参考链接：https://99designs.com/blog/tips/ppi-vs-dpi-whats-the-difference/
PPI describes the number of square pixels that show up in an inch of digital screen (usually between 67-300). DPI, on the other hand, is a printing term referring to the number of physical dots of ink in a printed document.
PPI表示每英寸上像素的个数。DPI是关于打印的术语，表示印刷中打印文件的物理油墨的点数。
在弄懂上面两者的区别是，我们需要搞清楚什么是像素？
Pixel stands for “picture element”. It’s the smallest physical element of a digital display device that the eye can discern.
像素代表的是图像的元素，是人眼能看到的电子显示屏上最小的物理单元。
In fact, pixels are actually made up of “sub-pixels” — red, green and blue light elements that the human eye cannot see because additive color processing blends them into a single hue which appears on the pixel level. But this fact is not directly relevant to designers.
实际上，还有比像素更小的元素存在，像素是由这些sub-pixels组成，这些是人眼看不到的。加色处理将这些颜色混合成一种单一的色调。
像素是物理单位且是固定的大小。
Printers do not reproduce an image by tiling pixel squares directly on top of one another. Rather, they reproduce an image by spitting out tiny dots consisting of a mix of four colors, Cyan, Magenta, Yellow and Key (black), which combine to create a range of hues by the subtractive color model.
打印机不会通过平铺像素单元的方式来重现图像。相反，它们通过由四种颜色混合的小点来再现图像。
例如，一个150ppi的图片在600dpi的设备上显示，每个pixel有16个dot (600dots/150pixels)
