Light Through a Window

[Dylan]

Ok, it may seem a simple question, but could you tell me how I would go about rendering an interior scene with light coming through a window.
If I model a room, add furnitre and a window, will turning the Sun on be suffice, or are there methods to implement that give more light and better results?

Additionally, if the window is in view of the camera and I wish to have an outside background, is the best way to addd this to a face in SU, or do this in PS afterwards?

Thanks

[Fletch]

Hi Dylan! Welcome

Absolutely:

• 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 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.

There is also something in Twilight called a "Light Portal" material. I will go into more detail on that if you are still interested.
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, I highly recommend using a spherical sky image. But I have also used 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. I can give more info on this if you are interested.

[Dylan]

Thanks Fletch.

I'm definately interested in the Light Portal, but only when you have chance to explain, don't burn yourself out!

[Fletch]

OK, Hope you are ready for some reading

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 Light 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 "Light 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 Light 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 Light 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.

[Dylan]

For inserting background, I highly recommend using a spherical sky image. But I have also used 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. I can give more info on this if you are interested.

Thanks Fletch.

You mention inserting a spherical background. I did download a few to try out, one seemed to work while the others where way out of scale (too large) so you only got a tiny piece of the background image. Is this user error, or is there a trick I should be using to get around this. Or am I downloading the wrong things?

Talking about the alpha mask, did you once create a tutorial for something similar in Kerkythea? I'm sure I remember seeing something a few years ago!

[Fletch]

Hi Dylan,

Be sure, if using a Spherical Sky mapped image to choose "Spherical Sky" in the sky type in the light dialog.

Just using "Background Image" is really not useful for much, in my humble opinion... the User Manual explains this more in-depth

Background Color, Centered Image, Tiled Image, Fit Image
These options have the least control over placement of image, and do not contribute as lighting from the sky.

If Spherical Sky choice does not work, try the "Probe Sky" option. This works with the HDR probes available for free from the generous and talented folks at the Light Probe Image Gallery

If those do not work, then what you have may not be a true spherical sky image.

[Chris]

To add to Fletch's reply, with spherical images, you are really at the mercy of the photographer. How it shows up in your render is entirely dependent on how the photo was taken. The only thing you can do to adjust the apparent size of your background is to change your Field of View in SU ( or Lens Focal Length in Camera Tab) which will also obviously effect your model.