Getting the elements you packed from the matrix can be done using the data from the matrix without storing any additional data.
Pseudo code:
line <- W
i <- n
while (i > 0):
if dp[line][i] - dp[line - weight(i)][i-1] == value(i):
// the element 'i' is in the knapsack
i <- i-1 // only in 0-1 knapsack
line <- line - weight(i)
else:
i <- i-1
The idea behind it is that you iterate the matrix; if the weight difference is exactly the element’s size, it is in the knapsack. If it is not, the item is not in the knapsack, go on without it.