Importing CAD geometry into FLACS

Most industrial FLACS simulations are based on geometry imported from CAD models of the facility being analysed. CASD provides a CAD import workflow that converts external geometry files into the FLACS native format using the geo2flacs utility. This article explains how the import process works, what file formats to prefer, why complex models can be slow or incomplete, and how to prepare and optimise geometry for reliable, efficient import.

Quick fixes — try these first

If you have an import problem right now, these are the highest-impact actions to try:

1. Use a better source format — if your geometry originated in AVEVA PDMS or E3D, request or export an RVM file instead of NWD or FBX. RVM preserves solid geometry and dramatically reduces import complexity. See Choosing the right file format.

2. Enable mesh optimisation — in the import dialogue under Advanced options, tick Optimise meshes. This reduces polygon count during import at minimal cost to accuracy.

3. Filter out small objects — in the import dialogue, set the minimum diameter filter to approximately 50 mm (roughly 2" pipework). Objects below this size have negligible influence on results at typical grid resolutions and are the most common cause of bloated geometry files.

4. Split into external files — use Import as external nodes to split a large import into manageable linked files rather than one monolithic geometry. See Splitting geometry into external files.

5. Check MDI rendering is active — if the model opens but CASD is slow and unresponsive, verify that MDI (Multi Draw Indirect) has not been disabled in the CASD OpenSceneGraph viewer settings. MDI is on by default and uses the GPU for display culling; without it, large geometry will be sluggish.

6. On laptops with both integrated and dedicated GPU, set CASD to use the dedicated GPU — systems with both an Intel integrated GPU and a separate NVIDIA (or similar) GPU may run CASD on the integrated GPU by default. The integrated GPU shares system RAM rather than having its own dedicated memory, which limits how much geometry it can hold and can significantly degrade performance with large models. Fix this by setting CASD to High Performance in Windows Advanced Graphics Settings (Settings → System → Display → Graphics → Custom options for apps), or right-click the CASD executable, select Properties, and look for an option to specify which GPU to use.

7. Fall back to the command line — if the CASD GUI import fails or runs out of memory, the geo2flacs command line utility imports without the visualisation overhead and can handle models that the GUI cannot. See Optional: using geo2flacs from the command line.

Recommended geometry workflow

For a new project, the recommended sequence is:

1. Obtain or prepare the source CAD file — ideally RVM, DWG, or DGN; see What to request from your client or CAD team
2. Run the initial import — using the CASD import dialogue (Geometry > Import CAD file(s)) with size filtering and mesh optimisation enabled
3. Verify completeness and geometry quality in CASD — check for missing objects, incorrect solids, open meshes
4. Clean up — mark non-physical objects as visual only, fix open meshes, add porosity to gratings, convert mesh pipes to cylinders where possible
5. Build the grid — geometry should be substantially correct before grid generation; major geometry changes after grid generation require the grid to be rebuilt
6. Calculate porosities — use Verify porosities to confirm geometry is being processed correctly before committing to a simulation run

Geometry work and grid work are iterative — expect to cycle through steps 3–6 more than once on a complex project.

Understanding how FLACS represents geometry

Before choosing a file format or configuring import options, it helps to understand the two types of geometry object in a FLACS model and how each is handled. This directly explains why some imports are fast and reliable while others are slow or incomplete.

Solid primitives

Solid primitives — boxes, cylinders, tori, ellipsoids, GTCs (general truncated cones), and CP8 (convex polyhedrons) — are described using a small number of mathematical parameters. A cylinder, for example, is fully defined by its position, diameter, length, and axis direction. This makes primitives:

• Compact: very small file sizes relative to the geometry they represent
• Fast to process: porosity calculation (FGC) can resolve their properties analytically
• Reliable: geometric properties are unambiguous and well defined

Mesh objects

A mesh primitive is a triangulated surface — a collection of flat polygons that together approximate a shape. Mesh is used when the geometry cannot be represented as any of the standard primitive types. Mesh objects:

• Require significantly more resources: every triangle in the mesh must be individually examined during porosity calculation
• Are sensitive to file size: high polygon counts lead to very large geometry files and long processing times
• Require correct surface normals: the solver uses normal direction to determine inside versus outside of a solid; inconsistent normals can cause incorrect or missing blockage

The fundamental rule for efficient FLACS geometry is: primitives are always preferable to mesh. The larger the proportion of your model that is represented as solid primitives, the faster and more reliable the import, porosity calculation, and simulation will be.

