3dt — STL / STEP / OBJ Viewer

A fast, dependency-free 3D model viewer written in C++17 with native Win32 + OpenGL 3.3. No GLFW, no GLEW, no Assimp, no OpenCASCADE: every parser, the WGL context bootstrap, the GL extension loader and the renderer are implemented from scratch. The result is a single statically-linked executable (~1.3 MB) that runs on any Windows machine with no runtime dependencies.

Features

File formats — all parsers written from scratch

• STL — ASCII and binary, with robust auto-detection (handles binary files whose header starts with solid), chunked reading for multi-million-triangle files.
• STEP (.step / .stp, ISO 10303-21, AP203/AP214/AP242) — a real B-rep tessellator:
  • Part 21 parser with full value trees, complex multi-type instances, malformed-input resync.
  • Surfaces: plane, cylinder, cone, sphere, torus, linear extrusion, surface of revolution, B-spline/NURBS (all knot variants, rational via complex entities).
  • Curves: line, circle, ellipse, polyline, trimmed curve, B-spline (all variants, rational).
  • Full trimming: edges projected to UV space, seam unwrapping on periodic surfaces, hole bridging, ear-clipping triangulation, conforming refinement (no T-junctions), analytic per-vertex normals for smooth curved surfaces.
  • Unknown faces are skipped gracefully — malformed files never crash the viewer.
• OBJ — all index forms (v, v/vt, v//vn, v/vt/vn, negative indices), quad/n-gon fan triangulation, smooth normals from vn with geometric fallback, fast buffer-based parsing.

Rendering

• OpenGL 3.3 core profile with a hand-written WGL bootstrap and extension loader, automatic fallback to legacy fixed-function on old GPUs.
• MSAA 8x (fallback 4x/off), vsync, gamma-correct two-light Blinn-Phong shading, gradient background, two-sided lighting (handles mixed-winding CAD meshes).
• Shadow mapping with 3×3 PCF soft shadows: self-shadowing plus a ground shadow-catcher plane. Toggle at runtime.
• Section views: three axis-aligned clipping planes (X/Y/Z), independently toggleable, position adjustable across the model, flippable side. Applied to shadows too.
• On-screen help overlay and model info panel (triangle/vertex count, bounding box) rendered with a GL text atlas.
• Model tree panel for multi-part STEP assemblies: expand/collapse groups, per-part show/hide checkboxes (shadows follow), scrollable, double-click a row to fit the view on that part.
• Single draw call per pass — smooth interaction with multi-million-triangle meshes.

Controls

Input	Action
Drag & drop a file onto the window (or onto 3dt.exe)	Open model
Left drag / Right drag / Wheel	Orbit / Pan / Zoom
Double click or F	Fit model to view
W	Wireframe on/off
S	Smooth / flat shading
G	Grid on/off
O	Shadows on/off
H	Help overlay on/off
I	Model info panel on/off
T	Model tree panel on/off (multi-part STEP assemblies)
X / Y / Z	Toggle section plane on that axis
Hold X/Y/Z + Wheel	Move the section plane
Shift + X/Y/Z	Flip the cut side
F11	Fullscreen on/off
Ctrl+O	Open file dialog
ESC	Quit

Building

No third-party libraries on any platform: the platform layer (window, GL context, input, drag&drop) is hand-written — Win32/WGL on Windows, Xlib/GLX on Linux, Cocoa/NSOpenGL on macOS.

Windows (MinGW-w64)

Requires a MinGW-w64 toolchain (tested with GCC 15).

# with GNU make
mingw32-make            # release -> build/3dt.exe
mingw32-make DEBUG=1    # debug build

# or with CMake
cmake -S . -B build-cmake -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Release
cmake --build build-cmake

The executable is fully static (-static): it can be copied to any Windows PC as-is.

build/3dt.exe path/to/model.step   # optional: open a file at startup

Linux (X11/GLX)

Requires a C++17 compiler, CMake, and the X11 + OpenGL development packages. On Debian/Ubuntu:

sudo apt install build-essential cmake libx11-dev libgl1-mesa-dev

Then build with CMake (links against -lX11 -lGL only):

cmake -S . -B build-cmake -DCMAKE_BUILD_TYPE=Release
cmake --build build-cmake
./build-cmake/3dt path/to/model.step

The Ctrl+O file dialog uses zenity or kdialog when installed; without them, open files via drag&drop (XDND) or the command line. Note: development happened on Windows — the Linux backend compiles against Xlib/GLX but still needs testing on a real Linux machine.

macOS (Cocoa)

Requires Xcode command line tools (xcode-select --install) and CMake — no other dependencies. The backend is Cocoa/AppKit with an NSOpenGLContext (OpenGL is deprecated on macOS but fully functional; the build silences the deprecation markers):

cmake -S . -B build-cmake -DCMAKE_BUILD_TYPE=Release
cmake --build build-cmake
./build-cmake/3dt path/to/model.step

The result is a plain command-line executable (no .app bundle). Retina displays are rendered at full backing resolution. Cmd+O (or Ctrl+O) opens the file dialog and Cmd+Q quits — the Command key doubles as Ctrl for every shortcut. Note: the macOS backend is built by CI and has not yet been tested on a real Mac (see limitations).

Project layout

src/
  mesh.h / mesh.cpp        shared triangle-soup mesh + bounds/normals helpers
  stl_loader.cpp           STL parser (ASCII + binary)
  obj_loader.cpp           Wavefront OBJ parser
  step_internal.h          STEP geometry model shared by the two STEP units
  step_loader.cpp          ISO 10303-21 (Part 21) parser + entity graph
  step_tess.cpp            surface evaluation, trimming, triangulation
  gl_loader.h / .cpp       manual OpenGL 3.3 function loader (proc address injected)
  renderer.h / .cpp        shaders, shadow mapping, camera, sections (platform-free)
  overlay.h / .cpp         2D text/panel pipeline over a platform font atlas
  platform.h               abstract platform interface (window, GL, input, dialogs)
  platform_win32.cpp       Win32/WGL/GDI backend
  platform_x11.cpp         Linux Xlib/GLX backend (XDND, EWMH)
  platform_macos.mm        macOS Cocoa/AppKit + NSOpenGLContext backend (ARC)
  bitmap_font.h / .cpp     embedded 5x7 overlay font shared by the X11/macOS backends
  app.cpp                  portable app logic: input commands, file dispatch
test/                      sample models (STL ASCII/binary, STEP incl. assemblies, OBJ)

Known limitations

• STEP: OFFSET_SURFACE and composite curves are not supported (such faces are skipped and counted); units are ignored (the viewer auto-fits the model).
• OBJ: materials/textures (.mtl, vt) are parsed but ignored; n-gons are fan-triangulated (convex polygons only).
• Section views render the interior hollow (no cap fill on the cut plane).
• The Linux (X11/GLX) backend compiles but was developed on Windows and still needs testing on a real Linux machine.
• The macOS (Cocoa) backend was written on Windows and is compiled by CI only: it has not yet been tested on a real Mac (rendering, retina scaling, input and the file dialog all need a first hands-on pass).

License

GNU General Public License v3.0 — see LICENSE.

Copyright (C) Manuel Finessi