docs: detailed ash-wallpaper usage guide with parameter reference and recipes

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+# `ash-wallpaper.sh` — guide
+
+Turns a color photo into an **Ash** wallpaper: the image is reduced to
+Ash-tinted greys, while warm *ember/gold* highlights (sunsets, fire, lamps,
+skin tones) are selectively kept — the "Pleasantville" effect.
+
+## Quick start
+
+```sh
+tools/ash-wallpaper.sh photo.jpg                  # -> photo-ash.png (dark base)
+PRESET=light tools/ash-wallpaper.sh photo.jpg     # -> light Ash base
+```
+
+Requirements: ImageMagick 6.9+ or 7 (`convert` / `magick`). 4K input takes ~1s.
+All knobs are environment variables read once per run — tune one, rerun,
+compare.
+
+## How it works
+
+1. **Hue band** — the photo's HSL hue channel is thresholded: pixels with
+   `hue ≤ HUE_MAX` or `hue ≥ HUE_MIN` form the "warm" band (reds, oranges,
+   golds; the wrap-around at HUE_MIN catches pure crimson reds).
+2. **Saturation gate** — inside that band, a saturation ramp decides how much
+   of the pixel survives: nothing below `SAT_MIN`, everything from `SAT_MAX`.
+   This is what keeps dull, near-grey warmish pixels as ash.
+3. **Softening** — the combined mask is blurred, then re-sharpened with
+   `SENSITIVITY` so highlights fade into the greys instead of showing hard
+   cut-out edges.
+4. **Base** — a desaturated copy is tinted along the Ash grey ramp
+   (`BASE_LO` shadows → `BASE_HI` highlights).
+5. **Composite** — the original photo, with saturation multiplied by
+   `EMBER_BOOST`, is laid over the base through the mask.
+
+## Parameters
+
+| Variable | Default | Meaning |
+|---|---|---|
+| `PRESET` | `dark` | Grey ramp of the base. `dark` = `#100f12 → #f3ede1` (void→chalk), `light` = `#b7ae98 → #f2ecdf` (stone→paper). |
+| `HUE_MAX` | `13%` | Upper edge of the warm band, in percent of the hue circle (360°). `13%` ≈ 47°, so reds, orange, ember and gold survive; yellows from ≈47° and all greens/blues fall out. |
+| `HUE_MIN` | `94%` | Lower edge of the wrap-around band (`≥ 338°`). Catches deep crimson/cherry reds whose hue sits just below 360°. |
+| `SAT_MIN` | `12%` | Saturation (HSL) below which a pixel becomes full grey, no matter its hue. Raise to make the image more monochrome (e.g. muted/desaturated photos: `18–25%`). |
+| `SAT_MAX` | `45%` | Saturation at which the highlight is fully kept. Pastel skies/candle-lit scenes peak below 45% — lower it (`25–35%`) to let soft colour through; raise it (`55–70%`) to keep only the most saturated embers. |
+| `SENSITIVITY` | `8x45%` | `-sigmoidal-contrast` applied to the mask: `contrast x midpoint`. Higher contrast → crisper, more cut-out-looking highlights; lower (e.g. `3x50%`) → dreamier, feathered bleed. The midpoint shifts the whole mask: `>50%` = only the most certain warm pixels, `<50%` = more generous. |
+| `EMBER_BOOST` | `130` | Saturation multiplier (%) on the surviving pixels, so the remaining embers read against all that ash. `100` = untouched colours, `160+` = intense glow. Applied to *all* channels of those pixels, so already-saturated fire can clip — reduce if it looks neon. |
+| `BASE_LO` / `BASE_HI` | from preset | Exact endpoints of the grey ramp. Any colour works, e.g. `BASE_LO=#1c1b1f BASE_HI=#e6dfd1` for a flatter panel→bone ramp. |
+
+### Where the Ash palette sits in the hue wheel
+
+For reference when widening/narrowing the band (hue in IM percent ≈ degrees/360):
+
+| Colour | Hex | Hue | In band at defaults? |
+|---|---|---|---|
+| blood | `#c15c46` | 11° (3%) | ✅ |
+| rust | `#a86a48` | 21° (6%) | ✅ |
+| ember | `#e0914f` | 27° (8%) | ✅ |
+| gold | `#d6b46a` | 41° (11%) | ✅ |
+| pure yellow | — | 60° (17%) | ❌ (needs `HUE_MAX=17%`) |
+| moss | `#8f9a5c` | 69° (19%) | ❌ |
+| steel | `#7c9aa1` | 187° (52%) | ❌ |
+| rot | `#b47e99` | 330° (92%) | ❌ (needs `HUE_MIN=88%` — usually you *don't* want this) |
+| violet | `#a585ac` | 292° (81%) | ❌ |
+
+## Recipes
+
+```sh
+# Warm sunset landscape, keep the drama (dark preset is the default)
+tools/ash-wallpaper.sh sunset.jpg
+
+# Night city: street lamps are soft yellow-orange, need a wider + softer gate
+HUE_MAX=17% SAT_MIN=8% SAT_MAX=60% SENSITIVITY=4x50% tools/ash-wallpaper.sh city.jpg
+
+# Portrait: only eyes/lips/warm skin glow, everything else ash
+SAT_MIN=18% EMBER_BOOST=150 tools/ash-wallpaper.sh portrait.jpg
+
+# Almost pure grey with faint traces of ember
+SAT_MIN=20% SAT_MAX=70% EMBER_BOOST=100 tools/ash-wallpaper.sh photo.jpg
+
+# Light-themed desktop
+PRESET=light tools/ash-wallpaper.sh beach.jpg
+```
+
+## Batch generation
+
+```sh
+for f in ~/Pictures/vacation/*.jpg; do
+  tools/ash-wallpaper.sh "$f"
+done
+```
+
+## Using the results
+
+Drop the generated files into the theme or user backgrounds folder — Omarchy
+cycles through them with `Super+Ctrl+Space`:
+
+```sh
+mkdir -p ~/.config/omarchy/backgrounds/ash-dark
+cp photo-ash.png ~/.config/omarchy/backgrounds/ash-dark/
+```
+
+## Troubleshooting
+
+- **Highlights cut with hard edges** → lower the `SENSITIVITY` contrast
+  (first number), e.g. `4x45%`.
+- **A warm object disappears** → its hue is outside the band (check the table
+  above) or its saturation is below `SAT_MIN`.
+- **Grey areas look slightly tinted/brown** → those pixels are just inside the
+  band; raise `SAT_MIN`.
+- **Fire/embers look clipped and neon** → lower `EMBER_BOOST` towards 100.
+- **Image too bright/dark after tinting** → adjust `BASE_LO`/`BASE_HI` instead
+  of editing the photo.