how to make a cylinder using displacements
ok I know some people might know all this, but this can be interesting for other.
Step 1

make a brush this one is 128x128x256
Step 2

clip it into 3 brushes.
Step 3

Select the outer faces (not the top and the bottom one)
Step 4

create a power 3 displacement
Step 5

click subdivide and pouf you have a smooth cylinder. you can delete the top and the bottom brush to have a simple cylinder.