To achieve the "Fullbright" effect in version 1.21.4, the mod likely employs one of two standard methodologies:
Minecraft relies on a complex lighting engine to determine visibility and atmosphere. The game calculates light levels from 0 (total darkness) to 15 (maximum light), interpolating rendering brightness accordingly. While this creates an immersive survival experience, it introduces usability challenges for players on specific hardware or those engaging in large-scale building projects where visibility is paramount. File name- Fullbright-Mod-Fabric-1.21.4.jar
Instead of rendering dark areas as pitch-black (light level 0-3), the Fullbright mod forces the game to render all areas as if they have a light level of 15—the maximum. This is not a simple gamma adjustment (like editing the options.txt file to increase brightness, which Mojang has limited in recent updates). This is a shader and lighting engine override. To achieve the "Fullbright" effect in version 1