FA-10556 / Raster compositing / Open access

Global opacity premultiplied · case 01

Layer opacity reduces alpha while leaving premultiplied color unchanged.

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

ROOT CAUSE

Layer opacity reduces alpha while leaving premultiplied color unchanged.

THE FAILURE

Layer opacity reduces alpha while leaving premultiplied color unchanged.

Unsuccessful approach: Scaling color alone leaves the old destination occlusion intact.

Case contract

Apply normalized layer opacity to premultiplied RGBA by scaling both RGB and alpha.

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, opacity):
    return rgba[:3]+[rgba[3]*opacity]
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, 0.5], 0.5)), [0.25, 0.125, 0, 0.25])
check('fixture 2', solve(*([1, 1, 1, 1], 0)), [0, 0, 0, 0])
check('fixture 3', solve(*([0.25, 0, 0, 0.5], 1)), [0.25, 0, 0, 0.5])
check('fixture 4', solve(*([0, 0, 0, 0], 0.5)), [0, 0, 0, 0])
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, 0.25]`	`[0.25, 0.125, 0, 0.25]`	Failed
fixture 2	`[1, 1, 1, 0]`	`[0, 0, 0, 0]`	Failed
fixture 3	`[0.25, 0, 0, 0.5]`	`[0.25, 0, 0, 0.5]`	Passed
fixture 4	`[0, 0, 0, 0.0]`	`[0, 0, 0, 0]`	Passed

SHA-256 / a399c0c4fa2a32009de51c242d2824197f0a7d2d6a6baca142c9b20497390538

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import math
N = 1
observations = []
def solve(rgba, opacity):
    return [x*opacity 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, 0.5], 0.5)), [0.25, 0.125, 0, 0.25])
check('fixture 2', solve(*([1, 1, 1, 1], 0)), [0, 0, 0, 0])
check('fixture 3', solve(*([0.25, 0, 0, 0.5], 1)), [0.25, 0, 0, 0.5])
check('fixture 4', solve(*([0, 0, 0, 0], 0.5)), [0, 0, 0, 0])
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	`[0.25, 0.125, 0.0, 0.5]`	`[0.25, 0.125, 0, 0.25]`	Failed
fixture 2	`[0, 0, 0, 1]`	`[0, 0, 0, 0]`	Failed
fixture 3	`[0.25, 0, 0, 0.5]`	`[0.25, 0, 0, 0.5]`	Passed
fixture 4	`[0.0, 0.0, 0.0, 0]`	`[0, 0, 0, 0]`	Passed

SHA-256 / b1509bf42b863b925dd7bb5a5f6e7cfb0e88023f50ce1d766a69eb6d6d539119

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 4 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

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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 2026-09-29T14:38:40.029078+00:00.

Case digest / 0feedc8eba7a6e559fe731cb93678e6384f4e94bacd48994417d924334dbb615