Tiling atlas textures correctly with a custom shader in Unity - unity3d

Question

This is a bit complicated, but it boils down to be quite a simple problem, I hope. So here is how it goes: I am using Unity to generate a map gameobject during runtime from a bsp file which has a whole bunch of vertices, faces, uvs, texture references, and so on. The meshes created come out exactly as they should be, and all the textures come out fine. There is one problem though, there are so many meshes created with so many materials leading to many draw calls making the program slow. So I searched on a way to reduce the draw calls and I found a solution. Combine all the meshes into one big mesh and create a texture atlas by combining all the textures used. Combining the meshes works fine and combining the textures comes out great as well. Then I faced the problem of uv mapping. So I found a solution from the NVidia white paper to make a custom shader which uses the tex2d function to interpolate the texel from the texture using the uv positions with their derivatives. I think this would have worked, but my meshes have really weird triangles and I think they are ruining this solution. In the images below you can see the difference when the meshes are combined from when they are separate:
Combined Meshes with Changed UVs and Custom Shader
Separate Meshes with original UVs
This is the code I am using in the shader to set the color of the model:
o.Albedo = tex2D (_MainTex, IN.uv2_BlendTex, ddx(IN.uv_MainTex), ddy(IN.uv_MainTex)).rgb;
As you can see, I have added a second UV which is the non-tiled version of the original UV. I do that by using the frac() function, but in the C# code rather than in the shader. Since the textures can be different sizes, I had to calculate the UV before getting to the shader because I have access to the texture sizes at that time.
Here is the code I used to calculate the 2 UVs:
Rect surfaceTextureRect = uvReMappers[textureIndex];
Mesh surfaceMesh = allFaces[i].mesh;
Vector2[] atlasTiledUVs = new Vector2[surfaceMesh.uv.Length];
Vector2[] atlasClampedUVs = new Vector2[surfaceMesh.uv.Length];
for (int j = 0; j < atlasClampedUVs.Length; j++)
{
Vector2 clampedUV = new Vector2((surfaceMesh.uv[j].x - Mathf.Floor(surfaceMesh.uv[j].x)), (surfaceMesh.uv[j].y - Mathf.Floor(surfaceMesh.uv[j].y)));
float atlasClampedX = (clampedUV.x * surfaceTextureRect.width) + surfaceTextureRect.x;
float atlasClampedY = (clampedUV.y * surfaceTextureRect.height) + surfaceTextureRect.y;
atlasTiledUVs[j] = new Vector2((surfaceMesh.uv[j].x * surfaceTextureRect.width) + surfaceTextureRect.x, (surfaceMesh.uv[j].y * surfaceTextureRect.height) + surfaceTextureRect.y);
atlasClampedUVs[j] = new Vector2(atlasClampedX, atlasClampedY);
if (i < 10) { Debug.Log(i + " Original: " + surfaceMesh.uv[j] + " ClampedUV: " + clampedUV); }
}
surfaceMesh.uv = atlasTiledUVs;
surfaceMesh.uv2 = atlasClampedUVs;
The array uvReMappers is an array of Rect created when using the Texture2D function PackTextures().
Sorry for taking so long, but here is my question: Why do the textures come out contorted. Is it because the way the meshes are triangulated or is it because of the way I wrote the custom shader. And finally how can I fix it.
Thank you for your time. I am sorry for writing so much, but I have never posted a question before. I always find answers to almost all my problems online, but I have been searching for days on how to fix this problem. I feel it might be too specific to be able to find an answer for. I hope I have provided enough information.

Answer 1

I finally solved the problem! So it turns out I should not calculate the UVs before the shader. Instead I passed the information needed by the shader through the UVs so that it can calculate the new texel positions directly.
Here is the code before the shader:
Rect surfaceTextureRect = uvReMappers[textureIndex];
Mesh surfaceMesh = allFaces[i].mesh;
Vector2[] atlasTexturePosition = new Vector2[surfaceMesh.uv.Length];
Vector2[] atlasTextureSize = new Vector2[surfaceMesh.uv.Length];
for (int j = 0; j < atlasTexturePosition.Length; j++)
{
atlasTexturePosition[j] = new Vector2(surfaceTextureRect.x, surfaceTextureRect.y);
atlasTextureSize[j] = new Vector2(surfaceTextureRect.width, surfaceTextureRect.height);
}
surfaceMesh.uv2 = atlasTexturePosition;
surfaceMesh.uv3 = atlasTextureSize;
Here is the shader code:
tex2D(_MainTex, float2((frac(IN.uv.x) * IN.uv3.x) + IN.uv2.x, (frac(IN.uv.y) * IN.uv3.y) + IN.uv2.y));

