Replace hand-made SDL GUI with Dear ImGui for interactive renderer

Integrates ImGui v1.92.6 with SDL2+SDL_Renderer backend, replacing all
manual slider/button rendering and hit-testing. UI now features collapsible
sections (Performance, Camera, Environment, Help), live SPS/ms graphs,
and ImGui input capture guards. Removes SDL2_TTF font dependency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pmudry

CMakeLists.txt

@@ -69,6 +69,13 @@ endif()
 set(EXTERNAL
     src/external/stb_image_write.h
     src/external/stb_image.h
+    src/external/imgui/imgui.cpp
+    src/external/imgui/imgui_demo.cpp
+    src/external/imgui/imgui_draw.cpp
+    src/external/imgui/imgui_tables.cpp
+    src/external/imgui/imgui_widgets.cpp
+    src/external/imgui/imgui_impl_sdl2.cpp
+    src/external/imgui/imgui_impl_sdlrenderer2.cpp
 )
 
 set(SOURCE_RAYON

src/external/imgui/imconfig.h

@@ -0,0 +1,147 @@
+//-----------------------------------------------------------------------------
+// DEAR IMGUI COMPILE-TIME OPTIONS
+// Runtime options (clipboard callbacks, enabling various features, etc.) can generally be set via the ImGuiIO structure.
+// You can use ImGui::SetAllocatorFunctions() before calling ImGui::CreateContext() to rewire memory allocation functions.
+//-----------------------------------------------------------------------------
+// A) You may edit imconfig.h (and not overwrite it when updating Dear ImGui, or maintain a patch/rebased branch with your modifications to it)
+// B) or '#define IMGUI_USER_CONFIG "my_imgui_config.h"' in your project and then add directives in your own file without touching this template.
+//-----------------------------------------------------------------------------
+// You need to make sure that configuration settings are defined consistently _everywhere_ Dear ImGui is used, which include the imgui*.cpp
+// files but also _any_ of your code that uses Dear ImGui. This is because some compile-time options have an affect on data structures.
+// Defining those options in imconfig.h will ensure every compilation unit gets to see the same data structure layouts.
+// Call IMGUI_CHECKVERSION() from your .cpp file to verify that the data structures your files are using are matching the ones imgui.cpp is using.
+//-----------------------------------------------------------------------------
+
+#pragma once
+
+//---- Define assertion handler. Defaults to calling assert().
+// - If your macro uses multiple statements, make sure is enclosed in a 'do { .. } while (0)' block so it can be used as a single statement.
+// - Compiling with NDEBUG will usually strip out assert() to nothing, which is NOT recommended because we use asserts to notify of programmer mistakes.
+//#define IM_ASSERT(_EXPR)  MyAssert(_EXPR)
+//#define IM_ASSERT(_EXPR)  ((void)(_EXPR))     // Disable asserts
+
+//---- Define attributes of all API symbols declarations, e.g. for DLL under Windows
+// Using Dear ImGui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.
+// - Windows DLL users: heaps and globals are not shared across DLL boundaries! You will need to call SetCurrentContext() + SetAllocatorFunctions()
+//   for each static/DLL boundary you are calling from. Read "Context and Memory Allocators" section of imgui.cpp for more details.
+//#define IMGUI_API __declspec(dllexport)                   // MSVC Windows: DLL export
+//#define IMGUI_API __declspec(dllimport)                   // MSVC Windows: DLL import
+//#define IMGUI_API __attribute__((visibility("default")))  // GCC/Clang: override visibility when set is hidden
+
+//---- Don't define obsolete functions/enums/behaviors. Consider enabling from time to time after updating to clean your code of obsolete function/names.
+//#define IMGUI_DISABLE_OBSOLETE_FUNCTIONS
+
+//---- Disable all of Dear ImGui or don't implement standard windows/tools.
+// It is very strongly recommended to NOT disable the demo windows and debug tool during development. They are extremely useful in day to day work. Please read comments in imgui_demo.cpp.
+//#define IMGUI_DISABLE                                     // Disable everything: all headers and source files will be empty.
+//#define IMGUI_DISABLE_DEMO_WINDOWS                        // Disable demo windows: ShowDemoWindow()/ShowStyleEditor() will be empty.
