PlanarUVMap.cs

using UnityEngine;
using System.Collections;

// modified from : http://docs.unity3d.com/ScriptReference/Mesh-uv.html

public class PlanarUVMap : MonoBehaviour {

	public bool flipX=false;

	void Start() 
	{
		UsePlanarUV();
	}
	
	void UsePlanarUV()
	{
		Mesh mesh = GetComponent<MeshFilter>().mesh;
		Vector3[] vertices = mesh.vertices;
		Vector2[] uvs = new Vector2[vertices.Length];
		
		Vector2 min = new Vector2(Mathf.Infinity, Mathf.Infinity);
		Vector2 max = new Vector2(Mathf.NegativeInfinity,Mathf.NegativeInfinity);
		
		for (int i=0; i < uvs.Length; i++) 
		{
			min = new Vector2( Mathf.Min(min.x,vertices[i].x), Mathf.Min(min.y,vertices[i].z));
			max = new Vector2( Mathf.Max(max.x,vertices[i].x), Mathf.Max(max.y,vertices[i].z));
		}
		
		float fixScaleX = 16; // temporary fix for X uv map
		
		for (int i=0; i < uvs.Length; i++) 
		{
			float uvx = Remap(vertices[i].x,min.x,max.x,0,1)*fixScaleX;
			float uvy = Remap(vertices[i].z,min.y,max.y,0,1);
			
			uvs[i] = new Vector2(flipX?1-uvx:uvx,uvy);
		}
		mesh.uv = uvs;
		
	}
	
	// remap value from range to another range
	float Remap(float current, float sourceRangeFrom, float sourceRangeTo, float targetRangeFrom, float targetRangeTo)
	{
		return targetRangeFrom + (current-sourceRangeFrom)*(targetRangeTo-targetRangeFrom)/(sourceRangeTo-sourceRangeFrom);
	}
	
}