It’s common in both software and especially hardware to render at a higher resolution and then filter & downsample. The 2x2 or 3x3 or 4x4 etc pixels in the higher resolution that correspond to the 1 pixel at final resolution, those are called subpixels. For example, you can read about subpixels in descriptions of traditional aliasing as well as GPU antialiasing.
The term subpixel is often referring to virtual pixels used to compute some final pixel value, as opposed to the LCD specific idea of a physical subpixel that’s red, green, or blue. Look around, for example, for discussions on subpixel resolution, subpixel positioning, subpixel animation, etc. Those are usually talking about the virtual kind used in traditional antialiasing, not the physical LCD subpixels.
Yes that’s an interesting point, the “fragment” terminology can be confusing at first. But I think of fragments as something slightly different. “Fragment” means basically pixel or subpixel geometry, it’s output from the rasterizer. The fragment shader is what takes a fragment as input and then outputs the pixel or subpixel color. Fragments aren’t necessarily involved in GPU antialiasing, you can have subpixels in OpenGL without fragments, if you’re doing image processing without rasterizing. In any case, it’s a good thing that they picked a different word to define, and didn’t just call it “subpixel”, right?
https://en.m.wikipedia.org/wiki/Spatial_anti-aliasing
https://en.m.wikipedia.org/wiki/Multisample_anti-aliasing
The term subpixel is often referring to virtual pixels used to compute some final pixel value, as opposed to the LCD specific idea of a physical subpixel that’s red, green, or blue. Look around, for example, for discussions on subpixel resolution, subpixel positioning, subpixel animation, etc. Those are usually talking about the virtual kind used in traditional antialiasing, not the physical LCD subpixels.