Differentiable inverse rendering based on radiative backpropagation

An apparatus for carrying out an inverse rendering of an image, the apparatus being configured to perform operations comprising:

• compute an adjoint image by differentiating an objective function that evaluates the quality of a rendered image, image elements of the adjoint image encoding the sensitivity of spatially corresponding image elements of the rendered image with respect to the objective function;
• sample the adjoint image at a respective sample position to determine a respective adjoint radiance value associated with the respective sample position;
• emit the respective adjoint radiance value into a scene model characterised by scene parameters;
• determine an interaction location of a respective incident adjoint radiance value with a surface and/or volume of the scene model;
• determine a respective incident radiance value or an approximation thereof at the interaction location, the respective incident radiance value expressing the amount of incident radiance that travels from the surrounding environment towards the interaction location;
• and
• update a scene parameter gradient without generating a transcript of the computation of the adjoint image by using at least the interaction location and the respective incident adjoint radiance value and the respective incident radiance value or its approximation at the interaction location.