sky-artifact-prevention

name	sky-artifact-prevention
description	Prevent and remove sky/tunnel mesh artifacts in road corridor photogrammetry. Covers five strategies from Metashape-side prevention (height field, region crop, point cloud classification, source selection) to post-mesh cleanup. The
user-invocable	false

Sky Artifact Prevention for Road Corridors

The Problem

Metashape's mesh generation creates "tunnel" or "dome" artifacts over road corridors. Even with proper sky masks:

• Depth maps ignore masks during mesh interpolation
• Interpolation fills gaps between opposite sides of the road, creating a closed surface overhead
• This produces tunnel effects, flashing artifacts, and enclosed environments where open sky should be

This is Metashape's most well-known limitation for road corridor captures. There is no single fix — use a combination of strategies.

When to Use

• Building mesh from road corridor captures
• Any long linear capture where sky/overhead closure is a problem
• After mesh building, if tunnel artifacts are visible
• When planning the dense reconstruction strategy (choose BEFORE building)

Strategy Overview

Five strategies, ordered by effectiveness for road corridors:

#	Strategy	When to Apply	Effectiveness
1	Point cloud source	Before mesh build	Best — avoids the problem entirely
2	Height field mode	Before mesh build	Good for flat terrain, loses vertical surfaces
3	Point cloud classification	Before mesh build	Good — requires classification step
4	Region cropping	Before mesh build	Moderate — simple but crude
5	Post-mesh cleanup	After mesh build	Fallback — fixes what got through

Recommended combination: Strategy 1 + 3 + 5 (point cloud source, with ground classification, followed by cleanup).

Strategy 1: Build Mesh from Point Cloud (Not Depth Maps)

Why it works: Point cloud generation DOES respect masks. If sky is masked, no points exist in the sky → no mesh in the sky. Depth map interpolation is what creates the tunnel — bypassing it avoids the problem.

# Step 1: Build point cloud with masks active
build_point_cloud(
    point_colors=True,
    point_confidence=True
)

# Step 2: Build mesh from point cloud
build_model(
    surface_type="arbitrary",
    source_data="point_cloud",
    interpolation="enabled",
    vertex_colors=True
)

Trade-offs:

• (+) Best preservation of open sky areas
• (+) Masks fully respected during point cloud generation
• (-) Point cloud mesh is slightly less detailed than depth map mesh
• (-) Still some interpolation at edges where point density drops

When NOT to use: When you need maximum mesh detail and will handle artifacts in post.

Strategy 2: Height Field Surface Type

Why it works: Height field mode generates only the surface visible from directly above — it cannot create tunnels, domes, or enclosed spaces.

build_model(
    surface_type="height_field",
    source_data="depth_maps",
    interpolation="enabled",
    vertex_colors=True
)

Trade-offs:

• (+) Completely eliminates tunnel/dome artifacts
• (+) Perfect for terrain-only workflows (DEM generation)
• (-) Loses vertical surfaces: building facades, retaining walls, canyon rock faces
• (-) Not suitable when you need full 3D geometry

Best for: Terrain/ground extraction where vertical surfaces don't matter. NOT suitable for driving simulator environments where canyon walls are important.

Strategy 3: Point Cloud Classification + Filtered Mesh

Why it works: Classify the point cloud to separate ground/building from vegetation/noise/sky, then build mesh from only the classes you want.

# Step 1: Build point cloud
build_point_cloud(
    point_colors=True,
    point_confidence=True
)

# Step 2: Classify ground
classify_ground_points(
    max_angle=15.0,
    max_distance=1.0,
    cell_size=50.0
)

# Step 3: Build mesh from ground + building classes only
build_model(
    surface_type="arbitrary",
    source_data="point_cloud",
    classes=[2, 6],              # 2=Ground, 6=Building
    interpolation="enabled",
    vertex_colors=True
)

Point cloud classes (ASPRS LAS standard):

Class	Code	Include?
Created/never classified	0	Maybe — contains unclassified real geometry
Unclassified	1	Maybe — same as above
Ground	2	YES — always include
Low vegetation	3	Depends on needs
Medium vegetation	4	Depends on needs
High vegetation	5	Usually NO for road corridors
Building	6	YES — retaining walls, structures
Noise	7	NO — sky artifacts live here

Trade-offs:

• (+) Precise control over what gets meshed
• (+) Removes sky noise AND vegetation noise
• (-) Classification can misclassify rock faces as buildings or vice versa
• (-) Extra processing step (classification takes time)
• (-) classify_ground_points is designed for aerial data — may not perfectly classify vehicle-level captures

Tuning for road corridors:

classify_ground_points(
    max_angle=25.0,     # Increase from 15 for steep canyon walls
    max_distance=2.0,   # Increase for rougher terrain
    cell_size=20.0      # Decrease for finer detail in narrow corridors
)

Strategy 4: Region Cropping

Why it works: Limit the reconstruction volume so nothing above a certain height gets meshed.

