Differentiable Rendering: Running Graphics Backwards

If a renderer is differentiable, recovering a 3D scene from a photograph becomes gradient descent. This path builds that idea properly: the rendering equation and why Monte Carlo path tracing is the only way to evaluate it, the visibility discontinuities that make naive autodiff return silently zero gradients for geometry, the adjoint and path-replay methods that made the backward pass affordable, and why the remaining difficulties are about the problem being ill-posed rather than about the gradients.