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Here's what he did. Pesci took a picture of the mural, reversed it (making a mirror-image), and then superimposed it back on the original picture. And what did he see? Two Knights Templar, one cradled baby, a strangely ostracized Judas, and the chalice that Christ was blessing at the last supper. (No partridge in a pear tree.)
Holy dialectic, DanBrown! You mean taking one image, getting the antithesis of it, and then synthesizing the two together creates a third image? Sure enough. A higher state of truth, even. You don't believe me, so look for yourself:
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Wait a minute. This is a trick! How about if we line them up with Christ's face, instead of with the physical center of the picture?
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Bam! The ghosts are gone. Do new ghosts appear though? That's your assignment. Take the Da Vinci Rorschach test and tell me what you see.