I build an ISO surface from a volume texture so I do thing a bit different but I had to find away to make the edges of the volume to be at y=0.
I use a falloff texture to attenuate the volume texture so the edges blend down to 0, I'm sure you could do sort of the same.
then blend in the volume texture shader like this:
float blend = permTexture.Sample(samANISOTROPICWrap, input.TexCoord2 / 32);
nFinal = clamp(lerp(-1, nFinal, blend), -1, 1);
return nFinal;
What the above is doing is:
taking the final pixel value from my noise generation and using lerp to blend between the final pixel value and -1 using the falloff texture as the lerp factor, I then clamp it to -1,1 just incase the falloff texture messes with the values.
My noise is in the -1 to 1 range and the falloff texture is in the 0 to 255 range but on the GPU all values are between 0 and 1 because I load it into an 8bpp texture2D so I remaps the values to 0 to 1 range.
Now if you are using the CPU then you will need to load it like a height map and sample the values as you would for height map terrain.
It would go something like this:
Keep in mind that this code will not just drop in and work, its here to give you an idea of how it should work.
VB.Net:
Public Class HeightFinder
Public HeightData As Single(,)
// the position of the heightmap's -x, -z corner, in worldspace.
Private heightmapPosition As Vector3
// the total width of the heightmap, including terrainscale.
Private heightmapWidth As Single
// the total height of the height map, including terrainscale.
Private heightmapHeight As Single
Private TerrainScale As Single = 32
Private Sub tex2Dlod_bilinear(image As Bitmap)
HeightData = New Single(image.Width - 1, image.Height - 1) {}
Dim texelSize As Single = 1.0F / image.Width
For y As Integer = 0 To image.Height - 1
For x As Integer = 0 To image.Width - 1
Dim uv As New Vector2(x, y)
Dim height00 As Single = image.GetPixel(uv.X, uv.Y).R
HeightData(x, y) = height00
Next
Next
End Sub
Public Sub New(texturePath As String, _TerrainScale As Single)
TerrainScale = _TerrainScale
Dim image As Bitmap = Bitmap.FromFile(DirectXDevice.ContentPath & texturePath)
tex2Dlod_bilinear(image)
heightmapWidth = (HeightData.GetLength(0) - 1) * TerrainScale
heightmapHeight = (HeightData.GetLength(1) - 1) * TerrainScale
heightmapPosition.X = -(HeightData.GetLength(0) - 1) / 2.0F * TerrainScale
heightmapPosition.Z = -(HeightData.GetLength(1) - 1) / 2.0F * TerrainScale
//image.Dispose()
End Sub
// This function takes in a position, and tells whether or not the position is
// on the heightmap.
Public Function IsOnHeightmap(position As Vector3) As Boolean
// first we'll figure out where on the heightmap "position" is...
Dim positionOnHeightmap As Vector3 = position - heightmapPosition
// ... and then check to see if that value goes outside the bounds of the
// heightmap.
Return (positionOnHeightmap.X > 0 AndAlso positionOnHeightmap.X < heightmapWidth AndAlso positionOnHeightmap.Z > 0 AndAlso positionOnHeightmap.Z < heightmapHeight)
End Function
//This function takes in a position, and has two out parameters: the
// heightmap's height and normal at that point. Be careful - this function will
// throw an IndexOutOfRangeException if position isn't on the heightmap!
Private Function GetHeight(position As Vector3) As Single
If IsOnHeightmap(position) Then
// the first thing we need to do is figure out where on the heightmap
// "position" is. This'll make the math much simpler later.
Dim positionOnHeightmap As Vector3 = position - heightmapPosition
// we'll use integer division to figure out where in the "heights" array
// positionOnHeightmap is. Remember that integer division always rounds
// down, so that the result of these divisions is the indices of the "upper
// left" of the 4 corners of that cell.
Dim left As Integer, top As Integer
left = CType(positionOnHeightmap.X, Integer) / CType(TerrainScale, Integer)
top = CType(positionOnHeightmap.Z, Integer) / CType(TerrainScale, Integer)
// next, we'll use modulus to find out how far away we are from the upper
// left corner of the cell. Mod will give us a value from 0 to terrainScale,
// which we then divide by terrainScale to normalize 0 to 1.
Dim xNormalized As Single = (positionOnHeightmap.X Mod TerrainScale) / TerrainScale
Dim zNormalized As Single = (positionOnHeightmap.Z Mod TerrainScale) / TerrainScale
// Now that we've calculated the indices of the corners of our cell, and
// where we are in that cell, we'll use bilinear interpolation to calculuate
// our height. This process is best explained with a diagram, so please see
// the accompanying doc for more information.
// First, calculate the heights on the bottom and top edge of our cell by
// interpolating from the left and right sides.
Dim topHeight As Single = MathHelper.Lerp(HeightData(left, top), HeightData(left, top), xNormalized)
Dim bottomHeight As Single = MathHelper.Lerp(HeightData(left, top), HeightData(left, top), xNormalized)
// next, interpolate between those two values to calculate the height at our
// position.
Return MathHelper.Lerp(topHeight, bottomHeight, zNormalized)
Else
Return 0
End If
End Function
End Class
The idea with this code is to load your falloff texture from a bitmap and then let you sample it with your cell\poly\whatever x and y values.
Once you have sampled the falloff texture you would use it like this:
noisevalue=lerp(0,noisevalue, GetHeight(cell.x,cell.y,cell.z)
will blend from 0 to noisevalue based on the falloff texture, the closer the falloff texture is to 1 the more of the noisevalue will be shown.
Ps. I haven't used this bit of code in a long time as I have changed my terrain to volumetexture based marching cube terrain.
