All of them. Just for example in the PRMan 3.9 release notes from 2001, it says under 'Miscellaneous Changes': "Some parts of the renderer now use double-preci

All of them. Just for example in the PRMan 3.9 release notes from 2001, it says under 'Miscellaneous Changes': "Some parts of the renderer now use double-precision floating point arithmetic, to avoid round-off error."[1] Your model-to-camera to camera matrix needs to be only single precision, usually. But your model-to-world matrix needs to be double precision to avoid jittering. So to calculate the former you use double precision and then you can truncate the resulting matrix to single precision for use e.g. in shaders. Everything dandy by then, even for GPUs. But first you need to get there somehow. So if you have a gazillion instances in your scene, particles, blades of grass, leaves of tress, spaceships, whatever, you need a gazillion matrix multiplications with double precision to build your acceleration structure and to actually start generating pixels. It's one of many reasons why GPU based renderers performance goes to shit, particularly on time to first pixel, when scenes of such complexity get thrown at them. Contemporary GPUs have comparatively shitty f64 performance. Edit: added example PRMan changelog backing up the claim in the 1st sentence. This was most likely considering just xforms. For ray-tracing specific issues and f64 see e.g. the PBR book on solutions to ray intersection precision challenges. [1] https://renderman.pixar.com/resources/RenderMan_20/rnotes-3.9.html#misc https://renderman.pixar.com/resources/RenderMan_20/rnotes-3....