The Sea And Moon Tutorial

Geomorph - The Sea and Moon Tutorial

Topics

Creating craters with a "meteorite shower"
Using a height field as a "bump map" on a sphere
Using subdivision #2 with a high distribution parameter
Eroding the terrain with a combination of rain erosion and "whimsical" erosion
Using the log transformation to raise the lower parts of the height field

The scene layout

The intended result is an imaginary moon, moving around an imaginary planet with no visible atmosphere. On the planet there is an ocean of some reflective liquid, which seems to have eroded the terrain (so there should be some atmosphere, shouldn't it?). Well, it's simpler to look at the result.

The center of interest is the moon and its craters. Reflection of the moon on the ocean increases the impact. So, we need a cleared area centered along the eye axis, in which the reflection would be visible. Furthermore, the cleared area can behave like a path driving the eye towards the moon. This cleared area would be given by a valley-like height field.

The moon is a sphere, over which we map a Geomorph height field acting like a normal. In Povray language, this is a "bump map normal".

The light is dimmed, giving a night effect.

The result

Steps

This tutorial is divided in two parts, one for building the moon, the other for rendering the final scene.

The moon part uses the moon.pov scene, the final scene is rendered with sea_n_moon.pov. Both are provided with Geomorph 0.12 and higher.

A. Building the moon

1. Height field creation Create a new height field, not too jagged. A smooth relief gives more realistic craters. Subdivision #1 with default settings is suitable and straightforward.
2. Power transformation We decrease the lower parts of the terrain. Craters seem more natural on a plain. The power transformation with an exponent of 3 does the job for the height field generated in step 1.
3. Adjusting the image density to allow "craters digging" The lower altitudes are a tad too dark to allow realistic "digging" of craters by the meteorites. Increase the brightness, and decrease the contrast so that the lower parts are seen as dark grey instead of black.
4. Spreading craters on the terrain This is the funny part. Hit "Repeat" until you have enough craters for your taste. If you hit "Apply" more than once, you'll only replace the 10 craters you have just drawn with 10 different craters. One important parameter in the "Diameter" subdialog is the distribution. This is, roughly, the exponent to the crater diameter, chosen randomly. At 1, all craters sizes between "Boundary 1" and "Boundary 2" have the same probability of being chosen. The distribution of diameters is uniform. The default is 2. In this case, the distribution of diameters shows a quadratic curve. There are more small craters, and less huge ones. For getting enough small craters here, 8 was used. Here is the formula, given: GD = generated diameter, in pixels (from random seed, uniformly distributed) RD = resulting diameter, in pixels B1, B2 = boundaries 1 and 2, in pixels E = exponent We normalize the difference between GD and B1, relatively to B2-B1, so that the difference fits between 0 and 1, apply the exponent to this value, then multiply the result by B2-B1 to reconstitute a value in pixels: RD = [ power( (GD-B1)/(B2-B1), E) x (GD-B1) ] + B1
5. Rendering and saving the result Render the result with moon.pov, using to choose the scene. This scene does not use the preview camera parameters, naturally, since it is only a bump map wrapped around a sphere. If you are satisfied with the result, save it under craters.png. It is the name of the height field used for the moon bump map in sea_n_moon.pov. Under Povray 3.1, the "spherical" map type was used. Under Povray 3.5, this gives an ugly stretched result, so a "planar" map type is used. The map seems to be applied perpendicular to the Z axis (the foreground to background axis, with the default Geomorph camera). When the camera is rotated around the Y axis (the bottom to top axis), the moon must be rotated too, otherwise the map is warped on the edges. sea_n_moon.pov manages this, and you'll have to care about this if you reuse this code in other scenes. I would welcome any advice about this mapping question.

