Wrong automatic Planckian fit, missing Planck axis information and synthetic images shown in pixels #400

Context

Feedback from DataLab 1.3.0 users about the blackbody signal, Planckian fitting and synthetic images. Most fixes belong to Sigima and SigimaX; this issue tracks the user-facing result in DataLab.

Symptoms

1. Processing > Fitting > Planckian fit (automatic) produces a curve about 150 times lower than the signal (e.g. blackbody at 296 K), while the interactive Planckian fit converges correctly.
2. The formula used by the interactive fit dialogs is not shown anywhere in the dialog; the documentation is the only place where it can be found.
3. Create > Blackbody (Planck's law) produces a signal with no X/Y titles or units, although X is a wavelength in meters and Y a spectral radiance (sine signals, by contrast, show s on the X axis).
4. Create > Sinusoidal grating (and the other images defined on a domain: Gaussian, 2D ramp, ring pattern, Siemens star, 2D sinc) is displayed with pixel axes instead of the [xmin, xmax] × [ymin, ymax] domain used to compute it, which makes frequencies, centers and widths confusing.
5. Exponential fitting fails when X is in seconds and the time constant is in the millisecond range.

Proposed changes

• Sigima and SigimaX fixes tracked in Planckian and exponential fits depend on data units; Planck signal and domain-based images lack axis information Sigima#74 and Interactive fit dialogs: show the model formula and evaluate models through Sigima SigimaX#9.
• DataLab: images created by earlier versions keep their pixel coordinates when their creation parameters are reused (history recompute, parameter re-edition).
• Documentation: formulas shown in the fit dialogs, how to derive the peak wavelength and temperature from Planckian fit parameters, new "Physical coordinates" option for image creation.
• Release notes for 1.4.0 and French translations.

Related

• Planckian fit is exactly non-identifiable: x0 and sigma cannot be determined separately Sigima#43 (Planckian model degeneracy, out of scope)
• Planckian and exponential fits depend on data units; Planck signal and domain-based images lack axis information Sigima#74, Interactive fit dialogs: show the model formula and evaluate models through Sigima SigimaX#9, Interactive fit dialog: show the model formula web#56