Visual secret sharing scheme (for short the VSS scheme) with access structure based on graph \cite{ABSS96} has been proposed and this can be considered as an extension of $(2, n) $-threshold VSS scheme. Ateniese et al. showed a decomposition method that we decompose star graphs from a given graph which edges are specified by qualified sets. Namely, two participants (vertices in a given graph) has a common edge if and only if participants can reconstruct the secret image by stacking the shares each other. In this paper, we expand the definition of black-white visual secret sharing scheme and propose new decomposition methods by splitting complete $n$-partite graphs. These methods improve the contrast of the decoded secret image. Moreover, we classify graph-based VSS schemes which pixel expansion is up to 3, and show several optimal examples.