Learn how to use the hidden computation capability of GPU texture units for general purpose computation. We describe GRASSY, a system for stellar spectral synthesis where the core problem is interpolation between pre-computed intensity value. We map these pre-computed tables to the GPU''s texture memory. Interpolation then becomes a texture lookup where the hardware automatically performs the interpolation, albeit at very low precision. Our mathematical framework reasons about the impact of this precision and our performance results show 500X speedups. This work generalizes the GPU texture units as computation engines and opens up new problems for GPU acceleration.