OSX specific: Fixed darker colors of objects inside multi-material gizmo on macOS running on Arm64 CPU.

For Apple's on Arm CPU computed triangle normals inside fragment shader using dFdx and dFdy has the opposite direction. Because of this, objects had darker colors inside the multi-material gizmo.
Based on https://stackoverflow.com/a/66206648, the similar behavior was also spotted on some other devices with Arm CPU.

resources/shaders/gouraud.fs

@@ -64,6 +64,9 @@ void main()
     float world_normal_z_fs = world_normal_z;
     if (compute_triangle_normals_in_fs) {
         vec3 triangle_normal = normalize(cross(dFdx(model_pos.xyz), dFdy(model_pos.xyz)));
+#ifdef FLIP_TRIANGLE_NORMALS
+        triangle_normal = -triangle_normal;
+#endif
 
         // First transform the normal into camera space and normalize the result.
         eye_normal_fs = normalize(gl_NormalMatrix * triangle_normal);

src/libslic3r/Platform.cpp

@@ -3,6 +3,12 @@
 #include <boost/log/trivial.hpp>
 #include <boost/filesystem/operations.hpp>
 
+#if defined(__APPLE__)
+#include <sys/types.h>
+#include <sys/sysctl.h>
+#include <mach/machine.h>
+#endif
+
 namespace Slic3r {
 
 static auto s_platform 		  = Platform::Uninitialized;
@@ -17,7 +23,38 @@ void detect_platform()
 #elif defined(__APPLE__)
     BOOST_LOG_TRIVIAL(info) << "Platform: OSX";
     s_platform        = Platform::OSX;
-	s_platform_flavor = PlatformFlavor::Generic;
+    s_platform_flavor = PlatformFlavor::GenericOSX;
+    {
+        cpu_type_t type = 0;
+        size_t     size = sizeof(type);
+        if (sysctlbyname("hw.cputype", &type, &size, NULL, 0) == 0) {
+            type &= ~CPU_ARCH_MASK;
+            if (type == CPU_TYPE_X86) {
+                int proc_translated = 0;
+                size                = sizeof(proc_translated);
+                // Detect if native CPU is really X86 or PrusaSlicer runs through Rosetta.
+                if (sysctlbyname("sysctl.proc_translated", &proc_translated, &size, NULL, 0) == -1) {
+                    if (errno == ENOENT) {
+                        // Native CPU is X86, and property sysctl.proc_translated doesn't exist.
+                        s_platform_flavor = PlatformFlavor::OSXOnX86;
+                        BOOST_LOG_TRIVIAL(info) << "Platform flavor: OSXOnX86";
+                    }
+                } else if (proc_translated == 1) {
+                    // Native CPU is ARM and PrusaSlicer runs through Rosetta.
+                    s_platform_flavor = PlatformFlavor::OSXOnArm;
+                    BOOST_LOG_TRIVIAL(info) << "Platform flavor: OSXOnArm";
+                } else {
+                    // Native CPU is X86.
+                    s_platform_flavor = PlatformFlavor::OSXOnX86;
+                    BOOST_LOG_TRIVIAL(info) << "Platform flavor: OSXOnX86";
+                }
+            } else if (type == CPU_TYPE_ARM) {
+                // Native CPU is ARM
+                s_platform_flavor = PlatformFlavor::OSXOnArm;
+                BOOST_LOG_TRIVIAL(info) << "Platform flavor: OSXOnArm";
+            }
+        }
+    }
 #elif defined(__linux__)
     BOOST_LOG_TRIVIAL(info) << "Platform: Linux";
 	s_platform 		  = Platform::Linux;

src/libslic3r/Platform.hpp

@@ -28,6 +28,12 @@ enum class PlatformFlavor
 	WSL2,
 	// For Platform::BSDUnix
 	OpenBSD,
+	// For Platform::OSX
+	GenericOSX,
+	// For Apple's on Intel X86 CPU
+	OSXOnX86,
+	// For Apple's on Arm CPU
+	OSXOnArm,
 };
 
 // To be called on program start-up.

src/slic3r/GUI/GLShadersManager.cpp

@@ -1,4 +1,5 @@
 #include "libslic3r/libslic3r.h"
+#include "libslic3r/Platform.hpp"
 #include "GLShadersManager.hpp"
 #include "3DScene.hpp"
 #include "GUI_App.hpp"
@@ -43,6 +44,16 @@ std::pair<bool, std::string> GLShadersManager::init()
     // used to render extrusion and travel paths as lines in gcode preview
     valid &= append_shader("toolpaths_lines", { "toolpaths_lines.vs", "toolpaths_lines.fs" });
     // used to render objects in 3d editor
+    // For Apple's on Arm CPU computed triangle normals inside fragment shader using dFdx and dFdy has the opposite direction.
+    // Because of this, objects had darker colors inside the multi-material gizmo.
+    // Based on https://stackoverflow.com/a/66206648, the similar behavior was also spotted on some other devices with Arm CPU.
+    if (platform_flavor() == PlatformFlavor::OSXOnArm)
+        valid &= append_shader("gouraud", { "gouraud.vs", "gouraud.fs" }, { "FLIP_TRIANGLE_NORMALS"sv
+#if ENABLE_ENVIRONMENT_MAP
+            , "ENABLE_ENVIRONMENT_MAP"sv
+#endif
+        });
+    else
         valid &= append_shader("gouraud", { "gouraud.vs", "gouraud.fs" }
 #if ENABLE_ENVIRONMENT_MAP
             , { "ENABLE_ENVIRONMENT_MAP"sv }