"Faithful by default" is easy to say. This is a story about how it gets checked.
The vector units
The PlayStation 2 had two small, fast vector processors that did the heavy math for the picture: turning, bending and lighting thousands of points a frame. The games carry their own programs for them, written in the chips' own language, and those programs depend on the chips' exact timing. Some results arrive a few steps after they are asked for, and a program that reads too early gets the old value.
OpenGOAL translated several of these programs into C++ by hand. That works, as long as every timing detail was translated right.
Two answers
Phaethon took a different route: an interpreter that runs the vector programs as written, built from the chips' own timing rules. Run against upstream's hand translation of Jak and Daxter's ocean, the two disagreed in 1,242 of 4,018 cases. One of them was wrong. Which one?
Asking the console
The answer came from a capture of the game running in PCSX2, the PlayStation 2 emulator, at Sentinel Beach. It sided with the interpreter. The hand translation read the result of a division one step before the hardware delivers it, which shifted the ocean's reflection at every fourth column of its texture. With the code generated from the console's own program, the ocean matched the capture on 512 of 512 points.
By October 13th no hand-translated vector program was left in the shipped game. Every one now comes from the disc's own program.
The frame oracle
The ocean was settled by hand, but most changes are checked by a machine. The frame oracle saves the full state of a moment in a game, draws that moment again later without the game running, and compares the picture with a reference a person approved. A change that moves pixels past tolerance is refused before it lands.
It started with seven scenes in September, all matching to the pixel. It now checks 797. Many compare the game with itself: one picture must be identical to another, or never brighter than it. When a reference picture changes, the approval is written down with its reason, and PCSX2 is named in those approvals 46 times. One example: sparks from an impact were approved at 129 strips, where the console draws 115 to 125 and the port had been drawing 239.
What it is checked against
The reference is PCSX2 and the discs' own code, not a recording from real hardware. No capture from a real console exists yet, and getting one is on the list. Until then, faithful means faithful to the best record of the console there is, checked on every change.
Alex (maintainer, and a mechanical engineer by trade)