Lighting Made Simple

The Twilight Render Team shares tips, ideas, helpful hints, and more on using Twilight Render
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Fletch
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Joined: Fri Mar 20, 2009 2:41 pm
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Lighting Made Simple

Post by Fletch » Tue Oct 06, 2009 11:40 pm

OK, lighting is a complex subject. So this may not really be "simple", however...
Lighting an interior scene in Twilight becomes easier when you follow a simple principle.

Put lights in places and with powers that are equivalent to the way you would light the space in real life.

First, Build reasonably accurate component for your lights. Insert a Twilight Light Object inside the Light Component. Put the lights in places and with powers that are equivalent to the way the space would be lit in real life.

Faking lighting with fill lights can lengthen render times, and detract from the 'real' look of the final image. It may actually make the rendering simply 'over lit' and blown out. If a scene is lit properly, using the gamma and exposure control is the best way to lighten the image. The best guideline is to think of lighting a scene like a photographer.

It's difficult to give a 'wattage' power to a spot light in Twilight. But for a rule of thumb, assume the default spot light is a 100w bulb in Lumen output. Lights don't emit "watts", they use Watts to emit light which is measured in "Lumens".

The closer the hotspot number is to the falloff number, the lower the light power will need to be, so a hotspot of 30, falloff of 31 can mimic a 100w bulb with a power of only .5 (approx). For a hotspot of 10 falloff of 179 the power will need to be about 1.5 to mimic the behavior of a 100w bulb (approx).

For the most accurate lighting with spotlights, insert a spot, set power to "1" and load the IES data for the fixture using the IES tab in the Twilight Light Editor. Rendering it in Easy 09 or 10 for interiors will be most accurate. If a light object is in a space with no walls, it will not bounce light around and will not give you an accurate understanding of the amount of light produced by the light. The light must be able to shine on a surface and to bounce around.

To calculate the power for light emitter surfaces is a bit more complicated.

Let's say you have a large space to light, and you want to render with Easy 1-7. Let's presume you want to light it all quickly and evenly with as few lights as possible.
:!: With Easy 09 and 10, 200 lights is going to render similar in speed to 10 lights. This is not so with Easy 1-7.

We just said above that we don't want to 'fake' lighting, typically, so what are we talking about here? Occasionally you will need to use some 'creative lighting' to render quickly yet still look good. So for your average space, just light it as is, but for a huge gymnasium with 50-200 light sources, there may be a better way...anything over 15 lights is considered a "lot" of lights for "Easy" render settings 1-8. 200 lights castings soft shadows for objects in a room can take a lot of calculation time in Easy 1-7.

The process for lighting large spaces for rendering with Easy 1-7 is 2 steps:
  1. Create a light emitting plane at the height of the lights for the space.
  2. Give an emittance value to the light emitting plane equivalent to all the lights in the room.
Step 1:
  • Create a plane below the ceiling at the height of the lights for the space should be. Hold if off from the side walls about 2 feet (60cm).
  • Paint it with a white color.
  • Rename the color to “Emit Ceiling”
  • Apply the Template “Emit 100w/m2” and set it to “Invisible” type emitter.
Step 2:
  • If the total lights in the space would equal 50 lights each at 200watts, then we need 50x200=10000w.
  • Select the face in SketchUp and look at the Entity Info to see the size in meters squared. Remember that this emittance has an efficiency of 2.1%, similar to a real world incandescent light bulb.
  • The Emit Ceiling Plane is 5540sq ft (515 sq meters) at 100w/m2 it will emit 515m2x100w = 51500watts. To make it 10000w, we take 10000w and divide by the size 515 sq meters = 19.4 w/m2
  • So set emitter to 19.42 w/m2 for a 515 square meter plane to make it mimick the light output of an emitter powered by 1000 total watts.
  • Now your ceiling just became a 10000watt light bulb and equals 2 lights in Twilight because a rectangle will be triangulated. This will render much more quickly than 50 spotlights in Easy 1-7.
  • For the ceiling fixtures now use a fake emitter material to make them appear lit.

Fletch
Posts: 11863
Joined: Fri Mar 20, 2009 2:41 pm
OS: PC 64bit
SketchUp: 2016-2021
Contact:

Re: Lighting Made Simple

Post by Fletch » Thu Oct 21, 2010 8:07 pm

This tip has now been updated to aid in clarity and to give it a step-by-step process.

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