Also there are quite a few high quality cloud brushes for photoshop floating around, quite a few crappy ones too but most of the time those are easy to spot.
I painted a bunch of skybox textures for The Ship way back when and I initially had a hard time of it.
Depending on the method used you might be better off using this Photoshop compatible piece of software. SkyPaint It looks horrendous but it REALLY helps get rid of the weird pinching effect on your skybox if you were to do it purely in Photoshop.
As for actually painting a pretty sky; it sounds obvious but I'd actually go outside and sit for a while and just study the sky. I used to commute to work and it helped to be able to sit on the bus and peacefully take mental notes.
When you're painting a skybox that will encompass your world you need to be able convincingly portray the effect that light has on clouds depending on their position relative to the sun. Pay attention to how they illuminate when the sun is either above or below, behind the clouds or shining on the front side. Clouds have Volume.
It's a really tough one to get right.. Also loads of reference. Don't just use 1 or 2 images.
For a quicker result, I just use a round brush with scattering and not so high opacity, and paint in many many layers. Usually I do it in black and white first, so I keep switching between one and another and panting until it looks good enough. Then you can throw a gradient map on top of it for a base color, then start coloring on top.
Dan Luvisi has a pretty good set of brushes he uses for his work. I started using his brush set for just about everything a couple years ago. Included in the set are some good atmospheric brushes that work well to make nice clouds.
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Also there are quite a few high quality cloud brushes for photoshop floating around, quite a few crappy ones too but most of the time those are easy to spot.
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Here are some good tips on painting a sky/clouds. It's traditional paint, but most of it can be implemented pretty easy into a digital workflow.
Depending on the method used you might be better off using this Photoshop compatible piece of software. SkyPaint It looks horrendous but it REALLY helps get rid of the weird pinching effect on your skybox if you were to do it purely in Photoshop.
As for actually painting a pretty sky; it sounds obvious but I'd actually go outside and sit for a while and just study the sky. I used to commute to work and it helped to be able to sit on the bus and peacefully take mental notes.
When you're painting a skybox that will encompass your world you need to be able convincingly portray the effect that light has on clouds depending on their position relative to the sun. Pay attention to how they illuminate when the sun is either above or below, behind the clouds or shining on the front side. Clouds have Volume.
It's a really tough one to get right.. Also loads of reference. Don't just use 1 or 2 images.
Good Luck
LuVisi's Brushes
my favorite collection, some are cloud and smoke specific
Nagel brush Series
Is it actually a skybox...if so, I have a good method of rendering a pre-distorted cubemap that you might be interested in.