One of the steps to verify whether or not we should cache a given picture is to check if the matrix has any anomalous conditions. It looks like the code only cares if the matrix is singular, but it uses the MatrixDecomposition class to determine this. Since that class first upgrades the 3x3 SkMatrix to a 4x4 SkM44 and then performs a lot of operations to classify various conditions about the full homogenous perspective matrix, and since the Prepare code does not use any of that data except for the test that tells it if the matrix is singular, this is a lot of overhead for a simple test.