Real-time ray tracing is limited to ~1 ray per pixel; denoising reconstructs a clean image from noisy 1-sample results

Offline path tracing can afford 10,000 or more samples per pixel to converge to a noise-free image. Real-time GPU ray tracing is limited to roughly 1–2 rays per pixel by performance constraints; sometimes less than one (via checkerboard or temporal reprojection). With only one sample, shadow and reflection calculations are extremely noisy — a shadow ray either hits the light or doesn’t, giving binary noise instead of a soft gradient. The solution is post-process denoising: a spatial filter (often AI/ML-accelerated) takes the noisy 1-spp image and blurs out the noise while preserving sharp edges. The result is usable but not perfectly accurate — fine details present in a multi-thousand-sample render are reconstructed, not actually computed. This is why rasterized+ray traced games don’t look identical to offline renders even though they use ray tracing.

Examples

1 spp path traced shadow → noisy salt-and-pepper over the penumbra. After denoising: smooth soft shadow that approximates the ground truth, but with detail reconstructed rather than computed.

Assessment

Explain why having only 1 ray per pixel produces noisy shadows even with physically correct ray tracing. Then explain what denoising does and why the output is an approximation rather than the true result.