+//#define IMGUI_DISABLE_DEBUG_TOOLS                         // Disable metrics/debugger and other debug tools: ShowMetricsWindow(), ShowDebugLogWindow() and ShowIDStackToolWindow() will be empty.
+
+//---- Don't implement some functions to reduce linkage requirements.
+//#define IMGUI_DISABLE_WIN32_DEFAULT_CLIPBOARD_FUNCTIONS   // [Win32] Don't implement default clipboard handler. Won't use and link with OpenClipboard/GetClipboardData/CloseClipboard etc. (user32.lib/.a, kernel32.lib/.a)
+//#define IMGUI_ENABLE_WIN32_DEFAULT_IME_FUNCTIONS          // [Win32] [Default with Visual Studio] Implement default IME handler (require imm32.lib/.a, auto-link for Visual Studio, -limm32 on command-line for MinGW)
+//#define IMGUI_DISABLE_WIN32_DEFAULT_IME_FUNCTIONS         // [Win32] [Default with non-Visual Studio compilers] Don't implement default IME handler (won't require imm32.lib/.a)
+//#define IMGUI_DISABLE_WIN32_FUNCTIONS                     // [Win32] Won't use and link with any Win32 function (clipboard, IME).
+//#define IMGUI_ENABLE_OSX_DEFAULT_CLIPBOARD_FUNCTIONS      // [OSX] Implement default OSX clipboard handler (need to link with '-framework ApplicationServices', this is why this is not the default).
+//#define IMGUI_DISABLE_DEFAULT_SHELL_FUNCTIONS             // Don't implement default platform_io.Platform_OpenInShellFn() handler (Win32: ShellExecute(), require shell32.lib/.a, Mac/Linux: use system("")).
+//#define IMGUI_DISABLE_DEFAULT_FORMAT_FUNCTIONS            // Don't implement ImFormatString/ImFormatStringV so you can implement them yourself (e.g. if you don't want to link with vsnprintf)
+//#define IMGUI_DISABLE_DEFAULT_MATH_FUNCTIONS              // Don't implement ImFabs/ImSqrt/ImPow/ImFmod/ImCos/ImSin/ImAcos/ImAtan2 so you can implement them yourself.
+//#define IMGUI_DISABLE_FILE_FUNCTIONS                      // Don't implement ImFileOpen/ImFileClose/ImFileRead/ImFileWrite and ImFileHandle at all (replace them with dummies)
+//#define IMGUI_DISABLE_DEFAULT_FILE_FUNCTIONS              // Don't implement ImFileOpen/ImFileClose/ImFileRead/ImFileWrite and ImFileHandle so you can implement them yourself if you don't want to link with fopen/fclose/fread/fwrite. This will also disable the LogToTTY() function.
+//#define IMGUI_DISABLE_DEFAULT_ALLOCATORS                  // Don't implement default allocators calling malloc()/free() to avoid linking with them. You will need to call ImGui::SetAllocatorFunctions().
+//#define IMGUI_DISABLE_DEFAULT_FONT                        // Disable default embedded font (ProggyClean.ttf), remove ~9.5 KB from output binary. AddFontDefault() will assert.
+//#define IMGUI_DISABLE_SSE                                 // Disable use of SSE intrinsics even if available
+
+//---- Enable Test Engine / Automation features.
+//#define IMGUI_ENABLE_TEST_ENGINE                          // Enable imgui_test_engine hooks. Generally set automatically by include "imgui_te_config.h", see Test Engine for details.
+
+//---- Include imgui_user.h at the end of imgui.h as a convenience
+// May be convenient for some users to only explicitly include vanilla imgui.h and have extra stuff included.
+//#define IMGUI_INCLUDE_IMGUI_USER_H
+//#define IMGUI_USER_H_FILENAME         "my_folder/my_imgui_user.h"
+
+//---- Pack vertex colors as BGRA8 instead of RGBA8 (to avoid converting from one to another). Need dedicated backend support.
+//#define IMGUI_USE_BGRA_PACKED_COLOR
+
+//---- Use legacy CRC32-adler tables (used before 1.91.6), in order to preserve old .ini data that you cannot afford to invalidate.
+//#define IMGUI_USE_LEGACY_CRC32_ADLER
+
+//---- Use 32-bit for ImWchar (default is 16-bit) to support Unicode planes 1-16. (e.g. point beyond 0xFFFF like emoticons, dingbats, symbols, shapes, ancient languages, etc...)
+//#define IMGUI_USE_WCHAR32
+
+//---- Avoid multiple STB libraries implementations, or redefine path/filenames to prioritize another version
+// By default the embedded implementations are declared static and not available outside of Dear ImGui sources files.
+//#define IMGUI_STB_TRUETYPE_FILENAME   "my_folder/stb_truetype.h"
+//#define IMGUI_STB_RECT_PACK_FILENAME  "my_folder/stb_rect_pack.h"
+//#define IMGUI_STB_SPRINTF_FILENAME    "my_folder/stb_sprintf.h"    // only used if IMGUI_USE_STB_SPRINTF is defined.
+//#define IMGUI_DISABLE_STB_TRUETYPE_IMPLEMENTATION
+//#define IMGUI_DISABLE_STB_RECT_PACK_IMPLEMENTATION
