I understand that unlike for ChIP-seq, with ATAC-seq MACS doesn't estimate the model, which means it doesn't determine distance 'd' between the tags and therefore it doesn't shift the tags by d/2 in 3´direction (and that is why with ATAC-seq the --nomodel --shift 0 parameters are set).
But if 'd' is not estimated then how can MACS slide a window of 2d across the genome to calculate peak enrichment?
Also, for ChIP-seq there is a control sample (input DNA or DNA pulled down with an unspecific antibody) that is used to calculate lambda local and determine peak enrichment. In ATAC-seq there isn't any control pulled down with unspecific antibody. What is that control in ATAC-seq?