Hi all,
   
  I have following problem :
   
  a = b = seq(1, 50000, by=500)
v = matrix(0, nrow=length(a), ncol=length(a))
for (i in 1:length(a))
   {
    for (j in 1:length(a))
       {
        d = c(17989*a[i], -18109*b[j])
        v[i,j] = t(d) %*% matrix(c(0.0001741, 0.0001280, 0.0001280, 0.0002570), nrow=2) %*% d
       }
   }
library("rgl")
open3d()
persp3d(a,b,v,col="green",alpha=0.7,aspect=c(1,1,0.5))

   
  Now I want to shed the portion with different color of the surface which satisfy following condition:
   
  0 < (a-b) < max(a)
   
  Can anyone please tell me how to do that?
   
  Regards,

       
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