# Get current bounds
get_chunk_bounds()

# Set tight region — limit vertical extent
set_region(
    center=[x, y, z],
    size=[width, length, limited_height]
)

How to determine limited_height:

• For a road canyon: tallest wall height + 2m margin
• For flat road: 5-10m above road surface
• For bridges/overpasses: height of the tallest structure + 2m

Trade-offs:

• (+) Simple, no extra processing
• (+) Works with any source (depth maps or point cloud)
• (-) Crude — a single height limit for the whole corridor
• (-) Clips tall features if the limit is too low
• (-) Requires manual coordinate inspection to set properly

Region rotation for corridors: If the corridor isn't axis-aligned, rotate the region to match:

set_region_rotation(yaw=corridor_heading)

Strategy 5: Post-Mesh Cleanup

When to use: After mesh is built, as a fallback for whatever got through.

5a: Component-based cleaning

clean_model(
    criterion="component_size",
    level=75
)

Removes disconnected mesh components smaller than the threshold. Sky artifacts are often disconnected from the main terrain mesh.

Level guide:

• 50: Conservative — only removes small fragments
• 75: Moderate — removes medium artifacts (good default)
• 90: Aggressive — removes everything except the largest component

5b: Confidence-based filtering

If point confidence was enabled:

# Remove low-confidence points first, then rebuild mesh
filter_points_by_confidence(min_confidence=3)

Sky interpolation artifacts tend to have low confidence scores.

5c: Manual region deletion

For stubborn artifacts that automated cleaning misses:

1. Identify the region in Metashape GUI
2. Select faces manually and delete
3. Or use execute_python to delete faces by height/normal criteria

Decision Tree

Building mesh for road corridor?
├── Need full 3D (walls, rock faces)?
│   ├── YES → Strategy 1 (point cloud source)
│   │         + Strategy 3 (classify, use classes [0,1,2,6])
│   │         + Strategy 5 (cleanup remaining artifacts)
│   └── NO (terrain only) → Strategy 2 (height field)
├── Have EXR masks?
│   ├── YES → Strategy 1 is especially effective (masks respected in point cloud)
│   └── NO → Strategy 3 + 4 + 5 (classify + region crop + cleanup)
└── Already built mesh with artifacts?
    └── Strategy 5 (cleanup) — then consider rebuilding with Strategy 1 or 3

Recommended Pipeline for Decker Canyon-Style Corridors

This project needs full 3D (road + canyon walls), has EXR masks, and outputs to a driving simulator:

# 1. GPU off for dense operations
set_gpu_config(cpu_enable=False)

# 2. Build point cloud (masks respected here)
build_point_cloud(point_colors=True, point_confidence=True)

# 3. Classify ground — tuned for steep canyon
classify_ground_points(max_angle=25.0, max_distance=2.0, cell_size=20.0)

# 4. Build mesh from point cloud, ground + unclassified + building
build_model(
    surface_type="arbitrary",
    source_data="point_cloud",
    classes=[0, 1, 2, 6],
    interpolation="enabled",
    vertex_colors=True,
    vertex_confidence=True
)

# 5. Clean remaining artifacts
clean_model(criterion="component_size", level=75)

# 6. Verify — check for tunnel artifacts
get_model_stats()
capture_viewport()

If artifacts remain after step 5, increase level to 90 or apply region cropping before rebuilding.

Common Mistakes

Mistake	Result	Fix
Building from depth maps with masks	Tunnel forms anyway — depth map interpolation ignores masks	Use source_data="point_cloud" instead
Height field on a canyon	Loses rock face walls	Use surface_type="arbitrary" with classification
Aggressive classification on vehicle data	Misclassifies walls as noise	Increase max_angle to 25-30 for steep terrain
Not cleaning after mesh build	Artifact fragments remain	Always run clean_model as final step
Region crop too tight	Clips legitimate tall features	Add 2-3m margin above tallest feature