Why do Optimum Realism textures flicker or shimmer in the distance

Sparkly, crawling, or shimmering textures in the distance are a side effect of high resolution: more pixels per block means more fine detail to alias as the camera moves. It's a render limitation, not a broken pack — and it's the flip side of distance blur (same mipmap cluster, opposite symptom).

Fixes, in order:

• Mipmap Levels → 4. Options → Video Settings → Mipmap Levels. Setting it to Off or low is what causes harsh distant shimmer; 4 smooths it. (Too aggressive and you get blur instead — 4 is the balance.)
• Anisotropic filtering. On OptiFine, Video Settings → Quality → Anisotropic Filtering → 4-8 keeps angled distant textures stable. On Iris + Sodium, Better Mipmaps restores the anisotropic detail Sodium drops.
• Shader anti-aliasing (TAA). For shimmer while moving, enable TAA / anti-aliasing in your shader's settings — it samples across frames to settle the sparkle. (Trade-off: TAA can add slight ghosting on fast motion.)

If it's flicker on specific blocks (two faces fighting), that's z-fighting — two surfaces at the same coordinate the GPU can't decide between. That's a model/texture issue, not mipmaps; a community pack like "No Z-Fighting" patches the worst offenders, and it usually shows on a small set of blocks rather than across the whole horizon.

Grass and leaves shimmering specifically? That's the same mipmap/AA fix — those have lots of high-frequency edges that alias first.