I mostly agree, and see my other top-level comment where I called out LCD panels as having physically square pixels.
It’s a good thought, and it is correct that ClearType is considering the sub pixel arrangement of LCD elements. But also remember the arrangement of pixels isn’t Alvy’s main point, he was mainly trying to convey how to think about the shape of samples (pixels).
Even with ClearType you don’t necessarily want to integrate the sub-sub-samples of an LCD sub-pixel with a box (square) filter.
For sub-pixel rendering in general, box filtering is definitely not the best answer. Though yes, lots of people do it and get away with it all the time when sampling quality is not a high priority. Games are a good example, even as they’re improving. Treating pixels as square when sub-sampling causes ringing artifacts that can never be cured by adding more samples. This is actually a really fun thing to do with a class of graphics students because it’s kind of surprising the first time you really get it. For some nasty antialiasing problems only high quality kernels like a Gaussian will integrate sub-samples without artifacts.
Note I’m not talking about LCD sub-pixels there, just normal supersampling. The Wikipedia article on ClearType calls that “grayscale antialiasing” to distinguish it from LCD red-green-blue subpixels. But IMO that’s a bad name since grayscale antialiasing is still referring to filtering color images.
I thought about it and came up with a counter-example. subpixel rendering in general and ClearType in particular. The algorithm works by considering the exact arrangement of RGB squares (rectangles actually) to improve the appearance of rendered text. Theoretically subpixel rendering could increase the horizontal resolution threefold which were very useful for DPI-starved screens. Font hinting were also used to fit the text into the confines of the limited pixel grid.