in easy-understanding way 30天从入门到精通
1.增加.ppm转.png，以两种形式输出 2.修改原书中的bug 3.实现玻璃，透明塑料，粗糙表面和粗糙度可调的金属表面以及景深
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Increase. Ppm to. PNG, output in two forms
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Modify the bug in the original book
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Realize glass, transparent plastic, rough surface and metal surface with adjustable roughness and depth of field