This is a common misconception. The difference is there, but it relates to the way that the two platforms consider fonts, not anything else. Logically, inside the graphics card, 1pt is defined as 1 pixel, and there are 72 pixels per inch because there are ~72 points per inch. A display pixel is a physical dimension, determined by the number of physical picture elements in the display device. (I’m deliberately ignoring “Retina” screens for this explanation.) All display pixels are different sizes depending on the hardware resolution of the display device itself (or OS-level tweekery). This means that when you draw a box that is defined as 1 inch square, it will be 72 pixels square. By no means will it be anything like 1 actual inch square, as if you put a ruler up to the screen. But if you put it next to something that is 2 inches square on the same screen, it will be half the size. It’s all relative.
Long ago, when Windows 2 and 3 were actual shipping products, the state of the art was this: PCs were for DOS, and had character-based displays. They may have had physical pixels, but there wasn’t any relationship between those pixels and the DOS, because all that DOS did was stream text at the video system, which turned it into text in some font defined by the graphics card. You can still see this text in a Windows < 8 computer when it crashes. Because the bar was set so low by graphics cart manufacturers, budget display manufacturers didn’t see any need to make their screens particularly fine-pitched or precise. This was okay as long as you were looking at ~80 characters per line of mostly upper-case text.
Mac monitors of the time were 72 pixels per inch or finer – true high-resolution displays for their time – and Mac color monitors were often massive Trinitron tubes with truly fine pitch, so you could make a 1pt rule look like a 1pt rule. And when you composed typography with a well-made set of screen fonts (or later, with ATM) you would see a reasonable representation of how your page would look when it spooled out of an ImageWriter or LaserWriter.
When Windows tried to duplicate this on the crappy PC hardware of the day, they found out that no matter how they tweaked their fonts, the bitmapped characters looked like they’d been through a shredder. The quick-n-hacky work-around they came up with was to tell the OS that the fonts should be composed at 96ppi instead of the 72ppi that the hardware was expecting. This made the fonts bigger on screen by 133%, which meant that the stair-steps in the curves of letters were effectively smoothed out a bit. It didn’t do anything to the other graphics on the screen, which meant that the pictures would look tiny compared with the text. As far as I know, at some point the decision was made to make the entire underlying graphics system compose things at 96ppi, perhaps as long ago as Windows 95 or 98. But the hardware has never changed – 72 pixels = 1 logical inch.
This difference between platforms only matters in the underlying OS – browser developers have worked around this difference; as long ago as IE 5 for Mac. By setting the base font size to 16pt instead of 12pt (as Netscape Mac originally defined the standard) they avoid the late-90s issue of too-large fonts on Windows (or too-small fonts on PCs). Mac shifted to match the PC, because the tables had turned. (In the mid 90s, an overwhelming majority of the computers on the Internet were Macs. By the late 90s, with PC TCP-IP configuration no longer requiring a Master’s degree from MIT, that percentage had shifted to reflect the much more dire PC / Mac sales figures.)
Walter
On Dec 18, 2013, at 10:25 PM, Jim Feeney wrote:
A related question: A Windows computer screen’s resolution is typically 96 ppi, and a Mac’s is 72 ppi. If I’m using two “Flow”-connected HTML text items and they look great in my Mac browser, will the text at the bottom of the first Flow-connected box be cut off when viewed on a PC? Or will the PC simply enlarge everything to reflect precisely how I have the text placed as seen on my Mac?
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