From 4b4a261787ee815fbde96adfe04f3871d88acc16 Mon Sep 17 00:00:00 2001
From: TheLegendTubaGuy <95944177+thelegendtubaguy@users.noreply.github.com>
Date: Sun, 4 Oct 2026 16:23:40 -0500
Subject: [PATCH] Fix small binary STLs failing to load as ASCII (#16130)

---
 deps_src/admesh/stlinit.cpp  | 13 +++++++++++++
 tests/libslic3r/test_stl.cpp | 25 +++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/deps_src/admesh/stlinit.cpp b/deps_src/admesh/stlinit.cpp
index 9d44cdf266..3b8f59866a 100644
--- a/deps_src/admesh/stlinit.cpp
+++ b/deps_src/admesh/stlinit.cpp
@@ -75,6 +75,19 @@ static FILE *stl_open_count_facets(stl_file *stl, const char *file, unsigned int
       		break;
     	}
   	}
+  	// Zero normals and coordinates like 10 or 15 have no byte above 127, so the test above can miss a binary file.
+  	// Its size still matches its facet count; text read as that count would need a file of gigabytes.
+  	if (stl->stats.type == ascii) {
+    	uint32_t header_num_facets;
+    	fseek(fp, custom_header_length, SEEK_SET);
+    	if (fread(&header_num_facets, sizeof(uint32_t), 1, fp) == 1) {
+#if BOOST_ENDIAN_BIG_BYTE
+      		stl_internal_reverse_quads((char*)&header_num_facets, 4);
+#endif /* BOOST_ENDIAN_BIG_BYTE */
+      		if (header_size + uint64_t(header_num_facets) * SIZEOF_STL_FACET == file_size)
+        		stl->stats.type = binary;
+    	}
+  	}
   	rewind(fp);
 
   	uint32_t num_facets = 0;
diff --git a/tests/libslic3r/test_stl.cpp b/tests/libslic3r/test_stl.cpp
index 0223392c3f..b121bfc122 100644
--- a/tests/libslic3r/test_stl.cpp
+++ b/tests/libslic3r/test_stl.cpp
@@ -1,10 +1,13 @@
 #include <catch2/catch_all.hpp>
+#include <algorithm>
 #include <string>
 #include "libslic3r/Point.hpp"
 
 #include <catch2/catch_test_macros.hpp>
 #include "libslic3r/Model.hpp"
 #include "libslic3r/Format/STL.hpp"
+#include "test_utils.hpp"
+#include <boost/nowide/fstream.hpp>
 
 using namespace Slic3r;
 
@@ -58,3 +61,25 @@ SCENARIO("Reading an STL file", "[stl]") {
 		}
 	}
 }
+
+TEST_CASE("A binary STL whose facet bytes never exceed 127 is read as binary", "[stl]")
+{
+    const indexed_triangle_set cube = its_make_cube(10., 10., 10.);
+    std::string stl(80, '\0');
+    const auto append = [&stl](const auto &value) { stl.append(reinterpret_cast<const char *>(&value), sizeof(value)); };
+    append(uint32_t(cube.indices.size()));
+    const stl_normal zero_normal = stl_normal::Zero();
+    for (const stl_triangle_vertex_indices &facet : cube.indices) {
+        append(zero_normal);
+        for (int i = 0; i < 3; ++i)
+            append(cube.vertices[facet[i]]);
+        append(uint16_t(0));
+    }
+    REQUIRE(std::none_of(stl.begin() + 84, stl.begin() + 84 + 128, [](unsigned char c) { return c > 127; }));
+
+    ScopedTemporaryFile file(".stl");
+    boost::nowide::ofstream(file.string(), std::ios::binary) << stl;
+    Model model;
+    REQUIRE(load_stl(file.string().c_str(), &model));
+    REQUIRE(is_approx(model.objects.front()->volumes.front()->mesh().size(), Vec3d(10, 10, 10)));
+}