Hay bale done!
It was a difficult asset because I never done objects composed of hundred little things. I really want to depict this complex volume and I think I found a satisfactory solution!
At the same time, I learned UE4 (I dislike the way Unity managed material) to be sure of the realtime result. So instead of sharing Substance Painter renders, I will post UE4 screenshots
Here is my process:
1/ I took photo of hay (I wanted higher quality as possible 'cause I didn't know what I could do).
Then, I quickly isolated them with Photoshop.
2/ I combined them to make 3 main blocks of hay and some simple piece to populate the floor around.
To give relief, I generated a normal map from the diffuse map (with the xNormal plugin), and a specular map to highlight some stalk. I only worked in Photoshop.
3/ Firstly, I made a basemesh with some polygons above its surface but it was too bad
So, I decided to only use floating polygon. I get a free plugin for Maya called "spPaint3d" which allows me to quickly place polygon objects onto a chosen mesh. I tested some density/scale/position/orientation configurations, exported them to UE4, tweaked materials and .... tadaaa !
I don't add a wireframe screenshot because it's beyond understanding
However, it's not a heavier solution (1434 tris / 1788 verts per bale) and I have fine details with 1024px texture maps.