Light through a window...

Frequently asked questions about Twilight Render
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Light through a window...

Post by admin » Sat Sep 12, 2009 4:42 pm

How to get the sun shining through your windows:
  • Be sure all windows are only modeled with a single pane of glass.
  • Be sure you have applied a material to the glass using SketchUp's materials, such as the typical "translucent_glass_grey" or whatever
  • Be sure you use the Twilight Material Tool to click that glass material on the windows and apply the Architectural Glass>"Common" or "No Shadow" set glass material color to white and alpha to Zero.
  • Set the sun in SketchUp to cast as much light as possible into your room... the more light the better.
  • Apply material templates that are appropriate to your other objects.
  • Render a small test render with Easy01-Prelim
  • Adjust tone mapping exposure in the camera panel of the Render Window if image is too dark.
  • When happy with camera, materials, etc. Render test render at 800x600-ish on Low or Medium easy settings.
  • When happy and ready for the "final" render on Easy09 progressive. This will give you best results with so little light.
Or, the classic "tricks" of placing an invisible light emitting plane outside your window, or adding spot lights outside to cast in light... and these work fine in Twilight, but are not necessary and take some tweaking/patience.

For inserting background, it is recommended to use a spherical sky image. But one may also try the old "picture on a plane" trick, at some significant distance from the window so that it will get enough light and not block light coming into the space through the windows. The other way that gives best control is to render with physical sky, then render an alpha mask of your scene after hiding all glass. This will let you mask in a hi-res background in your photo editing program. See the Twilight User Manual Mask Section on page 8.

There is also something in Twilight called a "Sky Portal" material.
How Do I Use A Sky Portal?
Install the Sky Portal Materials for Twilight Render from here.
Download a Sky Portal Test Room here.

The Sun and Sky cast light rays onto your model in a random way.
If some of those rays make it through an opening and bounce around inside your model, they will light up your scene.
Low render settings are fast because they tell the lights not to cast a lot of light rays. Therefore they will not work well for your scene where you are attempting to light an interior with light only coming from the Sun and Sky. You will need higher render settings, so that more rays are cast, and therefore, hopefully, more of these random light rays make it in through the openings in your model.

If only you had something that told the Sun and Sky exactly where the openings into the model were, and acted like a "door-man" for the light rays saying "come on in here, guys"... that would be useful. ;)
A Sky Portal is just that. It is a plane that covers any opening into an interior model.
When working properly it will take ALL rays of light from the Sun and Sky and bring them thru the "Sky Portal"
  • This plane must have the Front of the Face facing IN towards the INSIDE of the model.
  • Pick any color in SketchUp's material pallette and paint the Front of the face of that Light Portal Plane.
  • Now in SketchUp's material editor rename the material something like "Light Portal" :)
  • Now with Twilight's Material Tool apply the Sky Portal Material definition from the Library called "Light Portal"
  • Now when rendering with any method it will be working with all light info possible from only the Sun and Sky light outside.
  • Easy09 would still be the render method of choice.
  • However, even if rendered with "Easy 02 - Low" you will get high quality lighting, considering how little light set up has been done.
  • Rendering time will increase significantly for non-progressive render methods because of all the light bounces now being calculated, so start with the lowest render setting possible and work your way up when using light portals. Conversely, render times for Easy09 or 10 may actually decrease...difficult to quantify this
Keys to Sky Portals:
  • Use the least number of surfaces possible... no curving surfaces!
  • No light must be able to enter from the Sun and Sky into the model from any other way except through the Light Portal.
  • If 3 windows are on one wall, be sure to use a single plane to cover all 3 windows... the fewer portals the better/faster.
  • Be sure the portal covers completely/intersects the window or opening... not a crack of light may enter the opening without first going through the portal.
  • Be sure the face is pointing into the model... the direction you want the light to travel from the sun and sky to get into the model.

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