Yow, lots and lots of questions! I will try to answer each one *in brief*. I suggest you open a new thread to better discuss any more in-depth questions/answers you need.
First, as a preface, it is very important to know what you intend to do with your creations. If you just want to create procedural planets to make renders or animations in Terragen, then it's very possible. However if you have hopes to create things like procedural planets and then export the result to something like a game engine, that's not really a good workflow and you are better off not using Terragen in that kind of process at this time. So I'm answering your questions with the assumption that you intend Terragen to be your final "destination" program from which you create all renders.
Going in order of your questions...
Yes, you can apply Power Fractals and most other shaders on top of other data. Whether it will create a great-looking result is another question, but it's certainly possible. You *can* use Terragen to procedurally enhance raster/image-based data. For terrain data you need your other program(s) to output *heightfields*, where grayscale is used to indicate height. You can then add additional displacement on this for more detail. Alternatively you can output continent masks in another program and use these to create a base shape in Terragen and blend it with procedural displacement for better realism, bit is more ideal if your other program(s) create height to begin with, it will be a better base.
Augmenting textures is harder, they do not work in the same easy sort of "additive" way where a base image can be used to define the color of overlaid procedural color variation for detail. However you can use things like Merge Shaders and other more complicated approaches to try to take a lower resolution image-based texture and add procedural detail. It is challenging and I would generally recommend only doing your continent/large-scale *terrain* shapes (heightfield) in other programs, then do all texturing in Terragen.
For covering planets in rocks, trees, and other objects, there is actually no software that can do this without doing it using clever procedural hacks where everything is dynamically generated at runtime. To do it in Terragen, you need to customize your populations to cover the area that you plan to zoom to. If you want to zoom to any random area and have it covered in trees, that's pretty unlikely unless you have tons and tons of memory (think about 128GB, maybe even more), and you will be dealing with a lot of unnecessary overhead, so let's assume you do it using a more focused method. So for a single zoom-down animation from space to ground, what you would do is texture the terrain such that it has the appearance of, say, a forest from a distance (add tree-sized displacement and green texturing, for example), then you just create a population that covers the area that your camera will focus on as it descends, and use a mask to transition from the texture/displacement "forest" to the real 3D model forest, so that as your camera comes down, the transition zone is in the distance, hardly visible or not at all, by the time you are able to see the dimensions of trees.
The sun does have a glare if you enable Starburst in Filters (Terragen 4). Glare is a camera/eye effect, it does not exist in the real world.
If your cloud map generating software creates *masks* (black and white images that show where clouds should and should not appear), then you can easily use this as a mask for procedural clouds in Terragen.
v3 clouds (e.g. Easy Cloud) are always localized due to rendering demand and overhead. Use older v2 clouds (try "Global Cloud" presets in Add Cloud menu) for global clouds. Use the Mask input with any large-scale cloud map (you can use real-world NASA data for example to get good large-scale cloud shapes).
Emissive shader = Luminosity, a texture setting available in several shaders including Default Shader and Surface Layer. Use very high values (e.g. 1000 or 10,000 or more) to get a lot of light. You often need to use high Global Illumination (GI) settings to get this to look good, but it works.
Image Map Shader can be used for color as well as height (displacement). But as I said above, augmenting a color texture is harder to do well than augmenting height with more detail.
The "star" is an "infinite" light source, with an optional disc-shaped object. There is limited customization at this time, but you can change color, disc size, some glow options, and you can adjust Starburst settings to change some of its appearance. If you need more customization you will need to use workarounds like a secondary planet that does not cast shadows, set in front of the Sun (e.g. if you want to add a texture to the "sun").
Backgrounds can affect light if they are *luminous* (see Luminosity setting mentioned above), however you need very high GI values for it to work well, and Terragen is really not designed to work this way at present. It is not recommended.
To add a 360 background go into the Background Object, add an Image Map Shader with Spherical Projection and set the position of the Image Map Shader to be equal to the Background Object position.
Changing colors of plants can be done in any external 3D modeling program, of course. If you want to do it in Terragen, go into the internal network of any imported object node (the + symbol on the right of the node in the network view, or right-click it and go to Internal Network), then go inside the Parts Shader, find the Object Part and connected Default Shader that corresponds to the *part* of the object you want to adjust. In the Default Shader you can adjust Diffuse Color to tint the existing texture, or you can replace it by changing out Colour Image, or you can feed in a new procedural function (e.g. Power Fractal) to make more sophisticated procedural tinting changes.
As long as you have some way of receiving money on the Internet, you can just put up a website and sell anything you make. Whether people will want to buy it just depends on how high the quality is, so focus on that. You can talk to the people at New World Digital Art (NWDA), but I don't know if they can work with someone under 18 (up to them), and of course you would need to have some demonstration of the quality of your work. So that's probably something to work on and apply for later. Focus for now on making high quality content, and I'd advise starting by giving some things away, then as your quality increases you can consider charging.
Again I recommend opening dedicated threads for any in-depth discussion you want to have on the above topics. This is intended only as a quick response and this could quickly become an unwieldy thread to keep track of.
- Oshyan
