FA-10551 / Raster compositing / Open access

Unpremultiply transparent color · case 01

Premultiplied channels are exposed as straight channels and darken translucent pixels.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Premultiplied channels are exposed as straight channels and darken translucent pixels.

VERIFIED REPAIR

Use the stage contract: Input is valid premultiplied normalized RGBA. Divide RGB by nonzero alpha; represent fully transparent output as transparent black.

Unsuccessful approach: Clamping the divisor to at least one prevents recovery of every translucent color.

Case contract

Input is valid premultiplied normalized RGBA. Divide RGB by nonzero alpha; represent fully transparent output as transparent black.

Why this case matters

A deterministic software graphics stage with explicit channel and coordinate conventions; no hardware, device profile or API behavior is inferred.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import math
N = 1
observations = []
def solve(rgba):
    return list(rgba)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*([0.5, 0.25, 0.125, 0.5],)), [1, 0.5, 0.25, 0.5])
check('fixture 2', solve(*([0, 0, 0, 0],)), [0, 0, 0, 0])
check('fixture 3', solve(*([0.25, 0.5, 1, 1],)), [0.25, 0.5, 1, 1])
check('fixture 4', solve(*([0, 0.125, 0, 0.25],)), [0, 0.5, 0, 0.25])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 1	`[0.5, 0.25, 0.125, 0.5]`	`[1, 0.5, 0.25, 0.5]`	Failed
fixture 2	`[0, 0, 0, 0]`	`[0, 0, 0, 0]`	Passed
fixture 3	`[0.25, 0.5, 1, 1]`	`[0.25, 0.5, 1, 1]`	Passed
fixture 4	`[0, 0.125, 0, 0.25]`	`[0, 0.5, 0, 0.25]`	Failed

SHA-256 / c8cc2b137706f9ba920ae8bbac41bac2edc3aa0a0eb69149fa89d457144a2c07

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import math
N = 1
observations = []
def solve(rgba):
    return [x/max(rgba[3],1) for x in rgba[:3]]+[rgba[3]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*([0.5, 0.25, 0.125, 0.5],)), [1, 0.5, 0.25, 0.5])
check('fixture 2', solve(*([0, 0, 0, 0],)), [0, 0, 0, 0])
check('fixture 3', solve(*([0.25, 0.5, 1, 1],)), [0.25, 0.5, 1, 1])
check('fixture 4', solve(*([0, 0.125, 0, 0.25],)), [0, 0.5, 0, 0.25])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 1	`[0.5, 0.25, 0.125, 0.5]`	`[1, 0.5, 0.25, 0.5]`	Failed
fixture 2	`[0.0, 0.0, 0.0, 0]`	`[0, 0, 0, 0]`	Passed
fixture 3	`[0.25, 0.5, 1.0, 1]`	`[0.25, 0.5, 1, 1]`	Passed
fixture 4	`[0.0, 0.125, 0.0, 0.25]`	`[0, 0.5, 0, 0.25]`	Failed

SHA-256 / a81736c896c2763ce8b16b4ddf5a999bef199bceb1efcc9c21aac45ae437e6a0

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import math
N = 1
observations = []
def solve(rgba):
    return [x/rgba[3] for x in rgba[:3]]+[rgba[3]] if rgba[3] else [0,0,0,0]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*([0.5, 0.25, 0.125, 0.5],)), [1, 0.5, 0.25, 0.5])
check('fixture 2', solve(*([0, 0, 0, 0],)), [0, 0, 0, 0])
check('fixture 3', solve(*([0.25, 0.5, 1, 1],)), [0.25, 0.5, 1, 1])
check('fixture 4', solve(*([0, 0.125, 0, 0.25],)), [0, 0.5, 0, 0.25])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 1	`[1.0, 0.5, 0.25, 0.5]`	`[1, 0.5, 0.25, 0.5]`	Passed
fixture 2	`[0, 0, 0, 0]`	`[0, 0, 0, 0]`	Passed
fixture 3	`[0.25, 0.5, 1.0, 1]`	`[0.25, 0.5, 1, 1]`	Passed
fixture 4	`[0.0, 0.5, 0.0, 0.25]`	`[0, 0.5, 0, 0.25]`	Passed

SHA-256 / 9dc451a49115751130e4e202a1b3cd340f49d1bfa33be38650252427db1f37e7

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:38:39.947725+00:00.

Case digest / ef173c845c78137cbd55764e61e6cc5738cec818e18e5e53246f3672992912cd