Q:

Spreading micromirrors evenly across surface of a helmet model (Particle effect, procedural, instancing)

I am doing a pre-visualization of an art installation. It looks a bit qwirky ;)
It is to remember the fall of the berlin-wall etc. etc. 
So it will be old helmets plated with small mirrors arranged in a very large ball / flower. 
BUT the question is, how do i get the tiny mirrors to spread evenly without clipping into each other on the mesh evenly? 
I wanted to do that effect for a long time already on a couple of projects. 
I thought i could instanciate the micro mirrors onto the surface with a particle emitter and use the normals of the surface etc. 
OR alternatively spawn at each vertices a mirror plate. Yet it would be very hard to make them even like a disco ball. 
Do you have any ideas? here is a video of the status quo (sorry for the quality) VIDEO of the status quo
The first original helmet looks like this:

i tried your question. 
Though i think from the method it is very much like extruding individual faces right? Not entirely sure how it does it ;D to be honest. 
here is the result, i believe because of the (rather unclean) mesh topology I can not go that route if I would have to make all faces evenly first. 
MAYBE: i could use something like cloth simulation? I could make a "fabric" (just joined mirrors as plane ) and stitch it to the surface? Does anyone has experience with it?

A:

You can use the Tissue addon.

The addon will tesselate a given shape on a mesh.
Select a tile, shift select your mesh, and use 'tesselate':

From that, if you want to modify the original mesh, you can come back to the tesselation and refresh it.

A:

If the models geometry allows it, you can try that :

In Edit mode, select every face you want a mirror on. In the Face menu, select Exrtude Individual Faces, and move your mouse to give the mirrors some depth. Give them a nice shiny material and you're done.

A:

This is a partial shader-only solution, which may be good enough for your visualization.

It depends on a 'Tile UV' node group, available in the download.
'Tile UV' splits a given UV space into square cells,  tile size given by the 'Scale' input.
Its outputs are:

Cell UV, UV coordinates within each cell, -0.5 to 0.5 in X and Y
Dist. Center, the distance of the shading point from  the center of its cell
Dist. Edge, the same for the edges of the cell, here used to put the cracks between the tiles
Min, Mid and Max Tile UVS, which return the locations of the Min. Mid. and Max. X and Y of the shading point's cell in the original, given, UV space.

The steps to applying the shader to your object would be:

UV unwrap the object. The shape/distribution of the tiles will depend on your seams, UV islands and how you straighten / distort them. You could do this while looking at just the Base Color branch of this material, to see how your squares/fragments are coming out. This example just uses the default unwrap of Suzanne, you can see the tiles swell in her mouth region.
Having settled the map, bake an image of the object's  object-space normals for it to use: one way.. with the object at the origin and aligned the world, plug the 'Normal' output of a Geometry node into an emission shader, and bake the 'Emit' from that into a floating-point image, with your UV map active, using Cycles, 1 sample.

You can then use that normal map, as shown in the node tree, to look up the normal at the center of each tile, and use that for the normal of every shading point in the tile. 

'Partial', because so far. this is only a Bump/Normal effect. Building the tree to create actual flat-per-tile displacements would be a bit more involved, and need you to bake a position map, too. I don't know how well it would come out - you'd either need very high-res geometry to displace the model, or use Cycles to get the renderer to do it, but If you wanted it, given time, I'd have a look at that.