How much detail is actually needed?

A common question is how simplified the geometry needs to be. The practical rule of thumb is to model down to approximately 2" (50 mm) pipework — this captures the congestion that meaningfully influences explosion and dispersion results while excluding the fine mechanical detail (nuts, bolts, instrument tubing, cable tray brackets, small conduit runs) that has negligible influence at typical grid resolutions.

More precisely, the threshold is tied to grid cell size: objects significantly smaller than one grid cell contribute little to porosity calculations regardless of whether they are included. Geometry close to the region of interest, where the grid is finest, warrants more care than far-field geometry where coarser cells average out fine detail anyway.

Why large or complex models can be challenging to import

Geometry preparation is a demanding step in any CFD workflow, not just FLACS. Industry surveys consistently show that preprocessing and meshing account for 40–80% of total CFD project time across all major codes, including ANSYS Fluent, STAR-CCM+, and OpenFOAM. All of these tools have developed specialised geometry repair, wrapping, and simplification workflows specifically because handling complex CAD models is inherently difficult. Gexcon continues to invest in this area — the conversion algorithms in geo2flacs are continuously improved as new file examples and edge cases are encountered.

For FLACS specifically, the primary factors that make large model imports challenging are:

1. High mesh polygon counts: A detailed plant model may contain tens of millions of triangles. Every triangle must be examined individually during import and again during porosity calculation. Imports that are slow are most commonly traceable to mesh-heavy source file formats or highly detailed geometry that has not been simplified before export.

2. Memory pressure during GUI import: When running geo2flacs through the CASD graphical interface, CASD must simultaneously hold the geometry in memory for rendering in the 3D view. For large models, this can exhaust available system memory, causing the application to slow or terminate before completion.

3. Mixed solid and mesh geometry: When a file contains both solid primitives and mesh objects, import failures tend to affect the mesh components first. Because mesh objects require more complex conversion, they are more likely to be silently dropped or cause the import to terminate before completion.

4. Object grouping in the source file: Some CAD tools group many small objects — such as nuts, bolts, or grating details — into a single large mesh whose bounding box spans much of the model. This prevents size-based filtering from removing them, even though the individual components may be tiny.

5. Mesh retained rather than converted: mesh objects from the source file are retained as mesh geometry rather than being automatically converted to solid primitives. This is more geometrically accurate but more resource-intensive — a model that previously imported quickly may take longer if it contains substantial mesh content. The strategies in this article are the recommended response; a legacy conversion mode is available as a last resort. See The legacy "Generate FLACS model" option for the trade-offs.

Understanding these factors helps identify the most effective solutions, which are covered in the sections below.

What to request from your client or CAD team

Many FLACS users receive geometry from a client's engineering team rather than preparing it themselves. The format and level of detail in that file determines how much import work lies ahead. Asking the right questions upfront can save significant time.

Preferred formats to request

In order of preference: RVM (if the source is AVEVA PDMS or E3D), then DWG or DGN, then mesh-only formats such as NWD or FBX as a last resort. See Choosing the right file format for a full comparison and explanation. Avoid geometry that has been converted through multiple formats — each conversion tends to degrade solid geometry into mesh.

Level of detail

Request a model that has been filtered for engineering review, not a full construction model. Specifically, ask that the following be excluded or suppressed before export:

• Pipework and fittings below approximately 2" (50 mm) diameter
• Instrument tubing, cable trays, and small conduit runs (unless these form a significant congestion contribution)
• Annotation geometry, construction lines, and reference planes
• Access platforms and escape routes (these can be added manually in CASD if needed for visualisation)
• Objects hidden or suppressed in the current design revision

If the client's CAD team can split the export by area or module, request separate files per module or deck rather than a single combined file. This gives you flexibility to import and work with individual sections independently.

Coordinate system

Ask whether the model uses a project coordinate system or a local coordinate system. If coordinates are very large (e.g., national grid values), translation will be needed at import. Agree on a reference point to use as the origin for the FLACS model.

Choosing the right file format

The file format you supply to geo2flacs has a major influence on import reliability and performance. Formats that preserve solid geometry descriptions produce far better results than mesh-only formats.

