One trick I've used a few times is to really bump up the spec in the low areas of the normal map. gives the impression of water pooling, and works even in a pipeline that has only basic shaders. You might also be able to mask a cube map the same way depending on your rendering engine.
Can you use a cubemap for reflection? Do that. Otherwise just make the most of your spec, normal, and gloss. Glossiness should be high. Normal map should have plenty of strength, so you get those specular highlights.
start by analyzing what kind of material properties you see there
Reading through some of Neil Blevins material break downs in the CG Education section of his site should help you start thinking about how you approach and recreate materials. specifically the "wet materials" paper.
I'm a little confused.
What you have going on doesn't look like wet mud in the middle of a grass field.
It looks like a pressed down area of grass, with water pooling in it.
If it's supposed to look like mud, then make the decal more opaque. Make it 'take over' the grass more strongly. Put a lot more detail in that normal map. Mud isn't generally perfectly flat. The normal should reflect that.
Also make the diffuse a darker brown. All the colors melt together too much as it is now and you can't really tell what those pools are.
On a sidenote: you should do something about your grass alpha planes. I never make grass on a single non-subdived plane unless I -KNOW- that they will always face the camera (be a backdrop, be in a 2.5D game or whatever). One simple way of making the geometry for smaller kinds of vegetation is to take a pyramid primitive, remove the base face and flip the pyramid upside down. Dupe the mesh a few times and apply some variations to the clones. That is one very basic shape you can try out. (it's more suitable for smaller bushes and flowers though)
the diffuse color is wrong. make it dark brown, and make the spec color shift to a pale yellow. mud also has tight spec values... to play with that use a gloss map if you have that option. You also need to make the mud look soft like the reference.
I think texturing wet mud might not be a big deal, but I think that to make it believable there's a lot of shader work to do to get that wet/dry feeling
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The same way you do materials for any other surface... Reading through some of Neil Blevins material break downs in the CG Education section of his site should help you start thinking about how you approach and recreate materials. specifically the "wet materials" paper.
http://www.neilblevins.com/cg_education/cg_education.htm
Also added an environment cubemap like suggested.
What you have going on doesn't look like wet mud in the middle of a grass field.
It looks like a pressed down area of grass, with water pooling in it.
If it's supposed to look like mud, then make the decal more opaque. Make it 'take over' the grass more strongly. Put a lot more detail in that normal map. Mud isn't generally perfectly flat. The normal should reflect that.
This link is what I think you're going for. Tell me if I'm wrong.
http://users.ox.ac.uk/~sg/ridgeway/11%20Mud.jpg
On a sidenote: you should do something about your grass alpha planes. I never make grass on a single non-subdived plane unless I -KNOW- that they will always face the camera (be a backdrop, be in a 2.5D game or whatever). One simple way of making the geometry for smaller kinds of vegetation is to take a pyramid primitive, remove the base face and flip the pyramid upside down. Dupe the mesh a few times and apply some variations to the clones. That is one very basic shape you can try out. (it's more suitable for smaller bushes and flowers though)
that should help.