远裁剪平面之外的对象在透视图中渲染

Question

I see objects beyond the far clipping plane in perspective projection and I don't think this is how it's suppose to work, so can someone give me an explanation why do I see objects beyond the far clipping plane such as a grid in this example.

The orthogonal projections works fine btw

I cleared all shapes from this demo and added two grids by changing the following code in Luna Frank Shapes Demo

void ShapesApp::BuildRenderItems()
{
auto gridRitem = std::make_unique<RenderItem>();
gridRitem->World = MathHelper::Identity4x4();
gridRitem->ObjCBIndex = 0;
gridRitem->Geo = mGeometries["shapeGeo"].get();
gridRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
gridRitem->IndexCount = gridRitem->Geo->DrawArgs["grid"].IndexCount;
gridRitem->StartIndexLocation = gridRitem->Geo->DrawArgs["grid"].StartIndexLocation;
gridRitem->BaseVertexLocation = gridRitem->Geo->DrawArgs["grid"].BaseVertexLocation;
mAllRitems.push_back(std::move(gridRitem));

 gridRitem = std::make_unique<RenderItem>();
XMStoreFloat4x4(&gridRitem->World, XMMatrixTranslation(0,-1002,0)* XMMatrixRotationRollPitchYaw(1.5708, 0, 0));;
gridRitem->ObjCBIndex = 1;
gridRitem->Geo = mGeometries["shapeGeo"].get();
gridRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
gridRitem->IndexCount = gridRitem->Geo->DrawArgs["grid"].IndexCount;
gridRitem->StartIndexLocation = gridRitem->Geo->DrawArgs["grid"].StartIndexLocation;
gridRitem->BaseVertexLocation = gridRitem->Geo->DrawArgs["grid"].BaseVertexLocation;
mAllRitems.push_back(std::move(gridRitem));

gridRitem = std::make_unique<RenderItem>();
XMStoreFloat4x4(&gridRitem->World, XMMatrixTranslation(0, -1002, 0) * XMMatrixRotationRollPitchYaw(1.5708, 1.5708, 0));
gridRitem->ObjCBIndex = 2;
gridRitem->Geo = mGeometries["shapeGeo"].get();
gridRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
gridRitem->IndexCount = gridRitem->Geo->DrawArgs["grid"].IndexCount;
gridRitem->StartIndexLocation = gridRitem->Geo->DrawArgs["grid"].StartIndexLocation;
gridRitem->BaseVertexLocation = gridRitem->Geo->DrawArgs["grid"].BaseVertexLocation;
mAllRitems.push_back(std::move(gridRitem));

and change the grid size

GeometryGenerator::MeshData grid = geoGen.CreateGrid(200.0f, 200.0f, 60, 40);

and the projection matrix from on resize

 XMMATRIX P = XMMatrixPerspectiveFovLH(0.25f * MathHelper::Pi, AspectRatio(), .1f, 1000.f);
XMStoreFloat4x4(&mProj, P);

Now I can still see the grid even though its beyond the far plane and even if the far plane is 900 the still appears on rotation at the edge of the screen. so I need to reduce the far plane or move the grid further, and as I keep changing the value of the far plane I can see a shape works as a brush, it hides everything beyond it but as camera rotate and the grid is no longer behind it the grid re-appear

and here's what I meant by the brush

Answer 1

I think you're thinking of the maximum view distance as being consistently 900 units away from the camera/eye position. If that was the case, it wouldn't be a clipping plane at all, it would be a curve - a sector of a sphere.

In reality the view frustum is a truncated pyramid made up of 6 planes. When the far plane is set to 900, then the view distance for the pixel in the centre of the view is 900, but the view distance at the corners is much higher (how much higher depends on the FOVs - you could work it out with a bit of trig).

So as you turn your camera left and right, an object approx 900 units away from the camera will come in and out of view as it intersects the far plane.