D553 - An export writes a fragment output variable

Status: decided Date: 2026-09-04

A translated exp becomes a store of four bitcast lane-zero components to a Location 0 output of a Fragment module, whose epilogue skips the observation window. A module with no colour output, any target other than mrt0, and the compressed, done and write-mask fields are refused. Amended by D825: mrtK writes the output at Location K, bound to the Kth colour buffer SPI_SHADER_COL_FORMAT formats.

Why: a fragment module's oracle is the attachment, so the observation window is redundant there. A vector register holds the colour's bit pattern, so an arithmetic conversion would scale every channel. Which attachment another target selects is guest register state that needs a capture, and fields the decoder has not solved cannot be claimed as honoured.

Rejected:

• Mapping the export onto the storage buffer: invents a destination, and a plausible frame hides it.
• Translating every target onto attachment zero: appears to work while drawing to the wrong place.
• Converting components numerically: scales every channel.