I have created two methods of making a Gas giant planet. Both of these methods have GIMP scripts that automate the process. The first method, described below, makes a gas giant with fuzzy borders. To start, make an image that is much taller than it is wide. I usually use a 10:1 ratio. For this example, we'll make it 100x1000. The background color doesn't matter much at all, since we'll be rendering lots of plasma.
The next step is to determine how many layers of color you want in your planet. For this example, we'll make four different bands. Make a rectangular selection stretching from the top of the image to one-quarter of the way down. It should go all the way across the image. Using the Position in the Rectangle select dialog box (on the main GIMP tab) will help out considerably. You should have something that looks like the screen below. I've included the Rule of Thirds guidelines in the selection just to make it easier to see.
Now go into the Filters --> Render --> Plasma. Randomize the seed, and select a turbulence level. Two or three is usually a good value.
Once you have rendered the plasma in the selected area, it's time to select another empty area. Doing this manually, the easiest way to accomplish this is to go into Layer --> Transform --> Offset, and offset the layer 250 pixels (1000 layer height divided by four different bands = 250 pixels) in the y-direction. Make sure that your edge behavior is set to Wrap Around. If you are doing this manually, it should look like this:
with an active selection on the top white part of the image.
Repeat this process (Render plasma, offset image) until the entire image is covered with bands of plasma. You can use different turbulence values for each band if you are so inclined.
Once your entire image is filled with plasma, stretch the image so it has a 1:1 aspect ratio. Go into the Image –> Scale Image menu, click on the chain (so you can change the aspect ratio of your image), and set the width equal to the height. It should now look something like this:
Now make the image seamless by going into Filters –> Map --> Make Seamless. Although it's an optional step, it will prevent any jarring discontinuities in the atmosphere of the planet as we continue on.
The next step is to use the Ripple filter to create some sense of vertical turbulence between the layers. Go into Filters –> Distorts --> Ripple. Make sure that “Retain tilablity” is checked, your orientation is Vertical, and your Wave Type is sine.
Period and Amplitude are up to you. I usually use a Period equal to 1/50th of the image size, and an amplitude of 1/300th the image size, rounded down. For our 1000x1000 pixel image, this would be a period of 20 and an amplitude of 3.
If you want to run the Filters → Distorts –> IWarp filter, this would be the time to do so. This will let you create a Great Red Spot on your planet, manually shape the transitions between the bands of plasma, or do pretty much anything you want to. Use your imagination, because it's your planet!
Now we run Filters → Map → Map Objects filter to turn this plane into a sphere. This is why we made the picture seamless a few steps ago. If we hadn't, the borders would show up as discontinuities when certain rotation parameters were used. The values that I change from the default values are shown below:
Options tab:
Map to Sphere
Check the “Transparent background” box
Light tab:
Lightsource type - No Light
Orientation
Z – 0.9 (this makes it take up approximately 90% of the canvas size)
The X, Y and Z rotation values can be modified to get a good view of any features on your planet that you placed there. Mess around with these values a bit, and keep hitting Preview until you like what you see. When you like the picture, run the filter. You should now have something like this:
We're getting there, but it's still not quite right. Use Filters → Blur → Gaussian Blur on the planet to soften up the hard edges a bit. A value of (Image Size)/50 usually works well. Here, that would be a Gaussian blur of 20 pixels.
If you think the colors are too vivid, you can go into the Colors → Hue-Saturation menu, and decrease the saturation of the image by a certain amount. Usually, 30-40 is a good amount. If you like your gas giants to have vivid colors, then by all means skip this step.
That's it! Add in a background and any shadows you want to, and you're all set! I'll post a couple of sample planets in the next post, a PDF version of the tutorial, and a GIMP Script as well.
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Originally Posted by sambrookjm
I have created two methods of making a Gas giant planet. Both of these methods have GIMP scripts that automate the process. The first method, described below, makes a gas giant with fuzzy borders. To start, make an image that is much taller than it is wide. I usually use a 10:1 ratio. For this example, we'll make it 100x1000. The background color doesn't matter much at all, since we'll be rendering lots of plasma.