mirror of
https://github.com/IfcOpenShell/IfcOpenShell.git
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Dion Moult
b123ee69d6
## The bug
FZK-Haus windows rendered fully opaque despite every piece of the
data path carrying alpha correctly: vertex format is RGBA u8x4,
InstanceCpu/InstanceGpu carry color_override_rgba8 with its alpha
byte, fs_main returns vec4(rgb, in.color.a). Cause: the main render
pipeline's color target had `blend = nullptr`, which in wgpu disables
the blend stage entirely — fragment RGBA overwrites the back buffer
unmodified, alpha discarded.
## Why "just enable blend" isn't enough
Two failure modes that don't go away with a one-liner:
1. `depthWriteEnabled = True` on the main pipeline would make a
transparent window-frame pane occlude geometry behind it in
depth, so the wall behind the window then fails the depth test
and never draws — you'd see the silhouette of the window with
whatever colour was in the back buffer before, not the wall.
2. Order-dependent blending across transparent surfaces in arbitrary
cull order — overlapping transparent surfaces would shift colours
as the camera moves.
Standard fix for a BIM viewer is two-pass opaque-then-transparent.
## What this commit adds
### Per-mesh "has any alpha < 255" classifier
* `ModelGpuData::mesh_has_alpha` (uint8_t vector, parallel to meshes).
* Sized in `applyCachedModel`.
* Populated in `applyStreamedChunk` by scanning each in-chunk mesh's
vertex bytes for a vertex's alpha byte < 255 (offset 11 within
the 12-byte vertex record — the 4th byte of the third u32, which
the shader reads as `w2 >> 24`). Single chunk-arrival site covers
both sidecar streaming and the worker-result drain. First-load
IFC-without-sidecar geometry still routes opaque until the sidecar
bake completes; A-path scan is deferred.
### Per-chunk opaque/transparent partition during cull
* `Chunk::opaque_visible_vertices` / `opaque_visible_draws`
(per-frame counts).
* Transient `visible_draws_scratch_transparent` +
`transparent_per_draw_vertex_counts` filled alongside the existing
opaque half during the cull walk. Post-walk concat appends
transparent entries onto the opaque half and continues the
cumulative prefix-sum sequence — single buffer, single bind
group, no doubling.
* Classifier inside the cull lambda:
`xray_active ? always_transparent
: override_active ? (override.alpha < 255)
: mesh_has_alpha[mesh_id]`
### Per-chunk uniform layout extension
From `[total_draws, total_verts, 0, 0]` to
`[total_draws, total_verts, opaque_verts, opaque_draws]`. The third
slot is what `render()` passes as `firstVertex` to the transparent-
pass draw call so the shader's vid lands in the transparent range of
the same visible_draws_scratch buffer.
### `main_pipeline_transparent_`
Copy of `main_pipeline_` with `color_target.blend = SrcAlpha /
OneMinusSrcAlpha`. depthWriteEnabled stays True (see below).
### Two-pass `render()`
Opaque pass (`main_pipeline_`, firstVertex=0,
vertexCount=opaque_visible_vertices) then transparent pass
(`main_pipeline_transparent_`, firstVertex=opaque_visible_vertices,
vertexCount=total - opaque). Each loop skips empty halves so an
opaque-only chunk costs one draw call, transparent-only one draw,
mixed chunks two.
### depth_transparent.depthWriteEnabled = True (NOT off)
Initially set False (standard "let further-back geometry paint
through transparent front faces" trick) but that broke the edge-
detect pass: edge detection reads the depth buffer to find
silhouette discontinuities, and windows-without-depth meant the
glass had no silhouette at all (panes looked like framed holes) and
the edges of opaque geometry behind the glass painted through at
full intensity. Keeping the write avoids that — trade-off is depth-
test occlusion between transparent surfaces (closer occludes
farther), which for BIM panes that don't overlap in screen space
is invisible. Real fix for the overlap case is OIT or sort-back-
to-front, not depth-write toggling.
## Alt+X global X-ray (drops in basically free)
* `xray_alpha_cap` field on FrameUniforms + WGSL counterpart, default
1.0 (no effect). fs_main clamps `out.a = min(in.color.a, cap)`.
* `ViewportWindow::xray_alpha_cap_` member, default 1.0. Alt+X
toggles between 1.0 and 0.3.
