Fragment

A fragment is a sample of the final rendered image that the graphics card may or may not end up drawing. It's the fundamental unit of data processed by a fragment shader.

The fragment shader calculates each fragment's color, which ultimately determines the colors of pixels drawn on the screen or a texture.

In other words, while a pixel represents a single point of color in a rendered image, one way to think of a fragment is as a potential pixel that the graphics card may or may not end up drawing.

Each fragment also contains information like its position on the screen, UV coordinates, and other data passed from the vertex shader to the fragment shader.

Fragments vs. pixels

In practice, the words fragment and pixel are often used interchangeably when talking about shaders.

Technically, there can be multiple fragments per pixel, or there can be fragments that are discarded before being drawn so they don't end up in the final rendered image.

For example, the discard keyword tells the fragment shader to stop processing the current fragment. This code skips drawing any fragments that are not in the left half of the screen:

if (UV.x < 0.5) {
    discard;
}

This is different from setting the fragment to a transparent color. Unlike returning a transparent color, a discarded fragment isn't drawn at all.

Why is it called a fragment?

The term comes from the graphics card's rendering pipeline, where geometry gets 'fragmented' into individual screen pixels for further processing.