Adapter to add imgui to an existing vtkRenderWindow.

Provides two libraries:

• imgui-vtk a portion of imgui, with common headers and headers using sdl2 and opengl3.
• vtkImGuiAdapter that accepts an already populated vtkRenderWindow (with renderer, interactor, and actors) and permits mixing it with imgui components. It links to imgui-vtk.

The dependencies needed are VTK (9.0 at least) and OpenGL. If VTK has been compiled with VTK_USE_SDL2, we don't need anything more. Otherwise, files needed from VTK SDL2 module will be compiled here, and we will require SDL2 and GLEW as well. This is to avoid making you rebuild VTK with VTK_USE_SDL2=ON.

The libraries provide a vtkImGuiAdapterConfig.cmake in build and install tree to facilitate integration with other libraries using CMake.

Build example (bash)

mkdir vtkImGuiAdapter; cd vtkImGuiAdapter
git clone --recursive https://github.com/phcerdan/vtkImGuiAdapter src
mkdir build; cd build
cmake -GNinja -DCMAKE_BUILD_TYPE=Release ../src
# Or specify dependencies location
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release ../src -DVTK_DIR=/path/to
# It accepts building with BUILD_SHARED_LIBS ON or OFF.

Screenshot from example-imgui-vtk-sdl.cpp

#include "vtkImGuiSDL2OpenGLRenderWindow.h"
#include "vtkImGuiSDL2RenderWindowInteractor.h"

#include <vtkActor.h>
#include <vtkConeSource.h>
#include <vtkGlyph3D.h>
#include <vtkNew.h>
#include <vtkPolyData.h>
#include <vtkPolyDataMapper.h>
#include <vtkRenderer.h>
#include <vtkInteractorStyleSwitch.h>
#include <vtkSphereSource.h>

#include "imgui.h"
#include "imgui_impl_sdl.h"
#include "imgui_impl_opengl3.h"

#if (defined(__APPLE__) && (TARGET_OS_IOS || TARGET_OS_TV))
#include <OpenGLES/ES3/gl.h>
#else
#include <GLES3/gl3.h>
#endif

int main(int, char**)
{
  // VTK side, draw
  vtkNew<vtkRenderer> renderer;
  vtkNew<vtkImGuiSDL2OpenGLRenderWindow> renWin;
  renWin->AddRenderer(renderer);
  vtkNew<vtkImGuiSDL2RenderWindowInteractor> iren;
  iren->SetRenderWindow(renWin);

  vtkNew<vtkInteractorStyleSwitch> style;
  style->SetCurrentStyleToTrackballCamera();
  style->SetDefaultRenderer(renderer);
  iren->SetInteractorStyle(style);

  vtkNew<vtkSphereSource> sphere;
  sphere->SetThetaResolution(8);
  sphere->SetPhiResolution(8);
  vtkNew<vtkPolyDataMapper> sphereMapper;
  sphereMapper->SetInputConnection(sphere->GetOutputPort());
  vtkNew<vtkActor> sphereActor;
  sphereActor->SetMapper(sphereMapper);

  vtkNew<vtkConeSource> cone;
  cone->SetResolution(6);

  vtkNew<vtkGlyph3D> glyph;
  glyph->SetInputConnection(sphere->GetOutputPort());
  glyph->SetSourceConnection(cone->GetOutputPort());
  glyph->SetVectorModeToUseNormal();
  glyph->SetScaleModeToScaleByVector();
  glyph->SetScaleFactor(0.25);

  vtkNew<vtkPolyDataMapper> spikeMapper;
  spikeMapper->SetInputConnection(glyph->GetOutputPort());

  vtkNew<vtkActor> spikeActor;
  spikeActor->SetMapper(spikeMapper);

  renderer->AddActor(sphereActor);
  renderer->AddActor(spikeActor);
  renderer->SetBackground(0.2, 0.3, 0.4);
  renWin->SetSize(800, 800);

  iren->Initialize();

  // ImGui side. The window is created and handled by VTK, we pass it to ImGui.
  SDL_Window * window = static_cast<SDL_Window*>(renWin->GetGenericWindowId());
  IMGUI_CHECKVERSION();
  ImGui::CreateContext();
  ImGui::StyleColorsDark();
  ImGuiIO& io = ImGui::GetIO(); (void)io;
  iren->SetImguiIO(&io);
  SDL_GLContext imgui_context = SDL_GL_CreateContext(window);
  SDL_GL_MakeCurrent(window, imgui_context);
  SDL_GL_SetSwapInterval(1); // Enable vsync

  ImGui_ImplSDL2_InitForOpenGL(window, imgui_context);
  ImGui_ImplOpenGL3_Init();

  // Imgui state
  bool show_demo_window = false;
  // Main loop
  bool done =  false;
  while (!done)
  {
    renWin->Render();

    SDL_Event event;
    while (SDL_PollEvent(&event))
    {
      ImGui_ImplSDL2_ProcessEvent(&event);
      if (event.type == SDL_QUIT)
        done = true;
      if (event.type == SDL_WINDOWEVENT && event.window.event == SDL_WINDOWEVENT_CLOSE &&
          event.window.windowID == SDL_GetWindowID(window))
        done = true;
      renWin->PushContext();
      done = iren->ProcessEvent(&event);
      renWin->PopContext();
    }
    SDL_GL_MakeCurrent(window, imgui_context);
    ImGui_ImplOpenGL3_NewFrame();
    ImGui_ImplSDL2_NewFrame(window);
    ImGui::NewFrame();
    // 1. Show the big demo window (Most of the sample code is in ImGui::ShowDemoWindow()! You can browse its code to learn more about Dear ImGui!).
    if (show_demo_window)
      ImGui::ShowDemoWindow(&show_demo_window);
    // 2. Show a simple window that we create ourselves. We use a Begin/End pair to created a named window.
    {
      static float f = 0.0f;
      static int counter = 0;

      ImGui::Begin("Hello imgui-vtk!"); // Create a window called and append into it.

      if (ImGui::Button("Button"))  // Buttons return true when clicked (most widgets return true when edited/activated)
        counter++;
      ImGui::SameLine();
      ImGui::Text("counter = %d", counter);

      bool spike_actor_visible = static_cast<bool>(spikeActor->GetVisibility());
      ImGui::Checkbox("Show spikes", &spike_actor_visible);
      spikeActor->SetVisibility(static_cast<vtkTypeBool>(spike_actor_visible));

      ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
      ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / ImGui::GetIO().Framerate, ImGui::GetIO().Framerate);
      ImGui::End();
    }


    ImGui::Render();
    ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
    SDL_GL_SwapWindow(window);
  } // render loop

  ImGui_ImplSDL2_Shutdown();
  ImGui::DestroyContext();
  SDL_GL_DeleteContext(imgui_context);
  SDL_DestroyWindow(window);
  SDL_Quit();
}

Acknowledgments:

Improved from example initiated in https://github.com/trlsmax/imgui-vtk. We use sdl2 instead of glfw to take adventage of VTK's vtkSDL2OpenGLRenderWindow and Interactor. These VTK classes allow a better interaction between imgui and vtk.