irayt

an array selecting those ray groups listed in the array ray which are active for a particular shot point listed in the arrays xshot and zshot. If there are n ray groups listed in the array ray, then the jth ray group for the ith shot is referred to in the (2n(i-1)+j)th element of irayt for rays traced to the left , and in the (2n(i-1)+n+j)th element of irayt for rays traced to the right. You must specify values of irayt for each group and for each direction regardless of the values of ishot (default: 1)