Adds noise to image in A.
imnoise (A, 'gaussian' [, mean [, var]])- additive gaussian noise: B = A + noise defaults to mean=0, var=0.01
imnoise (A, 'salt & pepper' [, density])- lost pixels: A = 0 or 1 for density*100% of the pixels defaults to density=0.05, or 5%
imnoise (A, 'speckle' [, var])- multiplicative gaussian noise: B = A + A*noise defaults to var=0.04