From b1a641fa688c8d1d8651672d34e92f9a9da18fad Mon Sep 17 00:00:00 2001
From: Petru Conduraru <petru@bimvoice.com>
Date: Sat, 11 Jul 2026 07:21:38 +0300
Subject: [PATCH] Use NEGATIVE-side IfcSurfaceStyle as a fallback instead of
 dropping it (#5618)

The surface-style picker skipped every IfcSurfaceStyle whose Side was
NEGATIVE outright, so an item styled only with a NEGATIVE-side style was
left completely unstyled and fell back to the gray DefaultMaterial.

Keep preferring a POSITIVE or BOTH style for the visible front face
(returned immediately, unchanged), but remember a NEGATIVE-side style and
its shading and use it as a fallback when no POSITIVE/BOTH style supplies
the requested presentation type. A styled item is then no longer rendered
unstyled.

Verified on OCC 7.9.2 with a two-box IFC4 fixture (one box styled red on
the NEGATIVE side, one green on the POSITIVE side):
- before: NEG box -> DefaultMaterial (Kd 0.7 0.7 0.7), POS box -> green
- after:  NEG box -> its red (Kd 1 0 0), POS box -> green (unchanged)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 src/ifcgeom/mapping/mapping.cpp | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/ifcgeom/mapping/mapping.cpp b/src/ifcgeom/mapping/mapping.cpp
index 56df064c49..2fb209fd3b 100644
--- a/src/ifcgeom/mapping/mapping.cpp
+++ b/src/ifcgeom/mapping/mapping.cpp
@@ -488,20 +488,41 @@ namespace {
 #endif
 
         IfcSchema::IfcSurfaceStyle *surface_style_ = nullptr;
+        // A surface style with Side == NEGATIVE targets the back (negative) face.
+        // When a POSITIVE or BOTH style is also present it is preferred for the
+        // visible front face, but a NEGATIVE style must still be used as a
+        // fallback so a styled item is not left completely unstyled (see #5618).
+        IfcSchema::IfcSurfaceStyle *negative_surface_style_ = nullptr;
+        std::pair<IfcSchema::IfcSurfaceStyle*, T*> negative_fallback{nullptr, nullptr};
         for (auto& style : prs_styles) {
             if (auto surface_style = style->as<IfcSchema::IfcSurfaceStyle>()) {
-                if (surface_style->Side() != IfcSchema::IfcSurfaceSide::IfcSurfaceSide_NEGATIVE) {
+                const bool is_negative = surface_style->Side() == IfcSchema::IfcSurfaceSide::IfcSurfaceSide_NEGATIVE;
+                if (is_negative) {
+                    if (negative_surface_style_ == nullptr) {
+                        negative_surface_style_ = surface_style;
+                    }
+                } else {
                     surface_style_ = surface_style;
-                    auto styles_elements = surface_style->Styles();
-                    for (auto mt = styles_elements->begin(); mt != styles_elements->end(); ++mt) {
-                        if ((*mt)->template as<T>()) {
+                }
+                auto styles_elements = surface_style->Styles();
+                for (auto mt = styles_elements->begin(); mt != styles_elements->end(); ++mt) {
+                    if ((*mt)->template as<T>()) {
+                        if (!is_negative) {
                             return std::make_pair(surface_style, (*mt)->as<T>());
+                        } else if (negative_fallback.first == nullptr) {
+                            negative_fallback = std::make_pair(surface_style, (*mt)->as<T>());
                         }
                     }
                 }
             }
         }
-        return std::make_pair(surface_style_, nullptr);
+        if (negative_fallback.first != nullptr) {
+            return negative_fallback;
+        }
+        if (surface_style_ != nullptr) {
+            return std::make_pair(surface_style_, nullptr);
+        }
+        return std::make_pair(negative_surface_style_, nullptr);
     }
 
     bool process_colour(IfcSchema::IfcColourRgb* colour, double* rgb) {