Fig. 6

a–l Comparison of QPM and ALM for the ROF, VO, and DG3PD models: The images for ROF and VO models are obtained from [9]. The parameters for the DG3PD model are δ=0 and 20 iterations for the third and 60 for the fourth column, and the other parameters are the same as in Fig. 2. The relative error (y-axis) versus the number of iterations (x-axis) is illustrated in m. The first row shows that the VO and the DG3PD models can achieve good reconstructed images, see b, c, d, in comparison with the original magnified image (a). As mentioned in [9], the error image from the VO model (n) contains much less geometry and texture than the one from the ROF model. However, the error image from our model is much further reduced in comparison to the VO model. After 20 iterations some pieces of information still remain in the error image, see (o). As the number of iterations increases, the error numerically tends to 0; see p after 60 iterations