Recommended formats (solid-preserving)

RVM files from AVEVA PDMS or E3D typically give the best import result because plant design systems use a structured library of parametric objects — pipes, fittings, vessels, structural members — that map directly to FLACS primitive types. A piping network imported from RVM will typically arrive as cylinders and tori rather than as mesh.

DWG and DGN files can contain a mix of solid objects and mesh. The quality of the import depends heavily on how the geometry was drawn in the CAD application. Note that PDMS geometry can also be exported in DGN format using the ExPLANT-I export driver.

Mesh-heavy formats (higher resource requirements)

Formats such as FBX, NWD, OBJ, and STL are geometrically more flexible — they can represent any shape accurately — but they do so entirely using triangulated mesh. For a large facility model, these formats will produce geometry files that can be orders of magnitude larger than equivalent RVM or DWG imports, and porosity calculation times will increase proportionally.

If you are currently using NWD or FBX for large projects and experiencing very long import times or failures, switching to RVM (if the source is AVEVA) or DWG/DGN is the single most effective improvement you can make.

Textures in OBJ and FBX files

Materials and textures can be imported from DGN, OBJ, and FBX files. FLACS-CFD loads textures from separate image files — not embedded within the geometry file itself. If textures do not appear after import, check that the texture image files are present in the same directory as the geometry file and follow the expected naming convention (for example, basename_col.png for colour, basename_nrm.png for a normal map). The importer attempts to locate missing texture types automatically based on filename patterns; if paths are wrong, textures can be reassigned through the CASD materials dialogue.

Textures affect only the visual display in CASD and Flowvis — they have no effect on simulation results or porosity calculations.

When you have no choice but to use a mesh-heavy format

If your source data is only available in a mesh-heavy format, the strategies described in the Strategies for large and complex models section below become essential rather than optional.

Import workflow in CASD

Step 1: Open the import dialogue

In CASD, select Geometry > Import CAD file(s). The import dialogue allows you to add one or more input files, configure transformation parameters, set filtering options, and access advanced settings.

Step 2: Add files and set transformations

For each file added to the import list, you can configure transformations to correctly position and orient the geometry within the FLACS coordinate system.

Rotation — rotates the geometry before export. Use this when the facility's principal axes are not aligned with FLACS X, Y, Z directions.

Translation — shifts the geometry by a specified offset. This is particularly important for geo-referenced models: plant models designed in a national grid or site coordinate system may have X and Y values in the hundreds of thousands of metres (for example, X = 500,000 m, Y = 1,250,000 m). Coordinates this large can cause numerical precision issues in FLACS. Use translation to move the geometry so that the region of interest is close to the origin — for example, translate by −500,100 in X to shift a model centred near X = 500,100 close to zero.

Scaling — rescales the geometry by a fixed factor. The most common use is unit conversion: many CAD packages use millimetres as their internal unit, while FLACS expects metres. If the imported geometry appears one thousand times too large, apply a scale factor of 0.001. FLACS cannot detect file units automatically, so check which units the source CAD application uses before importing.

If multiple transformations are applied, the order of operation is: first scaling, then translation, then rotation.

Step 3: Configure split options

Before running a large import, consider whether to split the output into external files. See Splitting geometry into external files.

Step 4: Configure advanced options

Click Advanced options... to access additional settings that can significantly affect import quality. These are described in the next section.

Step 5: Run the import

Click Import to run. Progress is shown in the CASD status bar. For most models the GUI import is the recommended approach. If the import fails or CASD runs out of memory on a very large or mesh-heavy model, the geo2flacs command line utility can import without the visualisation overhead — see Optional: using geo2flacs from the command line as a fallback option.

Advanced import options

Options for DWG/DGN files

Options for RVM files

Options for NWD files

Geometry analysis options

These options control how FLACS interprets imported mesh geometry:

Geometry optimisation options

Splitting geometry into external files

For large facilities, splitting the geometry into multiple linked files is one of the most effective ways to manage import complexity and enable efficient post-import editing.

How external geometry works

Version note: external nodes require FLACS 24.1 or later.

