Levels (Histogram Transform)

Classic black / midtones / white levels: set the black and white points, then bend the midtones with a PixInsight-style Midtones Transfer Function. Live curve, histogram overlay, per-channel, and mask-aware.

1. Using the tool

Drag the three handles on the curve (or use the sliders/spin-boxes), watch the preview and the original-vs-result histogram, then Apply. The curve's black and white handles carry the midtone proportionally, so narrowing the range doesn't jump the midpoint.

Control	What it does
Channel	L (luminance, chroma preserved), K (brightness — same curve on R, G, B), or a single R/G/B. Mono images are locked to L.
Black point	Input level mapped to 0. Everything at or below it is clipped to black.
Midtones	The input level pulled to mid-grey (0.5) — the gamma-like bend between black and white.
White point	Input level mapped to 1. Everything at or above it is clipped to white.
Live preview	Recompute the preview on every change (debounced).
Curve + histogram	Draggable B/M/W handles; the histogram shows original (dim) vs result (bright) for R/G/B and luminance.
Apply / Apply as New / Reset	Write to the document, open the result as a new view, or return the handles to 0 / 0.5 / 1.
Grip (bottom-left)	Drag to the canvas to mint a Levels shortcut, or drop on an image to apply headlessly.

The clip readout reports, for the selected channel, how many pixels the current black/white points drive to pure black or white; a min/max readout shows the data's actual range.

2. Technical details

Two stages per channel: a linear black/white rescale, then a non-linear midtones transfer. Everything operates on float data in [0,1].

2.1 Black / white rescale

The black point b and white point w linearly remap and clip the input:

t=\mathrm{clip}\left(\frac{x-b}{w-b},\;0,\;1\right)

2.2 Midtones transfer

The rescaled value is bent by the PixInsight Midtones Transfer Function with a single midtone parameter m:

M(t;m)=\frac{(m-1)\,t}{(2m-1)\,t-m}

It is pinned at the ends and sends the midtone to mid-grey:

M(0;m) = 0 (black stays black) M(1;m) = 1 (white stays white) M(m;m) = 0.5 (the midtone input maps to mid-grey)

The Midtones handle is an absolute input level; it is converted to the MTF parameter relative to the black/white window before the curve is evaluated, so the midpoint stays meaningful as you move the end points:

m=\frac{m_{\mathrm{abs}}-b}{w-b}

This is the same midtones transfer that Statistical Stretch uses — here you place black/mid/white by hand (midtone → 0.5), whereas Statistical Stretch solves the midtone analytically to land a target median.

2.3 Channel modes

L (luminance) — the curve is applied to the luminance
Y=0.2126\,R+0.7152\,G+0.0722\,B
and re-applied to the colour by ratio, preserving chroma:
\mathrm{out}=\mathrm{rgb}\cdot\frac{M(t_Y;\,m)}{Y}
K (brightness / RGB linked) — the identical curve is applied to R, G and B directly (same black/mid/white on every channel).
R / G / B — the curve is applied to that one channel only.
Mono — the curve is applied to the single plane; channel selection is disabled.

2.4 Clipping stats

For the selected channel's values v (L and K use the luminance proxy), the readout counts what the current end points clip:

n_{\mathrm{low}}=\#\{v\leq b\}\,,\qquad n_{\mathrm{high}}=\#\{v\geq w\}

2.5 Masking

If the document has an active mask, the transform is blended back against the original over the mask (and the mask is resized to match — downsampled for the preview, full-res on apply, so what you see matches what you get):

\mathrm{out}=\mathrm{base}\cdot(1-\mathrm{mask})+\mathrm{out}\cdot\mathrm{mask}

2.6 Notes

The preview runs on a downsampled copy (longest side capped) for responsiveness; Apply recomputes on the full-resolution image.
Histograms are 256-bin, sampled, and show original (dim) against result (bright) for R/G/B and L.
After a successful Apply the handles reset to 0 / 0.5 / 1 so the next adjustment starts clean on the already-transformed image.
The headless/grip preset carries the relative midtone, so a dropped shortcut reproduces the live curve exactly even with the end points moved.