B. Creating the final scene

1. Creating the height field A height field with a huge valley can be created in two ways: by using the "height field pen" for digging the valley in a relatively smooth height field, or by using the random generator until we get a valley. The height field at left was generated with the random generator, clicking repeadly on the "Refresh" button (actually 15 times). The distribution parameter was set to a high value, to increase the probability of getting a large cleared area. See the desert tutorial for an explanation about the distribution parameter. We set the camera parameters so that the valley becomes parallel to the view axis. The little red arrow on the height field at right shows approximately were is the camera. Rendering the height field with sea_n_moon.pov shows that the water is hidden under the terrain. We must decrease the height of the valley bottom, or increase the height of the water plane in the Povray file if we want to see the satellite reflected in the water.
2. Decreasing the height of the valley bottom We choose to work with the brightness / contrast tool instead of decreasing the height of the water plane. A -12 brightness correction decreases the height enough for our needs. The rendered scene is better, but doesn't look eroded. You can stop there if you're satisfied.
3. Preparing to erode the terrain with rain, using "whimsical erosion" A realistic eroded terrain shows "ribs". The rain erosion provided with Geomorph gives a surface with delicate channels or gutters, which doesn't show a "ribs" effect. We can approximate "ribs" before applying the rain erosion, with the "whimsical erosion". You can use the default settings. You should adjust the radius and the rib smoothing radius to your image size. The defaults of 6 and 2 for a 512x512 image translate as 3 and 1 for a 256x256 image, 12 and 4 for a 1024x1024 image, and so on. This process has nothing to do with erosion, except some likelihood. It was discovered more or less by chance or at random, when experimenting with slope maps. The name is a translation of the French word "fantaisiste", which means something like "funny" and "weird" (as "whimsical" can mean, as I understand), but never "fanciful" or "capricious" (as "whimsical" does mean). But if you play with it a bit, maybe you'll find that it fully deserves the "whimsical" adjective... Nevertheless I would consider any interesting suggestion about this name. Hint: on the weird side, use a radius like 20 for a 512x512 image, slide the "Mix" scale towards the "Result", then increase the contrast with the brightness / contrast tool. If you find a use for the result, drop me a mail!
4. Adjusting brightness and contrast The contrast of the image is too weak, we don't even bother to render it. The proposed settings for the brightness / contrast tool give a valley ground which is black or almost black. It's preferable that you click on the "Accept" button in all tools, instead of relying on the auto-accept feature, at least as long Geomorph is in an alpha or a beta phase. Inconsistencies were observed when a new document is opened or created and the last transformation was not committed. The rendered image is not bad, but shows some strangeness. The "rain erosion" would get rid of most weird "holes" or "loops".
5. Applying the rain erosion The rain erosion is a simple but slow process. It drops randomly water on the ground, and move soil down the slope until the slope is too low. It doesn't add up drops to build creeks and rivers (not yet!). Here 300000 drops were thrown on the ground. Maybe you'll be happy with less (particularly if your computer is slow!). Hit the "Repeat" button three times to get this result. Remember that we started from an image of 512x512 pixels. For a 256x256 image, 75000 drops would give the same result. For a 1024x1024 image, you'll have to use 4 x 300000 drops. Hitting "Repeat" once is the same as hitting "Apply", but hitting "Apply" repeatedly never gives more than 100000 drops, since it reuses the original image as input. The rendered image is more natural now. However, we could get a smoother image with the "Crests" tool.
6. Using the "Crests" tool to smooth the eroded terrain The "Crests" tool implements a kind of "gravity erosion", or micro-subsidence. Parts of the soil fell on parts of lower altitude. Crests emerge where the slope is null and the height at its maximum. Contrarily to the rain erosion, this transformation involves a soil loss, so the average height of the terrain decreases a bit. Set the steps at will, reset if you are not satisfied. The 3 erosion tools and the order in which they were used here (whimsical - rain - crests) are not compulsory. Variations on this sequence can also give interesting results. Here is what we get with the steps parameter set at 6:
7. Using the Log transformation to create a pond by raising the lower parts of the terrain The proposed image, at the beginning of this tutorial, shows a pond with a smooth shore. Raising slightly the shore at the entrance of the cove obtained after step 6 would close the cove. The log transformation allows this. It raises the lower parts of the height field without raising the highest hills, while the deepest parts remain black (think about the log curve to get the figure). The parameter is interpreted as a percentage. Using 0,5, over a maximum of 10, means that we merge 5% of the resulting log transformation with 95% of the original image. 0,5 or so is enough here, except if you want another effect.

Written in October, 2003

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