512 pixels across is the sane and obvious choice for a 9" screen. It's 64 bytes/32 words across, which is a nice and even number. The next step up would be 640 pixels (40 words across), but it's probably not worth the memory tradeoff.
The Apple IIc shipped with the same 9" monochrome monitor and can do 560 pixels across (40 words, 14-pixels per word for historical reasons). It's hard to imagine shrinking the pixels much further on that screen.
The Apple IIgs can draw 640 pixels across in the same width that the Apple II draws 280 pixels. I suppose one could connect an Apple IIc monitor to the IIgs and experiment with monochrome 640x200 graphics to see how the smaller pixels look. (I own both, so maybe I'll try it out).
My theory: The article mentioned an earlier prototype used 384×256, a 3:2 ratio. 512×342 is the even height that gives closest to a 3:2 ratio with a pixel width of 512.
adjust the width control until the raster is 7 inches (177.8 mm) wide
adjust the height control until the raster is 4.7 inches (119.4 mm) high
This gives a physical aspect ratio of 1.489...:1, while the pixel ratio is 1.497...:1 (0.5% off square)
It makes a pixel 1/73.14...inch wide and 1/72.77in tall
Still, as a sibling comment points out: Once you decide you can hit your screen redraw time with around 170,000 pixels on the screen, that you will use a 9" 3:2 CRT, that you want one pixel to be close to one printer's point (1/72 inch), and that your framebuffer needs to be oriented around 16-bit or maybe 32-bit quantities, you aren't left with a lot of options. 480 pixels is not quite enough to show the width of a standard US letter page with 1/2 inch left and right margins, 512 pixels is (and can even fit a scroll bar)
Note: I don't have any evidence that the choice of a 9" screen or a 3:2 ratio preceded the choice of pixel resolution, it could have happened the other way around.
Also blowing a hole in one leg of my argument: MacWrite 1.0, as screenshotted at https://lowendmac.com/2006/macwrite-1-defining-word-processi..., used 80 pixels per inch for the onscreen ruler, not 72! With this scale, 7 inches (US letter paper with 1" left and right margins) plus a scrollbar fits in 512 pixels.
If you ever got to use the Mac emulator on a Mac XL - it highlighted how fixed Mac gui layout fit the 9” Mac screen like a glove. The extra screen resolution on the 12” screen was there but the menu stopped at the 9” mark. I had a Mac XL for a while but traded up to a Mac II when a/ux became available. A happier time in computing.
Back when I ran one of those machines, I was struck by just how on point the display was!
That resolution was dead on point for the CRT Apple used. It was capable of a bit more, say 700 lines, maybe even 800 horizontally. Vertically, maybe 400 to 480 would be pushing it.
Say they used 640x480. The user would have been happy with a pixel in the horizontal direction, and maybe less happy with it in the vertical one. And it would have been a bit less crisp all 9bwr the screen, IMHO.
Monochrome CRTs can be over driven to a crazy amount and they just work. And often, unless the overdrive is just crazy, the user will probably see the differences as the GUI changes too. I have run 1024x768 on a 7" amber screen.
Worked, but not well.
It is much happier with about 500 vertical lines and more like 700 horizontal ones.
PAL ish monochrome 720x586 looks fantastic and is 50hz. Slow phosphors = 50hz being no big deal. NTSC ish 720x480 at 60hz looks good too, maybe a bit more crisp.
All I am saying is for that CRT, the resolution Apple chose will look great! And that is due to that particular CRT and drive circuit.
I think Apple could have pushed it to 640, lime the GS machine could do, and go 400 lines vertical and that CRT would perform almost as well. We may not even be able to tell.
Maybe they did not do that as an overall balance between what the 68K could pixel wrangle, RAM and CRT performance.
The Apple IIc shipped with the same 9" monochrome monitor and can do 560 pixels across (40 words, 14-pixels per word for historical reasons). It's hard to imagine shrinking the pixels much further on that screen.
The Apple IIgs can draw 640 pixels across in the same width that the Apple II draws 280 pixels. I suppose one could connect an Apple IIc monitor to the IIgs and experiment with monochrome 640x200 graphics to see how the smaller pixels look. (I own both, so maybe I'll try it out).
The pixel resolution is 512x342. The Macintosh Service Source https://www.macdat.net/files/pdf/apple/servicesource/macinto... page 76 says to do the following:
adjust the width control until the raster is 7 inches (177.8 mm) wide
adjust the height control until the raster is 4.7 inches (119.4 mm) high
This gives a physical aspect ratio of 1.489...:1, while the pixel ratio is 1.497...:1 (0.5% off square)
It makes a pixel 1/73.14...inch wide and 1/72.77in tall
Still, as a sibling comment points out: Once you decide you can hit your screen redraw time with around 170,000 pixels on the screen, that you will use a 9" 3:2 CRT, that you want one pixel to be close to one printer's point (1/72 inch), and that your framebuffer needs to be oriented around 16-bit or maybe 32-bit quantities, you aren't left with a lot of options. 480 pixels is not quite enough to show the width of a standard US letter page with 1/2 inch left and right margins, 512 pixels is (and can even fit a scroll bar)
Note: I don't have any evidence that the choice of a 9" screen or a 3:2 ratio preceded the choice of pixel resolution, it could have happened the other way around.
Also blowing a hole in one leg of my argument: MacWrite 1.0, as screenshotted at https://lowendmac.com/2006/macwrite-1-defining-word-processi..., used 80 pixels per inch for the onscreen ruler, not 72! With this scale, 7 inches (US letter paper with 1" left and right margins) plus a scrollbar fits in 512 pixels.
https://www.bing.com/search?q=macintosh+xl+wikipedia&for...
That resolution was dead on point for the CRT Apple used. It was capable of a bit more, say 700 lines, maybe even 800 horizontally. Vertically, maybe 400 to 480 would be pushing it.
Say they used 640x480. The user would have been happy with a pixel in the horizontal direction, and maybe less happy with it in the vertical one. And it would have been a bit less crisp all 9bwr the screen, IMHO.
Monochrome CRTs can be over driven to a crazy amount and they just work. And often, unless the overdrive is just crazy, the user will probably see the differences as the GUI changes too. I have run 1024x768 on a 7" amber screen.
Worked, but not well.
It is much happier with about 500 vertical lines and more like 700 horizontal ones.
PAL ish monochrome 720x586 looks fantastic and is 50hz. Slow phosphors = 50hz being no big deal. NTSC ish 720x480 at 60hz looks good too, maybe a bit more crisp.
All I am saying is for that CRT, the resolution Apple chose will look great! And that is due to that particular CRT and drive circuit.
I think Apple could have pushed it to 640, lime the GS machine could do, and go 400 lines vertical and that CRT would perform almost as well. We may not even be able to tell.
Maybe they did not do that as an overall balance between what the 68K could pixel wrangle, RAM and CRT performance.