This single-page 3D viewer compares how well the J-20 and F-35A hide their engine exhaust from behind. You move an observer around the rear of either jet, and it shows which parts of the airframe block the line of sight to the nozzles, and by how much.
It only measures straight-line visibility. Radar and infrared signatures would need far more than geometry, so it makes no claims about them. The J-20 model uses the collision mesh from a J-20 mod for the DCS flight simulator, and I checked the models by comparing rendered outlines against reference photos.