SPIRE: A Differentiable Mesh Renderer for Closing the Export Gap

Triangle meshes are the standard representation for graphics and geometry processing, yet high-fidelity novel-view synthesis commonly relies on neural fields or Gaussian primitives. Converting these representations to meshes can reduce rendering fidelity, creating an export gap between native rendering and the reusable asset. Direct mesh optimization is challenging due to hard surface boundaries and occlusion. Conventional mesh renderers make binary coverage and visibility decisions at discrete pixel samples, yielding zero derivatives almost everywhere. A real camera, by contrast, integrates radiance over each pixel. This pixel-area integration can yield continuous, differentiable image values even for opaque meshes with hard boundaries. We introduce SPIRE, a differentiable mesh renderer that uses analytic anti-aliasing to realize pixel-area integration. The renderer integrates clipped triangle footprints, combines adjacent faces without mutual occlusion, and spatially tracks the remaining visible footprint when compositing depth-separated surfaces. A sparse backward pass efficiently differentiates these operations with respect to mesh geometry and spherical harmonic appearance. Given multiview RGB images, SPIRE reconstructs conventional opaque meshes with shared vertices and vertex colors, without external normal, depth, or diffusion priors. Our meshes achieve the highest post-export fidelity across the evaluated Blender and Filament settings on synthetic and real scenes, with small export gaps. Controlled ablations establish the importance of pixel-area coverage and surface-aware compositing for direct mesh optimization.