# RDK Ground Plane

This document describes how to use the RDK ground plane class in C/C++.

### Introduction

When rendering real-world scenes, it is very often the case that the rendering will include a large area of ground or flooring. This could be modeled by using a plane, but this is inconvenient because the ground tends to stretch as far as the eye can see. To circumvent this problem, the RDK provides a set of ground plane services to make it easier to add a ground or floor to your scene. A ground plane has an altitude, which is usually the same as its position along the z-axis. It can also have a material assigned to it, which will appear in renderings and in the viewport, much as materials assigned to objects do. If the auto-altitude option is enabled, the ground plane will adjust itself to sit below the objects in the scene.

Interstate 15, Nevada, USA ~ Photo by John Croudy.

### The Document Ground Plane

The RDK Document Ground Plane is a document-resident ground plane which affects viewports and renderings. If you have a Rhino document, you can read and write that document’s ground plane through the document’s IRhRdkGroundPlane interface. Any changes you make will appear in the main ground plane UI and will also be stored in the 3dm file. Getting the ground plane from a document always returns a const reference. To write to the ground plane, you must begin a batch of write operations and afterwards end the batch. This is done using the RDK’s standard BeginChange / EndChange system. The following is an example of how to access and change the document ground plane:

static class CGroundPlaneExampleCommand: public CRhinoTestCommand
{
protected:
virtual UUID CommandUUID() override { static const UUID uuid = { your_uuid_here } }; return uuid; }
virtual const wchar_t* EnglishCommandName() override { return RHSTR_LIT(L"MyGroundPlaneCmd"); }
virtual CRhinoCommand::result RunCommand(const CRhinoCommandContext& context) override;
}
theGroundPlaneExampleCommand;

CRhinoCommand::result CGroundPlaneExampleCommand::RunCommand(const CRhinoCommandContext& context)
{
auto* pDoc = context.Document();
if (nullptr == pDoc)
return failure;

// Get the document ground plane.
const auto& gp = pDoc->GroundPlane();

RhinoApp().Print(L"Ground plane altitude before: %.1f\n", gp.Altitude());

// Begin a change bracket on the ground plane.
auto& write_gp = gp.BeginChange(RhRdkChangeContext::Program);

// Set the ground plane's altitude manually.
write_gp.SetAutoAltitude(false);
write_gp.SetAltitude(10.0);

// Create a new custom material.
ON_Material mat;
mat.SetDiffuse(ON_Color(185, 14, 14));
auto* pMaterial = ::RhRdkNewBasicMaterial(mat, pDoc);
if (nullptr != pMaterial)
{
auto& write_contents = pDoc->Contents().BeginChange(RhRdkChangeContext::Program);
write_contents.Attach(*pMaterial);
write_contents.EndChange();

write_gp.SetOn(true);