A geometry file can reference separate .geo files as external nodes. These linked files are included in FGC porosity calculations regardless of whether they are visually loaded in CASD. This means you can:

• Work on individual modules without opening the entire facility geometry
• Enable multiple users to edit different parts of the model simultaneously
• Keep individual files small enough to open and edit without performance issues

Import options for splitting

Two options in the import dialogue control splitting:

• Import as external nodes: each input file is saved as a separate .geo file and linked to the main geometry file via an external node. This is the recommended approach when importing large multi-file models.
• Split on top groups (available only when "Import as external nodes" is enabled): the internal hierarchy of each input file is used to split it further — each top-level group becomes a separate external .geo file.

The --split-files and --split-on-groups command line options for geo2flacs provide the same functionality.

Strategic use of splitting for large projects

For a large multi-section facility, a recommended approach is:

1. Import each process module or area as a separate external file
2. In the region of interest (where the explosion scenario or dispersion release is located), use full-detail geometry
3. Simplify or filter geometry in far-field regions to reduce the total load on FGC porosity calculation

This allows you to construct the most realistic model where accuracy matters most while keeping the overall project manageable.

Optional: using geo2flacs from the command line

Most users will not need this section — the CASD GUI import handles the vast majority of projects. geo2flacs can also be run directly from the command line, which is useful in two situations: when the GUI import fails or runs out of memory on a very large model, and when automating geometry handling with scripts.

When run from the command line, geo2flacs does not render geometry during import. This removes the visualisation memory overhead and allows it to handle models that exhaust available memory in the GUI. All the same import options are available via command line flags.

Running geo2flacs

On Linux:

run geo2flacs [options] INPUT-FILES...

On Windows (from the cmd prompt):

geo2flacs [options] INPUT-FILES...

Input files may be individual files or directories; if a directory is specified, all recognised files within it (and subdirectories) will be imported.

Capturing diagnostic output

For troubleshooting import failures, redirect the verbose output to a file:

On Linux:

run geo2flacs -v --outdir=. INPUT-FILE > import_log.txt

On Windows:

geo2flacs -v --outdir=. INPUT-FILE > import_log.txt

This creates import_log.txt in the current directory. Review this file to identify which objects or geometry types are causing failures — it is usually far more informative than the CASD GUI progress messages. Include this file when contacting Gexcon support at flacs@gexcon.com.

Key command line options

Strategies for large and complex models

Filter small objects at import time

Plant CAD models frequently contain enormous numbers of small objects — nuts, bolts, valve actuator details, cable tray brackets — that have no meaningful influence on explosion or dispersion results at typical FLACS grid resolutions.

The practical rule of thumb is to filter to approximately 2" (50 mm) pipework as the smallest retained geometry. In the import dialogue, set the minimum diameter filter to 50 mm. This excludes fine mechanical detail while retaining all pipework and structural elements that meaningfully contribute to congestion.

Three filtering parameters are available (accessible via the import dialogue or as command line flags):

• Minimum diameter (-d): exclude objects with a bounding diameter below the threshold (mm)
• Minimum length (-l): exclude objects whose largest dimension is below the threshold (mm)
• Group small primitives (-n): consolidate small primitives into grouped objects rather than discarding them; a typical value is 10,000

Important limitation: size-based filtering uses the object's bounding box, not the actual object size. If small objects (such as nuts) are part of a single large mesh group that spans a wide area of the model, the bounding box will be large and the filter will not remove them. This is a known limitation that the development team continues to address.

Use mesh curvature analysis for pipe-heavy models

If your model contains many pipe systems represented as mesh (common in NWD, FBX, and some DWG files), enable mesh curvature analysis. This algorithm detects cylindrical and toroidal shapes within mesh objects and replaces them with native FLACS cylinder and torus primitives, substantially reducing file size and porosity calculation time.

After running an import with curvature analysis enabled, check the geometry carefully — in some cases the algorithm may produce large, incorrect objects. These can be deleted manually.

Apply load filters after import

If you have already imported a large geometry and it is too slow to work with in CASD, use a load filter to open only the relevant section:

1. Save the complete geometry to a database file and exit CASD
2. Restart CASD and apply a load filter (by position, material, size, or combination) before opening the geometry
3. Work only with the filtered subset

Use the "Optimise meshes" option

For mesh-heavy imports, enabling Optimise meshes (under Advanced options) reduces polygon count on every mesh object during import. While this reduces geometric detail slightly, the resulting model is much smaller, opens faster, and completes porosity calculation more quickly. For safety engineering purposes at typical grid resolutions, the simplified mesh usually provides adequate accuracy.

