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Shader floating point precision issue on large planets

Equinox shared this bug ago

Reported

STR: Create a 1250km planet, fly down close to the surface.

The triplanar shaders uses voxel-local coordinates for texture UVs, but passes these voxel-local coordinates around using single precision. This results in significant pixelization on large planets due to those single precision -- a radius of 1250km has +/- .125m precision when used at single precision, corresponding to the pixel size below.

The loss of precision is due to an implicit assignment from Vector3D to Vector3 in

MyVoxelCellComponent.CreateRenderableProxyForPart:

myRenderableProxy.VoxelCommonObjectData.VoxelOffset = this.m_offset;

This ultimately gets used in shader code as a texture coordinate in TriplanarSampling.hlsli:

float3 texcoords = (triplanarInput.texcoords + offset) * scale;

Since texture coordinates wrap, this is equivalent to:

float3 texcoords = frac(triplanarInput.texcoords * scale) + frac(offset * scale);

To resolve the pixelization is is necessary to hoist the high precision frac call out of the shader and into C# code. This is possible because the unique values for scale are limited, and it should be possible to pre-compute every possible frac(offset * scale) value that could be used, storing that information on the object constants structure.

There might be some clever molduar math that can be performed here to find a f(offset, scales) such that [ frac(f(offset, scales) * scale) == frac(offset * scale) for all scale in scales ], which could avoid some constant buffer size.

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