The bug

Planes inside of a block model will be darkened if said block is adjacent to another block which stops light. However, the extent of that darkening does not take into account where said plane is within the block, resulting in strange and often unsightly results.

This is the core issue responsible for the occurrence of MC-51113, MC-249087, and, by extension, MC-64678.

How to reproduce

1. Download the attached resource pack
2. Build the following arrangement of blocks:
3. Activate the resource pack

Expected results

The white horizontal bars comprising the model on the two glass blocks would be the same shade of white and would appear to be visually continuous due to being coplanar.

Actual results

The white bar from the glass block touching the solid stone block is much, much darker than the glass block with an air gap between it and the stone block: