Gfx GX Fn GXSetAlphaCompare - wowjinxy/libPorpoise GitHub Wiki

GXSetAlphaCompare

Category: Texture Environment
Matrix Status: Implemented
Matrix Notes: Implemented dual compare + logical combine state (comp0/ref0 op comp1/ref1) with hardware-width field masking for compare/op enums.
Matrix Link: Gfx-GX-API-Matrix

Manual page: reference/man/gfx/gx/Tev/GXSetAlphaCompare.html
SDK source: reference/ACGC-PC-Port/src/static/dolphin/gx/GXTev.c (GXSetAlphaCompare)
SDK behavior:
- Programs BP alpha-test register with:
  - ref0 in bits [7:0]
  - ref1 in bits [15:8]
  - comp0 in bits [18:16]
  - comp1 in bits [21:19]
  - op in bits [23:22]
- Uses alpha from the last active TEV stage and combines the two comparisons via GXAlphaOp (AND/OR/XOR/XNOR).

Public API declaration: include/dolphin/gx/GXTev.h
Runtime implementation: src/gx/pc_gx.c (GXSetAlphaCompare)
Shader path: src/gx/shaders/default.frag (alphaTest + logical combine in main)
Behavior:
- Stores both compare functions and refs plus combine op in GX state.
- Masks enum inputs to hardware register widths (comp* & 0x7, op & 0x3) before storing.
- Shader applies both comparisons and combine op, and discards fragments when alpha test fails.

Signature parity:
- Signatures match (GXCompare, u8, GXAlphaOp, GXCompare, u8).
Behavioral parity:
- Dual-compare + logical combine behavior is implemented.
- Bit-width field behavior for compare/op now mirrors BP register packing semantics.
Known divergence:
- SDK writes packed BP state directly; Porpoise stores decoded state and evaluates in shader.
- Equality/inequality comparison uses a small floating epsilon in shader to approximate 8-bit equality.