auto-tone

Auto Tone

Fix flat or under/over-exposed images without manual curves work. Three modes:

• level (default) — -auto-level on the luminance channel. Stretches the histogram to [0,1] without touching colour balance. Safe, reversible-feeling.
• gamma — -auto-gamma. Drives the image's mean brightness toward 50% grey. Lifts dark scans, tames blown-out daylight.
• punch — -normalize (1% black + 1% white clip) followed by -auto-gamma. Stronger; the "press the button" preset for batch gallery prep.

When to use

• Underexposed indoor / night phone shots.
• Faded scans of old prints.
• Bulk normalisation of a gallery shot under varying light.
• Pre-step before sharpening or upscaling — auto-tone first so the next stage isn't operating on a flat histogram.

Do not use this skill when:

• Source is a RAW file — fix tone in darktable-cli where you have full bit depth.
• Image is intentionally low-key (silhouettes, night scenes with crushed shadows) or high-key (snow, fashion editorials). Auto-gamma flattens the look.
• The histogram is already well-distributed (a quick magick identify -format "%[fx:standard_deviation]" "$IN" ≥ 0.22 → likely already toned; auto-level becomes a no-op or worse).

Inputs

1. Input — file or directory. Required.
2. Mode — level (default) | gamma | punch.
3. Recursive — --recursive for directory descent.
4. Output — default in-place suffix _tone. --output-dir <path> to write into a sibling folder. --overwrite to replace.
5. Strength — 0.0 to 1.0 (default 1.0). Blends corrected image with original via -compose blend.
6. Preserve hue — default on. Operate on the luminance channel only (HSL L), so colour relationships aren't shifted. --per-channel to apply auto-level independently to R/G/B (acts as combined tone + white-balance — usually you want auto-white-balance for that instead).
7. Clip (mode=punch) — black/white clip percent. Default 1. --clip 0.5 for gentler, --clip 2 for aggressive contrast.

Procedure

1. Verify ImageMagick 7+. magick -version | head -1. Else convert (IM6) with the same syntax.

2. Enumerate inputs (JPEG/JPG/PNG/TIFF/TIF/WebP). Skip RAW with note.

3. level (luminance-only auto-level, hue-preserving):

   magick "$IN" -colorspace HSL -channel L -auto-level +channel -colorspace sRGB "$OUT"
   

   With --per-channel:

   magick "$IN" -channel RGB -auto-level +channel "$OUT"
   

4. gamma:

   magick "$IN" -auto-gamma "$OUT"
   

   -auto-gamma computes the gamma needed to drive mean → 0.5 and applies it. No clipping, no hue shift.

5. punch:

   magick "$IN" -normalize -auto-gamma "$OUT"
   

   Or with explicit clip:

   magick "$IN" -channel L -contrast-stretch ${CLIP}%x${CLIP}% +channel -auto-gamma "$OUT"
   

   -contrast-stretch 1%x1% clips the darkest 1% to black and brightest 1% to white before stretching — the "Photoshop auto-contrast" behaviour.

6. Strength blend (when <1.0):

   magick "$IN" "$WB" -compose blend -define compose:args=$(awk "BEGIN{print $STRENGTH*100}") -composite "$OUT"
   

7. For batch, parallelise with xargs -P $(nproc). Skip files ending _tone to avoid recursive re-processing.

Output

• Tone-corrected images at the resolved paths.
• Summary:
  • Files processed / skipped (RAW or non-image) / failed / no-op (where the histogram was already well-distributed).
  • Per-mode count when mixed.
  • Average histogram-spread before/after (fx:standard_deviation) — sanity check that the run actually changed something.

Notes

• For "make my photos look better" without thinking: --mode punch --strength 0.7 is the sane default.
• Auto-tone amplifies noise. If the source is already noisy (high ISO phone shots), denoise first, then tone.
• Combined with auto-white-balance: run WB first (so tone correction operates on a neutral image), then tone. Reverse order is not order-equivalent.
• This skill is the tone-only primitive. The orchestrator apply-filters may chain it with sharpen/saturate; use this skill standalone when you only want tonal work.