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PPI:
Think of the PPI input as a way to adjust the physical size – not the resolution – of the eventual print-out. Decreasing the PPI, thus increasing the size of the printout, may seem to produce a lower quality image because the pixels are larger and more visible.
But remember, this is only a relative gauge of quality; if you were to stand further away, the image would appear as clear as it did before. The absolute resolution of the image has not changed; there are still as many “pixels” relative to the picture as there were before. So the way to increase the resolution of an image is to produce an image with more pixels, not increase the PPI.
DPI一般指喷墨打印的墨点deng。。。