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The raster matrix [a b c d tx ty] is an art-space matrix (y up) applied to the pixels with their rows running downwards, (u, -v). The reader flipped the matrix's result instead, which only agrees when b = c = 0, so a rotated image came in turned 180 degrees around its corner. The legacy version 7 writer and its prolog used the same convention; they now write and draw the matrix the reader takes.