always convert COLOUR_RGB (0.0-1.0) to your target system's range (e.g., 0-255) by multiplying by 255 and rounding.
2. Deduplication: In export, reuse the same COLOUR_RGB and SURFACE_STYLE_RENDERING instances for all faces that share the same color to keep the file size small.
3. Fallback: If a face has no style, check its parent Shell. If the Shell has no style, check the Solid. If all fail, apply a default gray (0.8, 0.8, 0.8).

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📚 Next Steps

• Anatomy of Product Entities - How parts are defined.
• Common Pitfalls - Common issues with colors.

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