Think of it this way: The image bits within the BMP structure define the shape of the image using that much data (that much information describes the image), but that information must then be translated to a target device using a measuring system to indicate its applied resolution in practical use.

For example, if the BMP is 10,000 pixels wide, and 4,000 pixels high, that explains how much raw detail exists within the image bits. However, that image information must then be applied to some target. It uses the relationship to the dpi and its target to derive the applied resolution.

If it were printed at 1000 dpi then it's only going to give you an image with 10" x 4" but one with extremely high detail to the naked eye (more pixels per square inch). By contrast, if it's printed at only 100 dpi, then you'll get an image that's 100" x 40" with low detail (fewer pixels per square inch), but both of them have the same overall number of bits within. You can actually scale an image without scaling any of its internal image data by merely changing the dpi to non-standard values.

Also, using 72 dpi is a throwback to ancient printing techniques (https://en.wikipedia.org/wiki/Twip) which are not really relevant in moving forward (except to maintain compatibility with standards) as modern hardware devices often use other values for their fundamental relationships to image data. For video screens, for example, Macs use 72 dpi as the default. Windows uses 96 dpi. Others are similar. In theory you can set it to whatever you want, but be warned that not all software honors the internal settings and will instead assume a particular size. This can affect the way images are scaled within the app, even though the actual image data within hasn't changed.

要计算显示器的每英寸像素值，首先要确定屏幕的尺寸和分辨率。

以屏幕尺寸为4吋的iPhone 5为例，分辨率为1136x640，像素密度为326PPI。而分辨率为1920x1080的家用21.5寸显示器，像素密度为103PPI。

另外，由于屏幕的尺寸有时并不代表其实际的大小（例如苹果公司生产的21.5寸iMac电脑实际对角线长度为21.465寸^[6]），因此单凭尺寸算得的每英寸像素值会有一定的误差。同样的一台显示器，如果分辨率设置的不同，像素点数也不同。分辨率越高，每英寸像素值也越高，图像的大小也越小。

根据屏幕每英寸像素值的不同，Android系统的开发者将平板电脑和手机的屏幕分成四类：^[11]