For Minecraft 1.12.2, you can achieve a "fullbright" effect without needing OptiFine or external mods by using a specialized resource pack or a simple configuration file tweak. Best Fullbright Resource Packs (No OptiFine Required)
Most servers allow Fullbright resource packs because they provide a visual convenience rather than a major gameplay advantage. How to Install for 1.12.2 Find a Compatible Pack: Look for packs like Simple Fullbright Fullbright Vanilla fullbright texture pack 1.12.2 no optifine
Return to the game, move the pack from the "Available" list to "Selected," and press Important Considerations Fullbright UB - Minecraft Resource Packs - CurseForge For Minecraft 1
Launch the game. Everything will be fully lit, even in deep caves. Fullbright Texture Pack Gamma Edit (Manual) Ease of Use High (Drag & Drop) Medium (Edit text file) OptiFine Required No (if using UB pack) Visual Quality Very bright, removes all shadows Very bright, removes all shadows Server Safety Generally allowed on most survival/creative servers Undetectable by servers Everything will be fully lit, even in deep caves
Save the file and restart the game. This will make caves perfectly clear without any mods or packs. Comparison Table: 1.12.2 Fullbright Methods Fullbright UB - Minecraft Resource Packs - CurseForge
Change the number to (or even 1000.0 for extreme brightness). Save the file and restart your game.