There are multiple background styles that can be chosen for your render.  Each has unique characteristics and it's important to understand the differences.

• All types starting with 'Background' do not contribute light to the scene!  This is important to remember.  These options only set the background color or image, like a billboard behind the scene.
  • Background Color sets a solid color behind the scene.
  • Background Centered Image places an image behind the scene, centered.
  • Background Tiled Image places an image behind the scene and repeats it as necessary
  • Background Fit Image places an image behind the scene and stretches or shrinks it to exactly fit the rendered image size
• Sky Color sets the sky to a solid color.  The difference from Background Color is that Sky Color lights your scene using the selected color.

• There are 3 kinds of image-based skies.  Each type expects the image file to be a specific shape.  It's important to remember that if you choose the an image that doesn't match your selected background type, your sky will be distorted!
  
  • Spherical Sky expects a rectangular image (usually twice as wide as it is high).  The image should show a complete 360 degree panorama and stretch from floor to ceiling.  This format is sometimes called Lat-Lon and is the same as Twilight Render's Sperical render mode.
  • Hemispherical Sky expects a rectangular image (usualy 3 or 4 times as wide as it is hight).  The image should show a complete 360 degree panorama, however, it only extends from the horizon centerline to the ceiling.  Basically the top half of a Spherical Sky.  When used in a render, the part of the image below the horizon stretches the last row of image pixels down.
  • Sky Probe expects a square image with the sky as a perfect circle in the middle, as if reflected in a silver ball.

• Physical Sky uses a physically-based mathematical formula to create a natural looking sky.
  • The 'Turbidity' property controls how dusty or dirty the horizon appears.  Note: values below 2.0 are generally not realistic.

Sometimes your background image doesn't line up with your scene.  Perhaps you want a more interesting treeline behind your scene, or to line up the image's sun glow with your scene's sun.  Twilight Render provides the ability to rotate the background image.

1. Open the Environment Editor.
2. Select a Spherical Sky, Hemispherical Sky, or Sky Probe and load a valid image, or select a Physical Sky.
3. Enter a rotation in degrees into Sky Rotation.  It may take a few adjustments to find the right alignment.
4. Depending on your goal, you may need to rotate the Sun along with the background.  If so, check 'Rotate Sun with Sky'.  Otherwise the sun and sun shadows will not rotate.

The position of the sun is important for realistically representing the time of day.  SketchUp provides controls to set both the date and the time of day.  Twilight Render uses this time information directly to set the sun position.

1. Open the Shadows dialog in SketchUp under Windows -> Shadows.
2. Set the date and time for your scene.
3. It is often helpful to turn on shadows in SketchUp to view the effects of your change.
4. Open the Twilight Render Environment Editor.  The Preview scene automatically captures the scene's day and time when rendering.
5. It may be helpful to open the Twilight Render Exploration Renderer when adjusting the date and time.  Changes in the SketchUp Shadows dialog will automatically be rerendered in the Exploration Renderer.