How to create solid height field from image? I want to simulate caustics generated by free form lens using Blender and Lux Core Render as a geometric optical simulator. I know how to create height field surface from a gray scale image using displacement modifier. But what I need is the one with volume, cube-like mesh solid object with arbitrarily height distribution on the top face and flat plane on bottom face. Could please anyone show me how to do it ?

Thank you for your cooperation.
You can still use the displace modifier for this, but with a vertex group to limit the influence to one face of your object.


Select the top face of a cube/suitable volume.
Create a vertex group in Properties > Object Data > Vertex Groups.
Assign the selection to the vertex group.


Then specify that vertex group in your displacement modifier: