Crab Nebula — palette & compositing gallery

Same four HST WFPC2 layers (F502N / F547M / F631N / F673N), many looks. Changing colors, the compositing mode (RGB vs Lab / HSL), and the background (black vs white) completely changes the feel of an image — useful for artistic preference, or for figures that need to sit on a light publication page or slide deck.

Panel	Mode	Notes
red orange	RGB / black	Classic m1_dark1 / big_M1_4color_dark1.jpg
purple orange	RGB / black	Classic m1_dark2 / big_M1_4color_dark2.jpg
gui	RGB / black	README / Qt splash palette
ember	Lab / max	Warm lava + violet/cyan accents (artistic)
ice lava	Lab / max	Hot filaments on an icy core (artistic)
aurora	RGB / black	Green / teal / violet / pink (artistic; classic additive is more vibrant than Lab)
copper gold	Lab / screen	Warm near-monochrome (artistic)
sunset	HSL / screen	Oranges / violet / blue (artistic)
on white	RGB / white	Same dark1 colors via the alpha Background path — for light slides / papers

Each stamp is a transparent-sky RGBA cutout built on top of a black-background composite (so classic RGB recipes keep the same colors as big_M1_4color_dark1.jpg / dark2.jpg). A soft sky matte (alpha_smooth=3, alpha_lo≈45, alpha_hi≈82, alpha_gamma=0.7) clears mosaic-edge junk while keeping the inner filaments opaque.

Note

Earlier cutouts looked neon / blown-out because export_transparent_cutout forced a transparent compose background, which routes classic RGB through the alpha compositor and changes the colors. That helper now preserves black-background math by default (render_background='auto').

Reproduce locally

Requires a local Crab HST WFPC2 crop directory (same layers as the README splash). With those FITS available:

# Maintainer regenerator (writes stamps + docs/_static/showcase/crab_gallery.png)
python hidden/make_crab_cutouts.py                 # all recipes + gallery
python hidden/make_crab_cutouts.py red_orange ember
# Interactive:
#   mcf.gui(state='…/crab_red_orange_session.json')