The problem arises when you have a heavy gradient in your sky and there are objects (like trees) that are in the way
This is where things lke the 'Darken only' can really work. I often find that I don't need any complex masking beyond the horzon grad, but I tend to work at completely different resolutions to film (my most recent composit was an 8000x10000 px headache of a Gimp-crasher.... :S), and obv theres no frame by frame variations jittering around with stills.
I find fore/mid ground trees can be more challenging still. The detail levels and exposure require more finesse than blunt masks can offer.
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edit: PS I Didn't really have time yesterday to type what I wanted to, and if I had the time I would upload some examples to show you the effects of some of the layer blend modes over horizon objects (or lack of), and how they work for me. Just realised that I might have come across as teaching you how to suck eggs. It's because I've seen lots of different appraoches to working with compositing, and unless we can really get into editing on a pixel scale (something that's a lot easier with stills!), I often find that the simpler it's kept, the easier on the eye it looks.
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