The legacy "Generate FLACS model" option

The Generate FLACS model option (under Advanced options > Geometry analysis) is a legacy import mode that converts all geometry to solid FLACS primitives, with unrecognised shapes falling back to bounding boxes. It is not recommended for new work — FGC operates directly on the model as imported, whether it contains primitives, meshes, or both.

If file size or processing time is a hard constraint and this option is used, note that bounding boxes are typically larger than the actual geometry, which overestimates blockage and congestion. In explosion assessments this tends to produce higher predicted overpressures — conservative, but potentially unrepresentative. Document the approach clearly if used.

MDI rendering for interactive work

MDI (Multi Draw Indirect) rendering is enabled by default in the CASD OpenSceneGraph viewer. MDI uses the GPU to determine which objects are in view and need to be drawn, including occlusion culling to avoid rendering hidden geometry — this is what allows large models to remain interactive. It requires OpenGL 4.3 or newer, supported by most graphics cards manufactured after 2012.

If CASD is performing poorly on a large model, check that MDI has not been inadvertently disabled in the viewer settings (Options menu > OpenSceneGraph viewer settings).

Post-import geometry verification and cleanup

The geo2flacs import process cannot guarantee perfect results. Always perform a thorough visual verification in CASD after import.

Verify completeness

Compare the imported geometry against the source CAD model. Check that all major structural elements, walls, decks, and equipment are present. The diagnostic output file from geo2flacs identifies which objects were successfully converted and which were not — review this for any categories of missing geometry.

Check the geometry is fit for purpose

After verifying that the geometry looks correct visually, confirm it will produce reliable results in porosity calculation:

• Run Calculate (Porosities menu) to generate the porosity file using FGC, then use Verify porosities (Porosities menu or toolbar) to inspect the results. Verify porosities opens Flowvis in porosity verification mode, showing a 2D cut-plane plot of volume and area porosities. Click inside any control volume to read its porosity values. This is a visual inspection tool — it lets you check whether walls, decks, and equipment are being represented as expected, but it does not automatically flag problems.
• Spot-check blockage in the region of interest. Compare the congestion level to what you would expect from experience with similar facilities. Unexpectedly high or low blockage values often indicate geometry problems — oversized bounding boxes, missing objects, or unclosed mesh objects that are not being treated as solid.
• For explosion assessments, pay particular attention to walls and decks: these must produce zero area porosity on their faces to correctly confine an explosion.

Mark non-physical objects as visual only

CAD models often contain annotation geometry, centre lines, construction planes, and access route markers that should not influence the simulation. Mark these as visual only in the object properties so FGC ignores them. This is important for correct porosity calculation.

Handle mesh objects representing closed solids

For a mesh object to produce volume blockage in the simulation, it must be recognised as a closed solid. CASD automatically analyses imported mesh objects and sets the Is closed flag where it can determine the mesh is a closed 2-manifold.

If a mesh object represents a closed solid but is not automatically detected as such, set the Force closed flag manually. A mesh that is not closed and not force-closed will produce no volume blockage — the simulator will treat it as completely open. This is one of the most common causes of unexpectedly low congestion or missing blockage in imported geometry models.

Convert mesh pipes to cylinder primitives

Where pipe geometry has been imported as mesh rather than recognised as cylinder and torus primitives, use the CASD mesh tools to convert mesh pipes to cylinders. Cylinder and torus primitives are processed far more efficiently by FGC and produce more accurate drag coefficients for the sub-grid turbulence model.

Merge and split mesh objects appropriately

• Merge closed solids: use the Merge mesh option to join mesh surfaces that together represent a single solid object but were imported as separate primitives.
• Split meshes: use Split mesh or Convex decompose mesh when a single mesh primitive contains multiple distinct objects. Note that splitting may produce "inside-out" mesh objects (representing the inner surface of a hollow object); these should be deleted.

Add porosity to grating and perforated surfaces

Grated floors, perforated plates, and louvers are typically modelled in CAD as solid plates. In FLACS, these must have an appropriate area porosity value set to correctly model partial flow obstruction. Porosity information is not exported by geo2flacs — it must be added manually in CASD after import.

