Intensity scaling#

Per-panel stretch and min/max control how each FITS layer is mapped to greyscale before colorization. This is separate from compositing — see Color compositing for how tinted layers are merged.

Built-in stretches#

to_grey_rgb and rescale_image accept any name in mcf.stretch_functions:

linear, sqrt, squared, log, power, sinh, asinh.

The browser and Qt GUIs expose the same list in each panel’s stretch dropdown (signed stretches below are scripting-only for now). Zscale / percentile sliders set vmin/vmax the same way as PanelState.apply_zscale() and min_max= on to_grey_rgb.

Signed flux through zero#

Background-subtracted maps often span negative noise through positive signal. log cannot handle negatives; asinh works but treats the near-zero region differently than symmetric logarithmic scaling.

rescalefn

Dependency

Behaviour

'asinh'

core

Smooth; already in GUI

'symlog'

core (matplotlib SymLogNorm)

Piecewise linear + log through zero

'symmetric_log'

optional [pysymlog]

C¹-continuous symlog (smoother near zero)

 
import multicolorfits as mcf

grey = mcf.to_grey_rgb(
    data,
    rescalefn='symlog',
    scaletype='abs',
    min_max=[vmin, vmax],
)

# For histograms / residual side panels
norm = mcf.make_norm('symmetric_log', vmin=-0.5, vmax=2.0)  # needs pysymlog

Install the optional extra:

pip install multicolorfits[pysymlog]

Colormap-heavy display (diverging maps, grey wedges for shallow negatives) is better handled in skyplothelper or a dedicated colormap workflow — mcf colorizes single-hue layers, not scalar colormap images. For a minimal matplotlib diverging map over a signed stretch, use TwoSlopeNorm (or CenteredNorm) with a diverging colormap on the greyscale data before (or instead of) tinting.