Tutorial: Creating a shield volcano with calderas

[su_liam]

While trying to help with wdmartin's Erobelis Isle thread, here, I discovered a pretty good way using only the Wilbur Mound tool to create a shield volcano studded with calderas much like Olympus Mons on Mars. A variant on this might prove useful in creating impact craters, but I'm not really going to deal with that here.

I'll start with a 1024x1024 blank map. To keep things sane, I'm just going to have this thing pop up out of a flat plane.

First, using the freehand selection tool with feather set to 1.0, select a roughly circular base for your volcano.
FirstSelection.jpg

Because I want to give this shield a bit of a lip above the surrounding plains like the edge of Mons Olympus on Mars, I will set the minimum height to 300 meters. I arbitrarily choose a maximum height of 15000 meters, making this lower than Olympus Mons, but definitely more of a Martian volcano than Terrestrial. For a shield volcano on an earthlike planet 5 or 6 kilometers above sea level would be a really tall shield. Set the Operation to Add and Noise to zero.
FirstMound_Settings.jpg

Now click the Edit Profile button. Your goal here will be to create a fairly steep-sided flat-topped profile. I usually start by entering a value like 0.9 in the Non-Linearity and hit Apply several times till I like the shape. Next hand draw a flatter top to the curve, trying to keep it continuously rounded and monotonically increasing to the right. Repeated applications of Smooth, Non-Linearity and Normalize will help. With luck and a bit of work you'll end up with a curve similar to the one below. With more luck, experience and effort you could get better results. This is pretty quickly thrown together.
FirstMound_Profile.jpg

Next, we want to create a caldera. Select a large round area near the top of the previous mound. Invoke the Mound tool with the Minimum Height set to zero and the Maximum Height set to a negative value equal to the desired depth of your caldera. In this case, -3000. Most of the time you'll want a much steeper dropoff around the edge of the caldera than the edge of the shield. A few more applications of Non-Linearity, perhaps flattening off the top again, followed by more Smoothing and Normalize should do for that.
FirstCaldera_Selection.jpgFirstCaldera_Settings.jpgFirstCaldera_Profile.jpg

[su_liam]

First we'll use the freehand selection tool to select a roughly circular area completely within the larger caldera.
InnerCaldera_Selection.jpg

I'm going to make this one a bit shallower than the larger caldera, so let's use a maximum height of -800. Otherwise, use the same settings as before. For more interest, you could use a different Profile for each caldera, some could be steep, while others are gently sloping. For this exercise, the previously generated Profile will do just fine.
Inner-Caldera_Result.jpg

[su_liam]

For this exercise, let's make a larger subsidiary caldera that crosses the edge of the first caldera. It'll still be smaller than the main pit, but bigger than the other sub-caldera.
EdgeCaldera_Selection.jpg

Before I do anything else, I'll Select>Feather the selection twice with a Sigma of 1.0. After I feather the selection I'm going to Filter>Blur>Gaussian Blur the area pretty radically. Perhaps three times with a Sigma of 8. This will tend to make the existing edge a lot less visible in the final new caldera. You can definitely blur it more for better results...
EdgeCaldera_Blurred.jpg

Before making our Mound(hole), we want to harden the selection back up. Select>Modify>Binarize with a Threshold of 128, then Select>Feather with a Sigma of 1.0 to bring its hardness back in line with what we've done before.

I'm going to make this caldera deeper than the main one, so set the Maximum Height to -5000. You might have noticed that the maximum and minimum values have been reversed. They should probably have been something like Outer Height and Inner Height, but the programmer seems to have intended this as a tool for making hills not holes.
EdgeCaldera_Result.jpg

After doing all this, I tried to embellish here and there, adding inner subsidence to calderas and additional calderas. I even tried to add a smaller cinder cone to the side. When this failed miserably, I made a try at creating a landslip on the side of the shield. You can tell by the results that this is quite a ways short of being ready for primetime.
Final_Result.png

In spite of the trouble I have with using Windows OpenGL in Wine, I did create a simple perspective. It clearly shows the shame that is my "landslip."
Final_Image.png
It does almost seem like that could be on it's way to being something useful. Maybe, if someone reading would be inspired to try their hand at it. Let me know if you come up with anything even marginally usable...

Finally, here's the grayscale heightmap. Possibly in 16-bit...
Final_Heights.png

If you have any comments, questions or suggestions, please feel free to reply.
If you wish to rate this tutorial, I'd be much obliged.
I have a very slightly different(not quite word-for-word the same) version of this tutorial on my blog, here if you're interested.

Thank you. Happy worldbuilding!

[wdmartin]

This is cool. I didn't wind up using it on Erobelis, but I greatly appreciate the time and effort that went into figuring it out!