[Sad Catapella N7][Scientific correctness] Image summaries can miscount saturated pixels across mixed-dtype Z/T planes #54

Baseline and classification

Sad Catapella Normalization Batch 7, pinned f784c95e3049483f87466150ffc5764375fa631d. Confirmed source-level numerical/data-contract defect for a synthetic mixed-dtype image; no repository tests executed. Distinct from #51 (mixed RGB FOVs in fit).

Evidence

• image_summaries() lines 165–175 checks channel counts but never enforces one dtype across Z/T planes belonging to a single source image.
• _summary() lines 178–209 groups all those histograms, assigns the summary's dtype from the first plane's first channel, and uses _saturated_fraction(histograms, ...) for every channel.
• _saturated_fraction() lines 213–232 sets top = np.iinfo(histograms[0].dtype).max, but counts the last occupied singleton bin of every plane whose pixel_max >= top, regardless of that plane's own dtype. PlaneRecord._check_channels() lines 114–130 validates dtype equality within each XYC record, not across Z/T records. Existing saturation tests keep all planes in one dtype.

Counterexample

One logical source_relative_path='a.tif' with z=0 a uint8 plane whose values are all 100 (no pixel is 255), and z=1 a uint16 plane whose values are all 1000 (no pixel is 65535). Make exact one-value histograms for both. If uint8 is first, the summary's top=255; the uint16 plane's highest occupied singleton bin at 1000 is counted as saturated despite not being at its type maximum. The true saturated fraction is 0 for either plane's native dtype, but the algorithm can report a positive result. Swapping record order changes summary.dtype and behavior.

Fix and acceptance

Refuse mixed dtypes for one logical source image before pooling; report paths, Z/T planes and observed dtypes. Do not auto-convert dynamic ranges or guess bit depths. Optionally require global experiment dtype consistency only at the upstream preprocessing boundary (separate policy). Test both plane orders, uint8+uint16, uint16+int16, same-dtype varying Z/T, one all-nonfinite channel, and saturated type maxima. Ensure saturated fraction does not depend on input row ordering. No Numba opportunity.