I'm trying to make a roughened parchment-like material. I don't want to just use a texture, because I want to take a low-angle shot that lets me see the actual bumps in the surface.
I currently have the parchment represented using a single plane mesh. Is there a modifier I can use to deform the mesh in order to add roughness to it?
Yes, though you'll probably want to use two for maximum effect; I recommend applying a Subdivision surface modifier ('Simple', not 'Catmull-Clark'), and then applying a Displace modifier. If your geometry is sufficiently complex it should work fine.
The Subdivision Surface modifier adds vertices to your mesh, then the Displace modifier uses those vertices to add the roughness you're looking for.
Before Modifiers
After Modifiers
You can, of course, use different settings for the Displace texture if you want to achieve different kinds of roughness.
I don't want to just use a texture, because I want to take a low-angle shot that lets me see the actual bumps in the surface
Since recently, you can use a texture and have actual bumps. This works in combination with Cycles only, and makes use of the Material's Displacement property.
First, you should use the Experimental Feature Set in the Render panel:
Add a Subdivision Surface modifier and enable Adaptive:
The trick here is that, by using Adaptive, the size of subdivisions will be dependent on how much the surface is close to the point of view. So you'll have each screen-space pixel displace independently from the neighboring ones, but without the need to subdivide the whole mesh by the same amount.
In the Node/Shader Editor, plug the desired texture into Displacement. If Blender>2.80, you should use a Displacement node, otherwise (2.79) just plug the height in the Material Output.
(If Blender 2.79, under Material > Settings set Displacement to "Both")
The bumps are now real, i.e. affecting the geometry, but non destructive, i.e. not affecting Edit mode vertices. In the gif, the geometry is just a 4-vertex plane:
Once your surface is subdivided with sufficiently small faces, you can use Mesh Tools -> Deform: Randomize to add roughness. Fine-tune using the tool parameters shown below.
My result:
If you prefer to have bumps instead of spikes, you can again Mesh Tools -> Add: Subdivide (with four or five cuts) and select Smoothness = 1.