Align geometry with the grid axes (optional)

The FLACS structured Cartesian grid represents axis-aligned geometry most accurately. If your facility has a clear dominant orientation — for example, a structure rotated at an angle to the cardinal directions — you may wish to use the CASD Auto-align function to rotate the geometry so that principal structural elements are aligned with the X, Y, and Z axes. This is not always necessary and many projects proceed without it. If you do align the geometry, do so before building the grid as changing the rotation afterwards requires the grid to be rebuilt.

Optimise high polygon-count meshes

If the Optimise meshes option was not used at import time, polygon count can be reduced after import using the mesh decimation tools in CASD. Target a polygon count that is appropriate for the grid resolution being used — detail finer than one grid cell has no influence on the simulation result.

Troubleshooting common import failures

Import completes but geometry is missing

• Check the geo2flacs diagnostic output for conversion errors or skipped objects.
• If mesh objects are missing, the model may have run out of memory during conversion. Try using --split-files to import in smaller pieces, or use geo2flacs from the command line instead of the CASD GUI.
• Enable Import fragments as instances (NWD) to reduce memory use.
• If using DWG/DGN with "Assume all objects are solid" enabled, try disabling it — this can cause non-geometry objects to be incorrectly imported as solid objects, which may prevent other geometry from loading correctly.

Import fails or takes an unacceptably long time

Work through these steps in order:

1. Set the minimum diameter filter to 50 mm in the import dialogue and enable Optimise meshes under Advanced options — these two settings alone can dramatically reduce import complexity
2. Split by file — use Import as external nodes to process each input file to a separate output rather than combining everything into one geometry file
3. Split in the source application — divide the model into smaller sections before exporting; import each section separately
4. Fall back to the command line — run geo2flacs from the terminal to eliminate the visualisation memory overhead; see Optional: using geo2flacs from the command line

If you cannot modify or split the source file (for example, the geometry was provided by a client):

• Enable mesh curvature analysis under Advanced options to convert mesh pipes to primitives, reducing mesh density
• If the file is NWD, request the model as RVM or DWG from the client — this is often far quicker than trying to force a mesh-heavy file to import reliably
• As a last resort, enable the legacy Generate FLACS model option to convert unrecognised objects to bounding boxes — this trades geometric accuracy for tractability; see The legacy "Generate FLACS model" option for the consequences
• Contact Gexcon support at flacs@gexcon.com with the diagnostic log file (see Capturing diagnostic output) — providing examples of problematic geometry directly informs future geo2flacs improvements

Pipes imported as large, incorrect torus primitives

This is a known issue with some DWG files where pipe bend geometry uses an unusual internal representation that geo2flacs maps to an oversized torus. Workarounds include:

• Enable mesh curvature analysis to attempt better detection of the pipe geometry
• Disable curvature analysis and manually replace incorrectly sized tori with correct primitives in CASD
• If possible, convert the pipe bends to standard geometry in the source CAD application before export

The conversion algorithms are continuously improved in newer FLACS versions as new examples of problematic geometry are identified.

Primitive geometry from AutoCAD or MicroStation fails to import

Some DWG and DGN files use non-standard or unusual geometry constructions that geo2flacs does not recognise. Try the following:

• Enable Assume circles are connected to fix pipe import issues
• Enable Merge non-solid shapes to help reconstruct solid boxes from individual surface planes
• Review the diagnostic output for specific element types that are failing; this can help identify whether a preprocessing step in the CAD application is needed
• For complex cases, exporting to a different format (e.g., RVM if AVEVA tools are available) may produce better results

Geometry preparation in the source CAD application

For large or complex projects, preprocessing the geometry in the source CAD application before import can significantly reduce import time and improve result quality. Common preprocessing steps include:

• Split into modules: divide the model into process areas or functional sections and export each as a separate file. Import each file separately using --split-files.
• Remove unnecessary detail: delete or hide objects that will not influence the simulation — ladders, handrails, cable trays, instrumentation details, escape route markings. This reduces polygon count before import.
• Simplify mesh objects: if using mesh-heavy formats, use the CAD application's mesh simplification or decimation tools to reduce polygon count, targeting objects that are smaller than the expected FLACS grid resolution.
• Standardise geometry types: where possible, replace mesh representations of pipes and standard equipment with parametric solid objects that will be recognised by geo2flacs as FLACS primitives.
• Separate near-field from far-field: create separate exports for the region of interest (full detail) and surrounding areas (simplified geometry).