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I'm trying to make a procedural landscape using the displacement node. It works fine until I try to put it through a color ramp to cap the top and bottom. Instead of making the height cap shorter, it stretches the mesh to the top and bottom.

Color ramp not used: enter image description here

Color ramp in use: enter image description here

The pictures are from the same perspective. I'm in cycles and have material settings displacement set to displacement and bump. Any clues what I'm doing wrong? Also I was following this tutorial: https://www.youtube.com/watch?v=jyvbRJJTzY4&t=660s

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It does seem strange, that when you take a 0-1 range, and then map, say, the 0 -> 0.33 part to 0, and the 0.66 -> 1 part to 1, suddenly values appear that weren't there before.

It's because the output of the Noise node does not fill the 0-1 range. The bulk of its output is between 0.25 and 0.75. Outside that range, the values are simply absent. Those values are not represented, either before or after the mapping through the color ramp.

What does happen with your color ramp settings, is values within the 0.25 -> 0.75 range are mapped to 0-1, and so 0 values and 1 values appear in the output, where there were none before.

So all you need to do is bring the color-ramp stops closer together and scale the result:

enter image description here

.. or if you're using 2.81, you can use the Map Range node, bringing the input 'From' and 'To' values inside the range that the Noise node is actually generating.

enter image description here

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When you are shifting the color ramp tabs like you do in the 2nd picture, what you are doing is telling blender to make most of the texture either black or white with only a little bit of greys in the middle. Only the colors that are mid-grey will remain grey, every other color will become either black or white.

The problem with this in this case is that, for displacement, blender interprets black as the very low and white as the very top. Lots of black means lots of low aid lots of white means lots of peaks with very little in between. If what you want is to make the peaks less high, what you want to do instead is make the whites less white. Change the white tab on your color ramp to a grey and the peaks will be lower. The darker the grey, the lower the peaks.

light grey peaks: light grey mid grey peaks: mid grey dark grey peaks: dark grey

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Your result corresponds to the ramp used, everything is OK. Remove the Ramp as it's not needed and connect the Noise's Fac the the Displacement's Height. Or connect The Noise's Fac to the Ramp's Fac if you insist on having Ramp. In this case the ramp's color is your height and you need factors. And greyscale displacement maps should be used for this.

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