Why fragments do not necessarily correspond to pixels one to one?

Question

Here is a good explanation what is a fragment:

https://gamedev.stackexchange.com/questions/8977/what-is-a-fragment/8981#8981

But here (and not only here) I have read that "I want to stress the fact that one pixel is not necessarily one fragment, multiple fragment can be combined to make one pixel....". But I don't understand clearly what are fragments and why they are not necessarily correspond to pixels one to one?

EDIT: When multiple fragments form one pixel it is only in the case when they overlap after projection, or it is because the pixel is bigger than the fragment, hence you need to put together next to each other multiple fragments with the same color to form a pixel?

Answer 1

A fragment has a location that can be queried via its built-in gl_FragCoord variable where the x and y component directly correspond to pixels on your screen. So you could say that a fragment indeed corresponds to a pixel.

However, a fragment outputs a color and stores that color in a color buffer at its coordinates. This does not mean this color is the actual pixel color that is shown to the viewer.

Because a fragment shader is run for each object, it could happen that other objects are drawn after your first object that also output a fragment at the same screen coordinate. When taking depth-testing, stencil testing and blending into account, the resulting color value in the color buffer might get overwritten and/or merged with new colors.

Think of it like this:

Object 1 gets drawn and draws the color purple at screen coordinate (200, 300);
Object 2 gets drawn and draws the color red at same coordinate, overwriting it.
Object 3 (is blue) has transparency of 50% at same coordinate, and merges colors.
Final fragment color output is then combination of red and blue (50%).

The final resulting pixel could then be a color from a single fragment shader run, a color that is overwritten by many other fragment shader runs, or a combination of colors via blending.