* Cull classifier sees `xray_alpha_cap_ < 1.0` and forces every
instance into the transparent pass so the blend stage actually
fires (an opaque-pass fragment with capped alpha would still
overwrite the back buffer).
* No per-instance state mutation needed — toggle is a single float
in a uniform plus a re-cull. Excluding objects from x-ray later
would mean tagging them so the classifier skips the force-
transparent branch for them, also small.
Stress-tested on FZK-Haus: window glass visibly translucent with
correct silhouette edges; Alt+X turns the whole scene to a tinted
ghost of itself and back without artefact.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
IfcOpenShell
IfcOpenShell is an open source (LGPL) software library for working with Industry Foundation Classes (IFC). Complete parsing support is provided for IFC2x3 TC1, IFC4 Add2 TC1, IFC4x1, IFC4x2, and IFC4x3 Add2. Extensive geometric support is implemented for the IFC releases IFC2x3 TC1 and IFC4 Add2 TC1. Extending with support for arbitrary IFC schemas is possible at compile-time when using C++ and at run-time when using Python.
In addition to a C++ and Python API, IfcOpenShell comes with an ecosystem of tools, notably including IfcConvert (an application to convert IFC models to other formats), Bonsai (an add-on to Blender providing a graphical IFC authoring platform), and many other libraries, CLI apps, and more. Support is also provided for auxiliary standards such as BCF and IDS.
For more information, see:
Development is sponsored through your generous donations!
Contents
| Name | Description | License | Service |
|---|---|---|---|
| bcf | Library to read and write BCF-XML and query OpenCDE BCF-API modules | LGPL-3.0-or-later |   |
| bonsai | Add-on to Blender providing a graphical native IFC authoring platform | GPL-3.0-or-later |    |
| bsdd | Library to query the bSDD API | LGPL-3.0-or-later |  |
| ifc2ca | Utility to convert IFC structural analysis models to Code_Aster | LGPL-3.0-or-later | |
| ifc4d | Convert to and from IFC and project management software | LGPL-3.0-or-later |  |
| ifc5d | Report and optimise cost information from IFC | LGPL-3.0-or-later |  |
| ifcbimtester | Wrapper for Gherkin based unit testing for IFC models | LGPL-3.0-or-later | |
| ifcblender | Historic Blender IFC import add-on | LGPL-3.0-or-later* | |
| ifccityjson | Convert CityJSON to IFC | LGPL-3.0-or-later |  |
| ifcclash | Clash detection library and CLI app | LGPL-3.0-or-later |  |
| ifcconvert | CLI app to convert IFC to many other formats | LGPL-3.0-or-later* |   |
| ifccsv | Library and CLI app to export and import schedules from IFC | LGPL-3.0-or-later |  |
| ifcdiff | Compare changes between IFC models | LGPL-3.0-or-later |  |
| ifcedit | CLI wrapper for ifcopenshell.api IFC model mutation functions | LGPL-3.0-or-later |  |
| ifcfm | Extract IFC data for FM handover requirements | LGPL-3.0-or-later |  |
| ifcmax | Historic extension for IFC support in 3DS Max | LGPL-3.0-or-later* | |
| ifcmcp | MCP server for querying and editing IFC building models | LGPL-3.0-or-later |  |
| ifcopenshell-python | Python library for IFC manipulation | LGPL-3.0-or-later* |         |
| ifcpatch | Utility to run pre-packaged scripts to manipulate IFCs | LGPL-3.0-or-later |  |
| ifcquery | CLI tool for querying and inspecting IFC building models | LGPL-3.0-or-later |  |
| ifcsverchok | Blender Add-on for visual node programming with IFC | GPL-3.0-or-later |  |
| ifctester | Library, CLI and webapp for IDS model auditing | LGPL-3.0-or-later |  |
The IfcOpenShell C++ codebase is split into multiple interal libraries:
| Name | Description | License |
|---|---|---|
| ifcgeom | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| ifcgeom_schema_agnostic | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| ifcgeomserver | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| ifcjni | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| ifcparse | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| ifcwrap | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| qtviewer | Internal library for IfcOpenShell | LGPL-3.0-or-later* |
| serializers | Internal library for IfcOpenShell | LGPL-3.0-or-later* |