Discussed in #17

Feature description
This feature introduces a "World Mode" as an alternative to the current "Box Mode." While the default box-shaped space is excellent for standard data visualization, a more open and exploratory environment is needed for applications involving large or unbounded datasets, such as GIS data or spatial simulations.

World Mode removes the bounding box in favor of a ground plane and introduces free-camera navigation, similar to 3D modeling software. This mode includes support for both orthographic and perspective projections and ensures the camera remains upright during navigation.

This is implemented through a set of new plot flags:

• ImPlot3DFlags_GroundOnly: Hides the side and top/bottom planes of the plot box, leaving only the XY "ground" plane visible.
• ImPlot3DFlags_LockGround: Clamps the camera's elevation to prevent it from flipping upside down, keeping the ground plane always below the horizon.
• ImPlot3DFlags_Perspective: Switches the rendering from an orthographic projection to a perspective projection.
• ImPlot3DFlags_WorldMode: A convenience flag that enables all three flags above for the complete world mode experience.

Current Implementation Status
The foundational elements for World Mode have been implemented. The plot box can be collapsed to a ground plane, perspective projection is functional, and camera controls now manipulate an NDCOffset and NDCScale for free-form panning and zooming. However, there are known issues and areas for improvement.

Known Issues & Next Steps

• Camera Movement: The current free-camera panning and zooming behavior can feel unintuitive or buggy at times and needs refinement.
• "Infinite" Ground Plane: The GroundOnly flag currently flattens the existing plot box into a finite plane. A truly infinite grid that extends to the viewport edges regardless of zoom or pan is still needed. This might require a new, separate flag or a modification to the existing grid rendering logic.

Tasks
Credit for the initial implementation of these tasks goes to @haiha95.

• Public API (implot3d.h)

  • Add new flags to the ImPlot3DFlags_ enum: ImPlot3DFlags_GroundOnly, ImPlot3DFlags_LockGround, and ImPlot3DFlags_Perspective.
  • Define the convenience flag ImPlot3DFlags_WorldMode as a combination of the three new flags.
  • Add a ToElAz method to the ImPlot3DQuat struct to convert a quaternion back to elevation and azimuth angles.
  • Add public getter functions GetPlotScale and GetBoxRotation.

• Internal State (implot3d_internal.h)

  • Add an NDCOffset member to the ImPlot3DPlot struct to manage camera panning.
  • Add a ClampGroundRotation method to the ImPlot3DPlot struct.

• Core Logic (implot3d.cpp)

  • Rendering Logic:
    • In ComputeActiveFaces, handle the ground_only case to force plane_2d to ImPlane3D_XY.
    • Update RenderPlotBox and ComputeBoxCorners to only render the XY plane when GroundOnly is active.
  • Input Handling (HandleInput):
    • Modify panning logic to update NDCOffset instead of axis ranges.
    • Update zoom logic to modify NDCScale and NDCOffset for free-camera zooming.
    • Implement camera elevation clamping by calling ClampGroundRotation when ImPlot3DFlags_LockGround is enabled.
  • Projection Pipeline:
    • Integrate NDCOffset into PlotToNDC and NDCToPlot transformations.
    • Implement a conditional perspective projection in NDCToPixels.

• Demonstration (implot3d_demo.cpp)

  • Add a "Plot Flags" section to the demo window.
  • Add checkboxes to toggle the new flags.
  • Include a sample plot to demonstrate the features.

• TODO

  • Refine camera panning and zooming logic to be more intuitive.
  • Implement a truly infinite ground grid that extends across the viewport.