Occasional missing polygons drawing a sky sphere (at far plane) - directx

Question

I am drawing a sky sphere as the background for a 3D view. Occasionally, when navigating around the view, there is a visual glitch that pops in:
Example of the glitch: a black shape where rendering has apparently not placed fragments onscreen
Black is the colour the device is cleared to at the beginning of each frame.
The shape of the black area is different each time, and is sometimes visibly many polygons. They are always centred around a common point, usually close to the centre of the screen
Repainting without changing the navigation (eye position and look) doesn't make the glitch vanish, i.e. it does seem to be dependent on specific navigation
The moment navigation is changed, even an infinitesimal amount, it vanishes and the sky draws solidly. The vast majority of painting is correct. Eventually as you move around you will spot another glitch.
Changing the radius of the sphere (to, say, 0.9 of the near/far plane distance) doesn't seem to remove the glitches
Changing Z-buffer writing or the Z test in the effect technique makes no difference
There is no DX debug output (when running with the debug version of the runtime, maximum validation, and shader debugging enabled.)
What could be the cause of these glitches?
I am using Direct3D9 (June 2010 SDK), shaders compiled to SM3, and the glitch has been observed on ATI cards and VMWare Fusion virtual cards on Windows 7 and XP.
Example code
The sky is being drawn as a sphere (error-checking etc removed the the below code):
To create
const float fRadius = GetScene().GetFarPlane() - GetScene().GetNearPlane()*2;
D3DXCreateSphere(GetScene().GetDevicePtr(), fRadius, 64, 64, &m_poSphere, 0);
Changing the radius doesn't seem to affect the presence of glitches.
Vertex shader
OutputVS ColorVS(float3 posL : POSITION0, float4 c : COLOR0) {
OutputVS outVS = (OutputVS)0;
// Center around the eye
posL += g_vecEyePos;
// Transform to homogeneous clip space.
outVS.posH = mul(float4(posL, 1.0f), g_mWorldViewProj).xyzw; // Always on the far plane
Pixel shader
Does't matter, even one outputting a solid colour will glitch:
float4 ColorPS(float altitude : COLOR0) : COLOR {
return float4(1.0, 0.0, 0.0, 1.0);
The same image with a solid-colour pixel shader, to be certain the PS isn't the cause of the problem
Technique
technique BackgroundTech {
pass P0 {
// Specify the vertex and pixel shader associated with this pass.
vertexShader = compile vs_3_0 ColorVS();
pixelShader = compile ps_3_0 ColorPS();
// sky is visible from inside - cull mode is inverted (clockwise)
CullMode = CW;
}
}
I tried adding in state settings affecting the depth, such as ZWriteEnabled = false. None made any difference.

Answer 1

The problem is certainly caused by far plane clipping. If changing the sphere's radius a bit doesn't help, then the sphere's position may be wrong...
Make sure you're properly initializing g_vecEyePos constant (maybe you've mispelled it in one of the DirectX SetShaderConstant functions?).
Also, if you've included the translation to the eye's position in the world matrix of g_mWorldViewProj, you shouldn't do posL += g_vecEyePos; in your VS, because it causes a vertex to be moved twice the eye's position.
In other words you should choose one of these options:
g_mWorldViewProj = mCamView * mCamProj; and posL += g_vecEyePos;
g_mWorldViewProj = MatrixTranslation(g_vecEyePos) * mCamView * mCamProj;