Answer 2

I took a different approach and created a texture atlas on the cpu, from there UV mapping was just like normal UV mapping all I had to do was assign a texture to the vertex info from my atlas ...
My scenario is a custom voxel engine that can handle anything from minecraft to rendering voxel based planets and I haven't found a scenario it can't handle yet.
Here's my code for the atlas ...
using UnityEngine;
using Voxels.Objects;
namespace Engine.MeshGeneration.Texturing
{
/// <summary>
/// Packed texture set to be used for mapping texture info on
/// dynamically generated meshes.
/// </summary>
public class TextureAtlas
{
/// <summary>
/// Texture definitions within the atlas.
/// </summary>
public TextureDef[] Textures { get; set; }
public TextureAtlas()
{
SetupTextures();
}
protected virtual void SetupTextures()
{
// default for bas atlas is a material with a single texture in the atlas
Textures = new TextureDef[]
{
new TextureDef
{
VoxelType = 0,
Faces = new[] { Face.Top, Face.Bottom, Face.Left, Face.Right, Face.Front, Face.Back },
Bounds = new[] {
new Vector2(0,1),
new Vector2(1, 1),
new Vector2(1,0),
new Vector2(0, 0)
}
}
};
}
public static TextureDef[] GenerateTextureSet(IntVector2 textureSizeInPixels, IntVector2 atlasSizeInPixels)
{
int x = atlasSizeInPixels.X / textureSizeInPixels.X;
int z = atlasSizeInPixels.Z / textureSizeInPixels.Z;
int i = 0;
var result = new TextureDef[x * z];
var uvSize = new Vector2(1f / ((float)x), 1f / ((float)z));
for (int tx = 0; tx < x; tx++)
for (int tz = 0; tz < z; tz++)
{
// for perf, types are limited to 255 (1 byte)
if(i < 255)
{
result[i] = new TextureDef
{
VoxelType = (byte)i,
Faces = new[] { Face.Top, Face.Bottom, Face.Left, Face.Right, Face.Front, Face.Back },
Bounds = new[] {
new Vector2(tx * uvSize.x, (tz + 1f) * uvSize.y),
new Vector2((tx + 1f) * uvSize.x, (tz + 1f) * uvSize.y),
new Vector2((tx + 1f) * uvSize.x, tz * uvSize.y),
new Vector2(tx * uvSize.x, tz * uvSize.y)
}
};
i++;
}
else
break;
}
return result;
}
}
}
And for a texture definition within the atlas ...
using UnityEngine;
using Voxels.Objects;
namespace Engine.MeshGeneration.Texturing
{
/// <summary>
/// Represents an area within the atlas texture
/// from which a single texture can be pulled.
/// </summary>
public class TextureDef
{
/// <summary>
/// The voxel block type to use this texture for.
/// </summary>
public byte VoxelType { get; set; }
/// <summary>
/// Faces this texture should be applied to on voxels of the above type.
/// </summary>
public Face[] Faces { get; set; }
/// <summary>
/// Atlas start ref
/// </summary>
public Vector2[] Bounds { get; set; }
}
}
For custom scenarios where I need direct control of the UV mappings I inherit texture atlas and then override the SetupTextures() method but in pretty much all cases for me I create atlases where the textures are all the same size so simply calling GenerateTextureSet will do the uv mapping calculations I believe you need.
The UV coords for a given face of a given voxel type are then ...
IEnumerable<Vector2> UVCoords(byte voxelType, Face face, TextureAtlas atlas)
{
return atlas.Textures
.Where(a => a.VoxelType == voxelType && a.Faces.Contains(face))
.First()
.Bounds;
}
In your case you probably have a different way to map to the texture of choice from your pack but essentially the combination of a face and type in my case are what determine the uv mapping set I want.
This then allows you to use your mesh with any standard shader instead of relying on custom shader logic.

Answer 3

You have to turn the passed in TEXCOORD0 from a percentage of the image space to a pixel value, use the modulus to figure out which pixel it is on the tiled texture, and then turn it back into a percentage of the image.
Here's the code:
You need the 2D variables _MainTex and _PatternTex to be defined.
struct v2f
{
float2 uv : TEXCOORD0;
float4 vertex : SV_POSITION;
};
float modFunction(float number, float divisor){
//2018-05-24: copied from an answer by Nicol Bolas: https://stackoverflow.com/questions/35155598/unable-to-use-in-glsl
return (number - (divisor * floor(number/divisor)));
}
fixed4 frag (v2f i) : SV_Target
{
fixed4 curColor = tex2D(_MainTex, i.uv);
fixed4 pattern = tex2D(_PatternTex,
float2(
modFunction(i.uv.x*_MainTex_TexelSize.z,_PatternTex_TexelSize.z) *_PatternTex_TexelSize.x,
modFunction(i.uv.y*_MainTex_TexelSize.w,_PatternTex_TexelSize.w) *_PatternTex_TexelSize.y
)
);
fixed4 col = curColor * pattern;
col.rgb *= col.a;
return col;
}