From PixieWiki

[edit] Displacements from textures

If you use a texture-map to displace the surface, you may find that grid cracks appear - small pinholes in the surface. This happens because the filter size chosen for adjacent grids may differ slightly, resulting in slightly different displacements which pull the surface edges apart.

Eg.

What can you do?

Turn on binary dicing, using

Attribute "dice" "binary" 1

Turn off the filtering on the texture lookup, by replacing your texture(name,s,t) with the following code:

   #define NUMPIXELS 1024
   #define SINGLEPIX (1/NUMPIXELS)
   P += amt*texture(texname,s,t, s+ SINGLEPIX,t, s,t+ SINGLEPIX, s+ SINGLEPIX,t+ SINGLEPIX)*normalize(N);

Additionally, you should try increasing the number of texture samples, ie.

   #define NUMPIXELS 1024
   #define SINGLEPIX (1/NUMPIXELS)
   P += amt*texture(texname,s,t, s+ SINGLEPIX,t, s,t+ SINGLEPIX, s+ SINGLEPIX,t+ SINGLEPIX,"samples",16)*normalize(N);