The spotlights - what is the difference between different Light Geometry forms?

handel_035c4ce6

I use a lot of spotlights lately and I presume using some light geometry other then "point" gives softer shadows. But there are several light geometry forms - what is the practical difference between those different forms?

maclean

I'm not sure why Point is the default, since it's the one I never use. Point will send out light in all directions, making shadow control a bit complicated (at least, if you come from a background of photography, like me). I prefer to use Disc or Rectangle and set Height and Width for the effect I want. If you want softer shadows, you have to use larger geometry, so a Disc that's 100x100 will be far softer than the default 10x10. Reducing it to say, 2x2 will give a much harder light.

As far as the practical difference goes, each shape has its own characteristics. Disc and Rectangle only light in one direction, whereas all the others light in multiple directions (for example, Cylinder). I can't swear to this, but I assume this increases render time, since the light is bouncing all over the place. Maybe not. I'm only going by what happens in the real world.

But say you want a round spotlight shape on a stage, then you'd use Disc and set the size/distance to suit.

handel_035c4ce6

I did some testing and indeed - the light geometry controls the form of the light spot. BUT - the default 10m size is completely useless - it makes the same light spot as the point light (even if it may soften the shadows a bit it is too hard to tell). So the size of the geometry must be increased significantly - from 50m and above. Also the Lumens value is not per sqm, but for the whole emision surface; and it seems the point light is actually calculated as having 10x10m emission surface for the purpose of light intensity. Even if there is some difference to small to make some noticeable difference.

JonnyRay

Just a quick note while I'm running some test renders myself. The units on the geometry are centimeters (cm) not meters (m). So it makes a little more sense that a 10cm diameter disc isn't that different from a single point.

JonnyRay

maclean said:

I'm not sure why Point is the default, since it's the one I never use. Point will send out light in all directions, making shadow control a bit complicated (at least, if you come from a background of photography, like me).

This isn't correct. In the image below I have a spotlight using point geometry.

Compare that to this image where I'm using a point light.

The lighting profile is considerably different. Especially pay attention to how the point light is lighting the two planes for the floor and wall compared to the point based spotlight which is mostly focusing it's light on the primatives in the center.

All the geometry on the spot light does is change the origin of the light rays. They still only travel in the direction of the spotlight beam. For instance, see the image below where I changed to a 10x100cm cylinder. Especially note the way the cube's shadow on the top edge is affected by the geometry of the emitter, spreading out laterially, but not longitudanly.