# 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.

```{image} ../_static/showcase/crab_gallery.png
:class: mcf-plot
:alt: Eight Crab Nebula stamps with different palettes and compositing modes
```

| 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:

```bash
# 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')
```