This shader uses a pass to create a slightly bigger mesh behind the original one.
This is a good solution (at least in Unity), but only for convex/non transparent object. The fragments of the outline will indeed appear behind the mesh:

We can remove the fragments behind the mesh modifying the depth buffer with a duplicated object.
The original object writes to the z-buffer, so the duplicated object (i.e. the one that act as an outline) will be partially culled by the original one.

In order to obtain this, we can use these shaders:

Shader"Outline/Outline"{Properties{_OutlineColor("Outline Color",Color)=(0,0,0,1)_Outline("Outline width",Range(.002,0.03))=.005}CGINCLUDE#include "UnityCG.cginc"structappdata{float4vertex:POSITION;float3normal:NORMAL;};structv2f{float4pos:POSITION;float4color:COLOR;};uniformfloat_Outline;uniformfloat4_OutlineColor;v2fvert(appdatav){// just make a copy of incoming vertex data but scaled according to normal directionv2fo;o.pos=mul(UNITY_MATRIX_MVP,v.vertex);float3norm=mul((float3x3)UNITY_MATRIX_IT_MV,v.normal);float2offset=TransformViewToProjection(norm.xy);o.pos.xy+=offset*o.pos.z*_Outline;o.color=_OutlineColor;returno;}ENDCGSubShader{Tags{"Queue"="Overlay"}Pass{Name"OUTLINE"Tags{"LightMode"="Always"}CullFrontZWriteOnZTestLessBlendSrcAlphaOneMinusSrcAlphaColorMaskRGBOffset15,15CGPROGRAM#pragma vertex vert#pragma fragment fraghalf4frag(v2fi):COLOR{returni.color;}ENDCG}}SubShader{Tags{"Queue"="Overlay"}CGPROGRAM#pragma surface surf Lambertsampler2D_MainTex;fixed4_Color;structInput{float2uv_MainTex;};voidsurf(InputIN,inoutSurfaceOutputo){fixed4c=tex2D(_MainTex,IN.uv_MainTex)*_Color;o.Albedo=c.rgb;o.Alpha=c.a;}ENDCGPass{Name"OUTLINE"Tags{"LightMode"="Always"}CullFrontZWriteOnColorMaskRGBBlendSrcAlphaOneMinusSrcAlphaCGPROGRAM#pragma vertex vert#pragma exclude_renderers gles xbox360 ps3ENDCGSetTexture[_MainTex]{combineprimary}}}Fallback"Diffuse"}

The result is pretty good:

Finally, here it is a Unity script that automatically creates the outline effect when applied to an object:

usingUnityEngine;usingSystem.Collections;publicclassOutliner:MonoBehaviour{publicColormeshColor=newColor(1f,1f,1f,0.5f);publicColoroutlineColor=newColor(1f,1f,0f,1f);// Use this for initializationpublicvoidStart(){// Set the transparent material to this objectMeshRenderermeshRenderer=GetComponent<meshrenderer>();Material[]materials=meshRenderer.materials;intmaterialsNum=materials.Length;for(inti=0;i<materialsNum;i++){materials[i].shader=Shader.Find("Outline/Transparent");materials[i].SetColor("_color",meshColor);}// Create copy of this object, this will have the shader that makes the real outlineGameObjectoutlineObj=newGameObject();outlineObj.transform.position=transform.position;outlineObj.transform.rotation=transform.rotation;outlineObj.AddComponent<meshfilter>();outlineObj.AddComponent<meshrenderer>();Meshmesh;mesh=(Mesh)Instantiate(GetComponent<meshfilter>().mesh);outlineObj.GetComponent<meshfilter>().mesh=mesh;outlineObj.transform.parent=this.transform;materials=newMaterial[materialsNum];for(inti=0;i<materialsNum;i++){materials[i]=newMaterial(Shader.Find("Outline/Outline"));materials[i].SetColor("_OutlineColor",outlineColor);}outlineObj.GetComponent<meshrenderer>().materials=materials;}}

In this post we will create a script for voxelize any kind of mesh in unity. Voxelization could be useful in physical simulations, terrain representation and every situation where we need to manipulate the hollow inside of a mesh.
A great post about Voxelization can be found “here”, on Wolfire blog. The post explains how the voxelization of a triangle mesh is done in Overgrowth, we will use the same method in unity.

