A few people have asked why they should turn off Scanline in the Maya globals and select Raytrace instead.
Simply put: Raytracing is more efficient than Scanline. Even if it’s just primary rays.
There is overhead introduced with Scanline rendering. It takes a moment to initialize the algorithm and there’s horizontal coherence but not vertical for tiles. There’s also the fact that Scanline algorithms may cause more objects to be built than necessary inside a tile. Z-buffering, etc may need to sort the triangles to decide who is in front. This means you may have an object built behind something opaque. The more geometry you have, the worse this becomes.
Generally Scanline performance is Linear. The more you add complexity, the slower it gets.
Raytrace performance tends to be logarithmic. The more complex the scene, the less it hurts overall performance. (The slope flattens out.)
Now, modern Scanline still attempts to speed up this process with Rasterization tricks. Early occlusion culling, etc are used to avoid tessellating some objects. But this only works to a point. You can see this in the fact that most raytracers allow a lot of “in render” processes while rasterizers much prefer you bake some things first (pointclouds).
Raytraced primary rays use the acceleration structure to find the necessary triangle to render. In scenes with plenty of triangles, this can be a substantial gain in performance. Scenes with multi-millions of triangles can shave multiple (if not dozens) of minutes off their rendertime. (We aren’t rendering in 1995 anymore. Render a single tree and voila, 1 million triangles. We’re generating entire forests at home now.)
In fact, raytracing performance is good enough that many renderers omit a scanline option (Vray for one) The only major argument for not using raytracing is the old argument of memory consumption. Memory is cheap and plentiful. So are 64-bit systems and software.
Also, you can control the amount of tracing where it might call more geometry by limiting ray distance and bounces in shaders and globals. (Do I really need the reflection of that tiny trashcan 6 blocks down the virtual street?)
So, try turning off that pesky “Scanline” button and select “Raytracing” and see how that goes.