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Create glass material.
I used a light blue color. Transparency now is 0, but Transparency Edge is 0, 4 so window will be a little bit transparent.
Later we can play with transparency, but now we see materials much better.
Make a render. Not bad, but boring. We could use a sky photo, but maybe have not any digital cameras or don't find CD with photos or simple don't want to use any photos (maybe the same as always), only shaders.
So we need some clouds. We can do it with some shader nodes (yes, there is a clouds node-but it not makes separated fluffy clouds), but I think fBm is the best for this (in my opinion)
So create an fBM at reflection color.
With basic settings we got too many clouds (as you see on image below)
So need to make some changes.
In next image you see some settings. At first set y_scale value a little bit smaller for rounder clouds. After you can make with Bias a little lighter (or darker) and adjust contrast with Gain.
Make some blur with a higher Fractal_Increment value.
Still too many clouds, so check off Signed checkbox.
Render and enjoy your sky reflection!
If you like, you can play with transparency value for more transparent look.
See some other settings:
On left image I attached fBm node to reflection_value too. Transparency is now 0, 3
On right image Transparency is 0, 3 and now I didn't attach fBm node to reflection_value, only to reflection_colors.
But I checked off Reflection_Lite_Mult checkbox and checked on Reflection_Kd_Mult checkbox.
See differences and decide what is better for you.
If you better like to use white color for glass, your sky reflection will gray. It is good for heavy clouds or a rainy day, but for a sunny sky, you need some blue.
In this case you need to attach some colors.
At first set Diffuse_Value to 0, 5 or less or your window will be too bright.
Create a ~ New node — Math — colorramp ~ node and attach to Reflection_Color. This will detach fBm what need to attach to ColorRamp Input line. In ColorRamp node you see 4 Colors.
This colors will replace fBm's grayscale colors. fBm's black parts will be replaced with Color1, dark grey with Color2, light grey with Color3 and white parts will be replaced with Color4.
For a simple cloudy sky you need to set Color1 to a light blue, Color2 a lighter (close to white) blue, Color3 a much lighter blue (almost white) and Color4 left White as you see on image below:
Render and see result
We have beautiful sky reflection on window but what about “real” sky ? I mean background.
Solution is: use same settings for background.
Attach ColorRamp to Background Color as you see below and fBm to ColorRamp Input as before.
And now you will have a really nice scene:
I hope you find this tutorial useful. If you can improve settings or you have another (maybe better) idea for clouds, please let me know.