Auto White Balance

Neutralise colour casts without per-image manual correction. Three algorithms, one CLI.

• gray-world (default) — assume the average of the scene is neutral grey. Scale each RGB channel so its mean equals the global mean. Robust on most natural scenes.
• white-patch — assume the brightest pixels are white. Per-channel -auto-level stretches each channel to full range. Strong correction; fails on scenes lacking a true white reference.
• combined — gray-world first, then a gentle white-patch pass at 50% strength. Good default for mixed-light indoor shots.

When to use

• Indoor photos under tungsten/fluorescent/LED with the wrong WB preset.
• Scanned prints / slides with age-related cast.
• Underwater or aquarium shots (heavy blue/green cast).
• Batch normalisation before publishing a gallery shot across multiple lighting conditions.

Do not use this skill when:

• Source is a RAW file (CR2/NEF/ARW/DNG/RAF) — use darktable-cli with --apply-custom-presets and the temperature module set to as shot to reference. White-balancing a baked JPEG throws away headroom that's still in the RAW.
• The cast is intentional (golden hour, sodium streetlamps for mood, blue-hour landscapes). Auto-WB will flatten it.
• The image is a deliberately monochromatic / toned edit.

Inputs

1. Input — file or directory. Required.
2. Mode — gray-world (default) | white-patch | combined.
3. Recursive — --recursive for directory descent.
4. Output — default in-place suffix _wb next to the source. --output-dir <path> to write into a sibling folder. --overwrite to replace.
5. Strength — 0.0 to 1.0 (default 1.0). Blends the corrected image with the original via -compose blend -define compose:args=<pct>. Use 0.5 for a softer touch.
6. Preserve luminance — default on. After channel scaling, rescale so the mean luminance matches the original (prevents brightness drift). --no-preserve-luma to disable.

Procedure

1. Verify ImageMagick 7+. magick -version | head -1. Fall back to convert (IM6) if magick is missing — the syntax below works on both with the leading binary swapped. If neither, point at install-deps.
2. Enumerate inputs. Accept JPEG/JPG/PNG/TIFF/TIF/WebP. Skip and note RAW formats.

3. gray-world:

   magick "$IN" \
     -colorspace sRGB \
     -channel R -evaluate multiply $(magick "$IN" -colorspace sRGB -format "%[fx:mean/mean.r]" info:) \
     -channel G -evaluate multiply $(magick "$IN" -colorspace sRGB -format "%[fx:mean/mean.g]" info:) \
     -channel B -evaluate multiply $(magick "$IN" -colorspace sRGB -format "%[fx:mean/mean.b]" info:) \
     +channel "$OUT"

   Each fx:mean/mean.<c> is the gain that drives that channel's mean to the global mean. Three info: calls is wasteful for batch — cache via a single magick identify -format "%[fx:mean] %[fx:mean.r] %[fx:mean.g] %[fx:mean.b]" "$IN" and compute gains in shell.

4. white-patch:

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

   Independent per-channel histogram stretch to [0,1]. Aggressive; clips highlights if any channel already saturates.

5. combined:

   Run gray-world to a temp file, then:

   magick "$TMP" \( +clone -channel RGB -auto-level +channel \) \
     -compose blend -define compose:args=50 -composite "$OUT"

6. Preserve luminance (when enabled): after step 3/4/5, measure luminance before and after and rescale:

   L_IN=$(magick "$IN"  -colorspace Gray -format "%[fx:mean]" info:)
   L_OUT=$(magick "$WB" -colorspace Gray -format "%[fx:mean]" info:)
   GAIN=$(awk "BEGIN{print $L_IN/$L_OUT}")
   magick "$WB" -evaluate multiply "$GAIN" "$OUT"

7. Strength blend (when <1.0):

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

8. For batch, parallelise with xargs -P $(nproc) over the file list. Skip files that already end in _wb to avoid recursive re-processing.

Output

• Corrected images at the resolved paths.
• Summary:
  • Files processed / skipped (RAW or non-image) / failed.
  • Per-mode count when mixed.
  • Average per-channel gain across the batch (sanity check — if R-gain ≈ G-gain ≈ B-gain ≈ 1.0, the batch had no cast and the run was a no-op).

Notes

• Gray-world fails on scenes dominated by one colour (snow, foliage, red brick wall). For those, white-patch is more honest, or skip auto-WB entirely and use a colour picker on a known-neutral patch.
• For batch mixed-lighting normalisation, run combined at --strength 0.7 — strong enough to neutralise obvious casts, weak enough to preserve scene character.
• Output colour profile: ImageMagick respects the input ICC profile. If the source has no profile, it's treated as sRGB. Use -profile sRGB.icc explicitly if the pipeline downstream is profile-strict.
• This skill is the single-image / batch primitive. The orchestrator apply-filters may call it as one step; this skill exists to do WB-only work without the rest of the pipeline.