3D Texture emulation in shader (subpixel related) - unity3d

Question

I am working on a Unity3D project which relies on a 3D texture momentarily.
The problem is, Unity only allows Pro users to make use of Texture3D. Hence I'm looking for an alternative to Texture3D, perhaps a one dimensional texture (although not natively available in Unity) that is interpreted as 3 dimensional in the shader (which uses the 3D texture).
Is there a way to do this whilst (preferably) keeping subpixel information?
(GLSL and Cg tags added because here lies the core of the problem)
Edit: The problem is addressed here as well: webgl glsl emulate texture3d
However this is not yet finished and working properly.
Edit: For the time being I disregard proper subpixel information. So any help on converting a 2D texture to contain 3D information is appreciated!
Edit: I retracted my own answer as it isn't sufficient as of yet:
float2 uvFromUvw( float3 uvw ) {
float2 uv = float2(uvw.x, uvw.y / _VolumeTextureSize.z);
uv.y += float(round(uvw.z * (_VolumeTextureSize.z - 1))) / _VolumeTextureSize.z;
return uv;
}
With initialization as Texture2D(volumeWidth, volumeHeight * volumeDepth).
Most of the time it works, but sometimes it shows wrong pixels, probably because of subpixel information it is picking up on. How can I fix this? Clamping the input doesn't work.

Answer 1

I'm using this for my 3D clouds if that helps:
float SampleNoiseTexture( float3 _UVW, float _MipLevel )
{
float2 WrappedUW = fmod( 16.0 * (1000.0 + _UVW.xz), 16.0 ); // UW wrapped in [0,16[
float IntW = floor( WrappedUW.y ); // Integer slice number
float dw = WrappedUW.y - IntW; // Remainder for intepolating between slices
_UVW.x = (17.0 * IntW + WrappedUW.x + 0.25) * 0.00367647058823529411764705882353; // divided by 17*16 = 272
float4 Value = tex2D( _TexNoise3D, float4( _UVW.xy, 0.0, 0.0 ) );
return lerp( Value.x, Value.y, dw );
}
The "3D texture" is packed as 16 slices of 17 pixels wide in a 272x16 texture, with the 17th column of each slice being a copy of the 1st column (wrap address mode)...
Of course, no mip-mapping allowed with this technique.

Answer 2

Here's the code I'm using to create the 3D texture if that's what bothering you:
static const NOISE3D_TEXTURE_POT = 4;
static const NOISE3D_TEXTURE_SIZE = 1 << NOISE3D_TEXTURE_POT;
// <summary>
// Create the "3D noise" texture
// To simulate 3D textures that are not available in Unity, I create a single long 2D slice of (17*16) x 16
// The width is 17*16 so that all 3D slices are packed into a single line, and I use 17 as a single slice width
// because I pad the last pixel with the first column of the same slice so bilinear interpolation is correct.
// The texture contains 2 significant values in Red and Green :
// Red is the noise value in the current W slice
// Green is the noise value in the next W slice
// Then, the actual 3D noise value is an interpolation of red and green based on the W remainder
// </summary>
protected NuajTexture2D Build3DNoise()
{
// Build first noise mip level
float[,,] NoiseValues = new float[NOISE3D_TEXTURE_SIZE,NOISE3D_TEXTURE_SIZE,NOISE3D_TEXTURE_SIZE];
for ( int W=0; W < NOISE3D_TEXTURE_SIZE; W++ )
for ( int V=0; V < NOISE3D_TEXTURE_SIZE; V++ )
for ( int U=0; U < NOISE3D_TEXTURE_SIZE; U++ )
NoiseValues[U,V,W] = (float) SimpleRNG.GetUniform();
// Build actual texture
int MipLevel = 0; // In my original code, I build several textures for several mips...
int MipSize = NOISE3D_TEXTURE_SIZE >> MipLevel;
int Width = MipSize*(MipSize+1); // Pad with an additional column
Color[] Content = new Color[MipSize*Width];
// Build content
for ( int W=0; W < MipSize; W++ )
{
int Offset = W * (MipSize+1); // W Slice offset
for ( int V=0; V < MipSize; V++ )
{
for ( int U=0; U <= MipSize; U++ )
{
Content[Offset+Width*V+U].r = NoiseValues[U & (MipSize-1),V,W];
Content[Offset+Width*V+U].g = NoiseValues[U & (MipSize-1),V,(W+1) & (MipSize-1)];
}
}
}
// Create texture
NuajTexture2D Result = Help.CreateTexture( "Noise3D", Width, MipSize, TextureFormat.ARGB32, false, FilterMode.Bilinear, TextureWrapMode.Repeat );
Result.SetPixels( Content, 0 );
Result.Apply( false, true );
return Result;
}

Answer 3

I followed Patapoms response and came to the following. However it's still off as it should be.
float getAlpha(float3 position)
{
float2 WrappedUW = fmod( _Volume.xz * (1000.0 + position.xz), _Volume.xz ); // UW wrapped in [0,16[
float IntW = floor( WrappedUW.y ); // Integer slice number
float dw = WrappedUW.y - IntW; // Remainder for intepolating between slices
position.x = ((_Volume.z + 1.0) * IntW + WrappedUW.x + 0.25) / ((_Volume.z + 1.0) * _Volume.x); // divided by 17*16 = 272
float4 Value = tex2Dlod( _VolumeTex, float4( position.xy, 0.0, 0.0 ) );
return lerp( Value.x, Value.y, dw );
}
public int GetPixelId(int x, int y, int z) {
return y * (volumeWidth + 1) * volumeDepth + z * (volumeWidth + 1) + x;
}
// Code to set the pixelbuffer one pixel at a time starting from a clean slate
pixelBuffer[GetPixelId(x, y, z)].r = color.r;
if (z > 0)
pixelBuffer[GetPixelId(x, y, z - 1)].g = color.r;
if (z == volumeDepth - 1 || z == 0)
pixelBuffer[GetPixelId(x, y, z)].g = color.r;
if (x == 0) {
pixelBuffer[GetPixelId(volumeWidth, y, z)].r = color.r;
if (z > 0)
pixelBuffer[GetPixelId(volumeWidth, y, z - 1)].g = color.r;
if (z == volumeDepth - 1 || z == 0)
pixelBuffer[GetPixelId(volumeWidth, y, z)].g = color.r;
}