The creation of a voxel model that reproduces the mesh is achived trough a 3d grid of cubes and an intersection test for each triangle against each cube.
The author states that he uses a AABB-AABB intersection test to check if a cube and a triangle are intersected. This is very fast and appropriate for most situations, but we want the general solution.

A slower but precise way to test the intersection is to use the Separating Axis Theorem. This paper explains the use of the SAT for Triangle-AABB intersection.

An implementation in C++ of this algorithm was written by Mike Vandelay and can be found planet-source-code.com. I rewrote the same code in unityscript.

Basically the SAT works like this

Take 13 axes: 3 axes are the cube face normals, 1 axis is the triangle face normal, 9 are the dot product between the first 3 axes and the triangles edges.

Project the AABB and the triangle on each axis. If the projections intersects on an axis, then the AABB and the triangle are intersected, otherwise they aren’t.
here a much more detailed explanation of the SAT.

Now, let’s see how implement all this in unity.

Mesh Voxelization

The complete script for voxelization can be found here on my github.

We are going to use a grid for creating a voxel model. Each Grid is formed by cubes of the same size, these are the grid properties:

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publicclassAABCGrid{privatevarside:float;privatevarwidth:short;privatevarheight:short;privatevardepth:short;privatevarorigin:Vector3;privatevarcubeSet:boolean[,,];privatevarcubeNormalSum:short[,,];privatevardebug=false;...}/*AABC stands for Axis Aligned Bounding Cube.For performance purpose, I didn't add a 3 dimension array of AABCs, otherwise each cube had to store the side length, the set value etc...However an AABC class is defined, but only for external use, while inside the AABCGrid class everything is evaluated starting from the class properties.e.g. to obtain the vertices of a cube is it possible to use the methodAABCGrid.GetAABCCorners(x : short, y : short, z : short) : Vector3[]or the method AABC.GetCorners().The AABC.GetCorners() is actually defined like this:*/publicfunctionGetCorners(x:short,y:short,z:short):Vector3[]{// grid is a reference to an AABCGridreturngrid.GetAABCCorners(x,y,z);}

All the public method call a CheckBound() function that check if the cube specified by the x, y, z variable is inside the grid, then the real implementation of the method is called.
e.g.

Off course, inside the AABCGrid class and in the possible inheritors, only the unchecked method should be called for faster code.

Creating the voxel shell

Once the grid is defined, we need to ‘set’ all the cubes that are intersected by a triangle of the mesh.
This is done in the AABCGrid.FillGridWithGameObjectMeshShell() method.

The result will be a voxel shell, an empty shape that reproduces the mesh.

Ignore the part relative to the normals of the triangles, I’m going to explain that later.

