Coordinate system in GLCanvas is flipped?

[VRichardJP]

I am trying to display a point cloud using guik::GLCanvas.

Inspired by the code from GLIM, I have come up with something like this:

  if (!flat_map_canvas_) {
    flat_map_canvas_.reset(new guik::GLCanvas(Eigen::Vector2i(width, height)));
    flat_map_canvas_->camera_control.reset(new guik::TopDownCameraControl());
  }

  ImGui::BeginChild(
    "flat_map",
    ImVec2(width, height),
    false,
    ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_AlwaysAutoResize | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoSavedSettings |
      ImGuiWindowFlags_NoNavFocus);

  if (ImGui::IsWindowFocused()) {
    flat_map_canvas_->mouse_control();
  }

  flat_map_canvas_->bind();

  flat_map_canvas_->shader->set_uniform("model_matrix", Eigen::Matrix4f::Identity().eval());
  flat_map_canvas_->shader->set_uniform("color_mode", guik::ColorMode::VERTEX_COLOR);
  
  flat_map_buffer_->draw(*flat_map_canvas_->shader);

  flat_map_canvas_->unbind();

  ImGui::Image((void*)flat_map_canvas_->frame_buffer->color().id(), ImVec2(width, height));

  ImGui::EndChild();

Where the "flat_map" is simply flatten submaps:

  std::vector<Eigen::Vector4d> flat_points;
  flat_points.reserve(nb_points);
  for (const auto& submap: submaps) {
    for (int i = 0; i < submap->frame->num_points; ++i) {
      flat_points.emplace_back(submap->T_world_origin * submap->frame->points[i]);
    }
  }

  std::vector<Eigen::Vector4d> flat_colors(nb_points, Eigen::Vector4d{0,0,0,0}); // default color = RED

  auto flat_map = std::make_shared<gtsam_points::PointCloudCPU>();
  flat_map->add_points(flat_points);
  flat_map->add_colors(flat_colors);

The map points looks ok, except the view is flipped:

I have tried both the top to down and xy orbit cameras, in both cases everything look like as if the camera was looking from below the map.

Here is the default view matrix when using guik::TopDownCameraControl:

  0  -1   0  -0
  1   0   0  -0
 -0  -0   1 -80
  0   0   0   1

And here the default projection matrix of the canvas guik::BasicProjectionControl:

3.73205       0       0       0
      0 3.73205       0       0
      0       0  -1.002  -2.002
      0       0      -1       0

Both look normal to me.

[koide3]

IIRC, this stems from the difference of image coordinates between ImGui (top left = origin) and OpenGL (bottom left = origin), and a workaround is to specify the texture coordinates when calling ImGui::Image as in the following line.

https://github.com/koide3/iridescence/blob/0aa860d5fdfd568c12d21a08c19321620621a8a2/src/guik/viewer/light_viewer_context.cpp#L150

[VRichardJP]

ImVec2(0, 1), ImVec2(1, 0) does the trick! Thank you!