Renderers can use any quantity of raw processing power of computers and the skill of rendering is balancing the quality of output against the speed of output.
Mental Ray is a very versatile renderer that handles a lot of optimisation on behalf of the user, switching automatically between appropriate render methods depending upon the complexity of the scene content.
The scanline method of rendering is the first resort in the trade off. The difference between scanline and ray trace is that scanline projects a single photon from the camera to the scenes geometry and records only the colour and alpha that is returned to the camera via the track directly back to the camera. There is no diffusion bounce or angular dispersion of the photon so thenumber of calculations required are much less than for raytracing. The downside is that as each pixel is effective emitting a colour at the camera in a way that cannot take into account relational light effects like reflections, shadowing, refraction, and caustics. Some relational lighting effects can be simulated with clever lighting, texture baking, which effectively simulates shadows with bump maps shadow maps that alter the output per pixel. or with tricks that mimic shadowing, like normal map shading that tricks the shadow map generation to get more shadow effect for a given geometry density. Some of these work well from certain angles and at a distance but they do not stand up to close scrutiny.
Ray tracing emulates realworld photons and tracks the path of the light rays from the light source to the camera. Picking up colour and alpha information at each bounce or bend in its path. Taking colour information from the reflections and refractions as it goes.
The advantage of raytracing is obvious in that it means you can simulate the real world behaviour of light sources. You can reproduce the bahaviour of light in transparent, translucent and reflective materials, so it is possible get more believable results. The downside is that raytracing exponentially increases the amount of calculations per pixel. And so uses up a lot of processing time.
Mental Ray gives a lot of options and controls over both types of render methods. And it’s strength as an application is that it can be used intelligently to use scanline for areas of a scene that are simple and switch to raytracing where it is needed.