publicfunctionFillGridWithGameObjectMeshShell(gameObj:GameObject,storeNormalSum:boolean){vargameObjMesh=gameObj.GetComponent(MeshFilter).mesh;vargameObjTransf=gameObj.transform;vartriangle=newVector3[3];varstartTime=Time.realtimeSinceStartup;varmeshVertices=gameObjMesh.vertices;varmeshTriangles=gameObjMesh.triangles;varmeshTrianglesCount=meshTriangles.length/3;varx:short;vary:short;varz:short;varignoreNormalRange=0;// In this method we can also evaluate stores the normals of the triangles // that intersect the cube.if(storeNormalSum){cubeNormalSum=newshort[width,height,depth];}if(debug){Debug.Log("Start:");Debug.Log("Time: "+startTime);Debug.Log(" Mesh Description: ");Debug.Log("Name: "+gameObjMesh.name);Debug.Log("Triangles: "+meshTrianglesCount);Debug.Log("Local AABB size: "+gameObjMesh.bounds.size);Debug.Log(" AABCGrid Description:");Debug.Log("Size: "+width+','+height+','+depth);}// For each triangle, perform SAT intersection check with the AABCs within the triangle AABB.for(vari=0;i<meshTrianglesCount;++i){triangle[0]=gameObjTransf.TransformPoint(meshVertices[meshTriangles[i*3]]);triangle[1]=gameObjTransf.TransformPoint(meshVertices[meshTriangles[i*3+1]]);triangle[2]=gameObjTransf.TransformPoint(meshVertices[meshTriangles[i*3+2]]);// Find the triangle AABB, select a sub grid.varstartX=Mathf.Floor((Mathf.Min([triangle[0].x,triangle[1].x,triangle[2].x])-origin.x)/side);varstartY=Mathf.Floor((Mathf.Min([triangle[0].y,triangle[1].y,triangle[2].y])-origin.y)/side);varstartZ=Mathf.Floor((Mathf.Min([triangle[0].z,triangle[1].z,triangle[2].z])-origin.z)/side);varendX=Mathf.Ceil((Mathf.Max([triangle[0].x,triangle[1].x,triangle[2].x])-origin.x)/side);varendY=Mathf.Ceil((Mathf.Max([triangle[0].y,triangle[1].y,triangle[2].y])-origin.y)/side);varendZ=Mathf.Ceil((Mathf.Max([triangle[0].z,triangle[1].z,triangle[2].z])-origin.z)/side);if(storeNormalSum){for(x=startX;x<=endX;++x){for(y=startY;y<=endY;++y){for(z=startZ;z<=endZ;++z){if(TriangleIntersectAABC(triangle,x,y,z)){vartriangleNormal=GetTriangleNormal(triangle);cubeSet[x,y,z]=true;if(triangleNormal.z<0-ignoreNormalRange){cubeNormalSum[x,y,z]++;}elseif(triangleNormal.z>0+ignoreNormalRange){cubeNormalSum[x,y,z]--;}}}}}}else{for(x=startX;x<endX;++x){for(y=startY;y<endY;++y){for(z=startZ;z<endZ;++z){if(!IsAABCSet(x,y,z)&&TriangleIntersectAABC(triangle,x,y,z)){cubeSet[x,y,z]=true;}}}}}}if(debug){Debug.Log("Grid Evaluation Ended!");Debug.Log("Time spent: "+(Time.realtimeSinceStartup-startTime)+"s");Debug.Log("End: ");}}

The code finds the AABB of each triangle (2), then performs the SAT intersection test on each cube intersected by AABB (3).

triangles
(1) the triangle in the grid. (2) the triangle with its AABB and the AABCs intersected by the AABB. (3) the AABCs intersected by the triangle

Filling the hollow inside

When this method is finished we will have a voxel model that reproduce the mesh. But we have not finished yet, we may need also to know which voxel (AABC) is inside the mesh and which is out.
In order to do that we use the scan fill algorithm like the post on overgrowth blog explains, except for a little thing: we don’t start to fill the cube when the normal of the last triangle faces to the left, instead we mark ‘Begin’ and ‘End’ cubes in FillGridWithGameObjectMeshShell().
If the z component of the triangle is positive, we decrease cubeNormalSum[x, y, z] by one, else we increase it. When all the triangles have been processed, a positive cubeNormalSum means that the cube is a ‘Begin’ cube, if it is negative then the cube is an ‘End’ cube.

We can’t just check the normal of the last triangle because we don’t know the order of the triangles, we neither traverse the entire grid during the creation of the voxel shell.

The method FillGridWithGameObjectMesh() does the real scan lining once that FillGridWithGameObjectMeshShell() ends. It traverses all the grid, starting from the cube at 0, 0, 0.
If a ‘Begin’ cube is found, an ‘End’ cube is searched. If an ‘End’ cube is found, all the cubes between the last ‘Begin’ and ‘End’ are set.

Performance are mainly determined by the number of triangles in the mesh and the side length of the AABCs.
Here they are some of the tests made on my laptop:

Laptop specs:
HP g6-1359el
Intel Core i5-2450M - 2,5 GHz
AMD Radeon HD 7450M

First Test

Mesh: construction_worke
Time spent: 0.4051636s
Triangles: 4020
Cube side: 0.05

Second Test

Mesh: construction_worker
Time spent: 1.088864s
Triangles: 4020
Cube side: 0.02

Third Test

Mesh: sphere

Time spent: 1.926165s
Triangles:760
Cube side: 0.03

Memory could be saved storing cubeSet using a 3D bitarray class and cubeNormalSum using a 3D array of bytes

Try it yourself

For testing purpose there is also a VoxelizationTest.js script on my github. Attach it to an object with a mesh to try this voxelization script. Remember to enable Gizmos in the game window, otherwise the AABCs will not appear!