Early-Z (Early Z Culling)

Early-Z (Early Z culling) is a technical term that is extremely important in rendering and optimizing game graphics (technical art). An explanation of the GPU's hardware function that automatically performs a depth comparison (Z-Test) at the "Early" stage of executing the heavy "Fragment Shader" that paints pixels, and immediately discards calculations for pixels in the back that are occluded and cannot be seen, thereby speeding up the calculation.

Real-world analogy: In a coloring book, a super clever coloring technique that automatically detects and completely skips the ``underpainting in the background (wasteful pixel calculation)'' before coloring, which disappears due to the overlapping of things that are drawn in the foreground

Normally, it is processed in the inefficient order of "coloring (shader execution) → finally checking the context and erasing invisible parts (Z-Test)". Early-Z checks whether the front distance from the camera (depth buffer) has already been determined ``just before'' coloring, and as soon as it realizes that ``the pixel is in the back where the wall is covering it from above and you can't see it anyway'', it throws it into the trash without painting for even a second, saving GPU stamina to the utmost.

Detailed mechanism and operating principle

Draw the opaque materials in order of "Front to Back" from the camera, and activate the GPU's "Early-Z" function to 100% to completely skip the calculation of pixels in the back from the beginning.