+//#define IMGUI_DISABLE_STB_SPRINTF_IMPLEMENTATION                   // only disabled if IMGUI_USE_STB_SPRINTF is defined.
+
+//---- Use stb_sprintf.h for a faster implementation of vsnprintf instead of the one from libc (unless IMGUI_DISABLE_DEFAULT_FORMAT_FUNCTIONS is defined)
+// Compatibility checks of arguments and formats done by clang and GCC will be disabled in order to support the extra formats provided by stb_sprintf.h.
+//#define IMGUI_USE_STB_SPRINTF
+
+//---- Use FreeType to build and rasterize the font atlas (instead of stb_truetype which is embedded by default in Dear ImGui)
+// Requires FreeType headers to be available in the include path. Requires program to be compiled with 'misc/freetype/imgui_freetype.cpp' (in this repository) + the FreeType library (not provided).
+// Note that imgui_freetype.cpp may be used _without_ this define, if you manually call ImFontAtlas::SetFontLoader(). The define is simply a convenience.
+// On Windows you may use vcpkg with 'vcpkg install freetype --triplet=x64-windows' + 'vcpkg integrate install'.
+//#define IMGUI_ENABLE_FREETYPE
+
+//---- Use FreeType + plutosvg or lunasvg to render OpenType SVG fonts (SVGinOT)
+// Only works in combination with IMGUI_ENABLE_FREETYPE.
+// - plutosvg is currently easier to install, as e.g. it is part of vcpkg. It will support more fonts and may load them faster. See misc/freetype/README for instructions.
+// - Both require headers to be available in the include path + program to be linked with the library code (not provided).
+// - (note: lunasvg implementation is based on Freetype's rsvg-port.c which is licensed under CeCILL-C Free Software License Agreement)
+//#define IMGUI_ENABLE_FREETYPE_PLUTOSVG
+//#define IMGUI_ENABLE_FREETYPE_LUNASVG
+
+//---- Use stb_truetype to build and rasterize the font atlas (default)
+// The only purpose of this define is if you want force compilation of the stb_truetype backend ALONG with the FreeType backend.
+//#define IMGUI_ENABLE_STB_TRUETYPE
+
+//---- Define constructor and implicit cast operators to convert back<>forth between your math types and ImVec2/ImVec4.
+// This will be inlined as part of ImVec2 and ImVec4 class declarations.
+/*
+#define IM_VEC2_CLASS_EXTRA                                                     \
+        constexpr ImVec2(const MyVec2& f) : x(f.x), y(f.y) {}                   \
+        operator MyVec2() const { return MyVec2(x,y); }
+
+#define IM_VEC4_CLASS_EXTRA                                                     \
+        constexpr ImVec4(const MyVec4& f) : x(f.x), y(f.y), z(f.z), w(f.w) {}   \
+        operator MyVec4() const { return MyVec4(x,y,z,w); }
+*/
+//---- ...Or use Dear ImGui's own very basic math operators.
+//#define IMGUI_DEFINE_MATH_OPERATORS
+
+//---- Use 32-bit vertex indices (default is 16-bit) is one way to allow large meshes with more than 64K vertices.
+// Your renderer backend will need to support it (most example renderer backends support both 16/32-bit indices).
+// Another way to allow large meshes while keeping 16-bit indices is to handle ImDrawCmd::VtxOffset in your renderer.
+// Read about ImGuiBackendFlags_RendererHasVtxOffset for details.
+//#define ImDrawIdx unsigned int
+
+//---- Override ImDrawCallback signature (will need to modify renderer backends accordingly)
+//struct ImDrawList;
+//struct ImDrawCmd;
+//typedef void (*MyImDrawCallback)(const ImDrawList* draw_list, const ImDrawCmd* cmd, void* my_renderer_user_data);
+//#define ImDrawCallback MyImDrawCallback
+
+//---- Debug Tools: Macro to break in Debugger (we provide a default implementation of this in the codebase)
+// (use 'Metrics->Tools->Item Picker' to pick widgets with the mouse and break into them for easy debugging.)
+//#define IM_DEBUG_BREAK  IM_ASSERT(0)
+//#define IM_DEBUG_BREAK  __debugbreak()
+
+//---- Debug Tools: Enable highlight ID conflicts _before_ hovering items. When io.ConfigDebugHighlightIdConflicts is set.
+// (THIS WILL SLOW DOWN DEAR IMGUI. Only use occasionally and disable after use)
+//#define IMGUI_DEBUG_HIGHLIGHT_ALL_ID_CONFLICTS
+
+//---- Debug Tools: Enable slower asserts
+//#define IMGUI_DEBUG_PARANOID
+
+//---- Tip: You can add extra functions within the ImGui:: namespace from anywhere (e.g. your own sources/header files)
+/*
+namespace ImGui
+{
+    void MyFunction(const char* name, MyMatrix44* mtx);
+}
+*/