Fullbright Texture Pack 1122 Link -

Most shader packs override the lightmap. Fullbright will not work if you have shaders enabled. Turn off shaders or use a shader that includes its own "fullbright" mode (e.g., Complementary Shaders has a toggle).

The Fullbright Texture Pack is designed to brighten up the Minecraft world. By adjusting the textures, this pack makes the game look more vibrant and, as the name suggests, full of light. It's particularly beneficial for players who find the default lighting a bit too dim, especially in caves or at night. The pack doesn't alter the gameplay mechanics, making it a perfect choice for those who want a more visually appealing experience without changing the game's balance. fullbright texture pack 1122 link

Even with a correct link or DIY pack, you may encounter these problems: Most shader packs override the lightmap