It can a little effort to get good Window Reflections.
- You need to make sure you have something to reflect,
- You need to balance to transparency and reflection of the windows to get the effect you want.
- You need to position the camera so that the windows reflect the Sky rather than the ground.
Here is a model a user sent in to try to get help with window reflections.
We added a mirror next to the window. This will help as we are trying to get the transparent/reflective settings adjusted.
We also made the building 5 times taller and repeated the window, because sometimes you expect a sky reflection when you are really reflecting the ground.
We also added a ground plane, to make it easier to see the horizon.
Backgrounds, such as this background sky are truly backgrounds - they are placed behind the rendered image, and do not effect lighting or reflections.
In this first image, the background is not reflected, and there is very little to reflect even in the mirror. (More on windows later.)
Use an HDRi Sky
We replaced the background with an HDRi sky - the Bear Mountain sky included with IRender.
We also turned off the sun, because with the sun on and an HDRi sky, the sun is too bright and often not in the same position as the sun in the HDRi sky.
The HDRi sky is reflected. However, in this case the model and view are positioned so that the stunning part of the HDRi sky is behind the building, and visible in our view. So the mirrors are reflecting the less-stunning part of the sky. (This particular HDRi has a great sky - but also has a mountain - se we are seeing the mountain reflected in the windows.
So we rotated the HDRi sky 180 degrees. This puts the mountain behind the building and the sky behind us, so that if reflects in the windows.
By rotating the sky 270 degrees instead of 180, I was able to put the mountains to the side, so we see more sky and still reflect some sky.
(If you look around on the internet, you can find some skys which will work well for both background images and reflections. But remember you get better reflections when the sun is behind you, not behind the building.
When working with rendering engine developers we often have discussions like "Can a 100% transparent object have reflections?". The problem here is what "100% transparent" means. If it means that 100% of the light passes through the window, then there is no light left to reflect back to the eye. No matter who wins the argument, you need to balance transparency and reflection to get the desired effect.
Also, if you look at a piece of glass which is reflecting light, you will notice that the glass is more opaque where the light is reflected, and more transparent where no light is reflected.
Instead of starting with glass for the window and adding reflection, we are now going to start with a mirror and add transparency. For this image, I made a mirror material 50% transparent.
You can adjust the reflection and transparency to get the effect you want.
Now, with a tall building, we want to position the camera well. If the camera is high above the ground, the reflections of the windows will see the ground and not the sky. So you want to place the camera near the ground.
(I also removed the mirrors, so we can see just the windows.)
For this first image, I placed the camera at 5' from the ground.
In order to see the whole building, you need to change the view.
If you try to rotate the view in SketchUp to see the top of the building, you will find that the camera moves below the ground.
If you pan up to see the top of the building, SketchUp will move the camera height to 1/2 the height of the building and most of the windows will reflect the mountains and the ground.
The trick is to turn on 2-pt perspective in SketchUp, and then pan. The camera wil stay at 5' for good reflections, but the building will be visible, and the vertical lines will stay vertical.
Showing Sky in Windows only
Another trick you can use is to turn off the sky, but leave the sky reflective.
This lets you sky reflections be stunning, while the visible sky is just a white background.