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Define legacy NC_proc image scaling #118

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gdevenyi merged 1 commit into from
Aug 27, 2026
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Define legacy NC_proc image scaling #118

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37 changes: 30 additions & 7 deletions FILE_FORMAT.md
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Expand Up @@ -2569,17 +2569,40 @@ image axes.

| Parameter | Description |
|-----------|-------------|
| `NC_proc` | Scaling exponent for pixel intensities |
| `YMIN_p` | Minimum pixel intensity |
| `YMAX_p` | Maximum pixel intensity |
| `NC_proc` | Optional TopSpin processing-status exponent for power-of-two scaling during floating-point-to-32-bit-integer spectrum storage |
| `YMIN_p` | Minimum intensity of the stored processed data |
| `YMAX_p` | Maximum intensity of the stored processed data |

The *TopSpin Processing Reference* (`procref`, “NC_proc — intensity scaling factor”) defines the
exponent's sign by examples: `NC_proc = -3` means the data were multiplied by 2 three times,
while `NC_proc = 4` means they were divided by 2 four times. For TopSpin-processed spectral data
such as `1r`/`1i`, therefore:

```
stored_value = unscaled_value * 2**(-NC_proc)
unscaled_value = stored_value * 2**NC_proc
```

`NC_proc` is one exponent for all stored intensities, not a per-frame array, and supplies no
additive offset. The same definition appears in the PV360 3.7 TopSpin processing reference.
It must not be used to scale a ParaVision `2dseq`: image scaling comes from
`VisuCoreDataSlope`/`VisuCoreDataOffs`, or from `RECO_map_slope` and `RECO_map_offset` when working
at the reconstruction-parameter level.

These are superseded by the Visu parameters where available — `NC_proc` by
`VisuCoreDataSlope`/`VisuCoreDataOffs`, and `YMIN_p`/`YMAX_p` by
`VisuCoreDataMin`/`VisuCoreDataMax` (which are themselves pre-scaling values, to be transformed
with the slope and offset). Note that on disk `YMIN_p`/`YMAX_p` are written into `procs`, not
into `d3proc` (zero occurrences across the public datasets' `d3proc` files); the manuals
document them under the D3 "Image Scaling" heading as Image Display & Processing state (D13
§13.4.11.3 / D02 §2.4.10.3).
with the slope and offset). On disk `YMIN_p` and `YMAX_p` are processing-status parameters in
`procs`, not `d3proc`; `NC_proc` may also occur in `procs`, but is absent from many ParaVision
image PROCNOs. In imaging-only cases where it occurs beside a `2dseq`, public PV5.1–PV7 data
write zero even when `VisuCoreDataSlope` is non-unit, confirming that it is not the image decode
slope. Non-zero examples occur in TopSpin-processed spectroscopy PROCNOs with `1r`/`1i`. Such a
PROCNO may also contain a `2dseq`; that co-location does not make `NC_proc` its decode exponent.

The parameter references group these fields under the D3 "Image Scaling" heading because they
describe Image Display & Processing state (PV5.1 D13 §13.4.11.3; PV6 D02 §2.4.10.3); D13 also
warns that the D3 values may no longer be set when Visu parameters are present. Native PV360
PROCNOs do not include `procs`, so this is a legacy/TopSpin path rather than a PV360 fallback.

---

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