ds-galo/backend/internal/photos/edit.go

package photos

import (
	"encoding/json"
	"errors"
	"math"
	"sort"
)

// Edit parameters are stored in the photo metadata and describe a
// non-destructive render: the original object is never touched, the edited
// image is produced as a separate object.

// LightParams holds the parameters of the "Свет" (light) tool. The slider
// ranges mirror the ones Lightroom exposes: exposure in stops, everything else
// in percent.
type LightParams struct {
	Exposure   float64 `json:"exposure"`
	Contrast   float64 `json:"contrast"`
	Highlights float64 `json:"highlights"`
	Shadows    float64 `json:"shadows"`
	Whites     float64 `json:"whites"`
	Blacks     float64 `json:"blacks"`
	Curve      CurveParams `json:"curve"`
}

// CurveParams holds one tone curve per channel. An empty channel means the
// identity (linear) curve.
type CurveParams struct {
	RGB   []CurvePoint `json:"rgb,omitempty"`
	Red   []CurvePoint `json:"red,omitempty"`
	Green []CurvePoint `json:"green,omitempty"`
	Blue  []CurvePoint `json:"blue,omitempty"`
}

// CurvePoint is a control point of a tone curve, in the 0..1 range.
type CurvePoint struct {
	X float64 `json:"x"`
	Y float64 `json:"y"`
}

// EditParams is the full set of non-destructive editing parameters of a photo.
type EditParams struct {
	Light LightParams `json:"light"`
}

// Limits of the editing parameters. They are enforced on write so a hand-made
// request can never push the renderer into an extreme state.
const (
	MaxExposureStops = 5.0
	MaxPercent       = 100.0
	MaxCurvePoints   = 16
)

// ErrInvalidEditParams is returned when the client sends unusable parameters.
var ErrInvalidEditParams = errors.New("invalid edit params")

// ParseEditParams decodes and normalizes the editing parameters sent by a
// client. Values outside the allowed range are clamped, and the curve points
// are sorted and de-duplicated.
func ParseEditParams(raw []byte) (EditParams, error) {
	var p EditParams
	if len(raw) == 0 {
		return p, nil
	}
	if err := json.Unmarshal(raw, &p); err != nil {
		return EditParams{}, ErrInvalidEditParams
	}
	p = p.Normalized()
	return p, nil
}

// Normalized returns a copy with every value clamped to the allowed range.
func (p EditParams) Normalized() EditParams {
	p.Light.Exposure = clampFloat(p.Light.Exposure, -MaxExposureStops, MaxExposureStops)
	p.Light.Contrast = clampFloat(p.Light.Contrast, -MaxPercent, MaxPercent)
	p.Light.Highlights = clampFloat(p.Light.Highlights, -MaxPercent, MaxPercent)
	p.Light.Shadows = clampFloat(p.Light.Shadows, -MaxPercent, MaxPercent)
	p.Light.Whites = clampFloat(p.Light.Whites, -MaxPercent, MaxPercent)
	p.Light.Blacks = clampFloat(p.Light.Blacks, -MaxPercent, MaxPercent)
	p.Light.Curve.RGB = normalizeCurve(p.Light.Curve.RGB)
	p.Light.Curve.Red = normalizeCurve(p.Light.Curve.Red)
	p.Light.Curve.Green = normalizeCurve(p.Light.Curve.Green)
	p.Light.Curve.Blue = normalizeCurve(p.Light.Curve.Blue)
	return p
}

// IsEmpty reports whether the parameters describe an untouched photo.
func (p EditParams) IsEmpty() bool {
	l := p.Light
	if l.Exposure != 0 || l.Contrast != 0 || l.Highlights != 0 || l.Shadows != 0 ||
		l.Whites != 0 || l.Blacks != 0 {
		return false
	}
	for _, pts := range [][]CurvePoint{l.Curve.RGB, l.Curve.Red, l.Curve.Green, l.Curve.Blue} {
		for _, pt := range pts {
			if math.Abs(pt.X-pt.Y) > 1e-6 {
				return false
			}
		}
	}
	return true
}

// JSON encodes the parameters for storage in the photo metadata.
func (p EditParams) JSON() []byte {
	if p.IsEmpty() {
		return []byte("{}")
	}
	data, err := json.Marshal(p)
	if err != nil {
		return []byte("{}")
	}
	return data
}

// normalizeCurve clamps the points to the unit square, keeps them sorted by X
// and drops duplicates.
func normalizeCurve(points []CurvePoint) []CurvePoint {
	if len(points) == 0 {
		return nil
	}
	out := make([]CurvePoint, 0, len(points))
	for _, pt := range points {
		out = append(out, CurvePoint{
			X: clampFloat(pt.X, 0, 1),
			Y: clampFloat(pt.Y, 0, 1),
		})
	}
	sort.SliceStable(out, func(i, j int) bool { return out[i].X < out[j].X })
	deduped := out[:0]
	for i, pt := range out {
		if i > 0 && math.Abs(pt.X-deduped[len(deduped)-1].X) < 1e-6 {
			continue
		}
		deduped = append(deduped, pt)
	}
	if len(deduped) > MaxCurvePoints {
		deduped = deduped[:MaxCurvePoints]
	}
	return deduped
}

func clampFloat(v, min, max float64) float64 {
	if math.IsNaN(v) {
		return 0
	}
	if v < min {
		return min
	}
	if v > max {
		return max
	}
	return v
}

func clamp01(v float64) float64 {
	return clampFloat(v, 0, 1)
}

func clampInt(v, min, max int) int {
	if v < min {
		return min
	}
	if v > max {
		return max
	}
	return v
}

// curveLUT builds a 256-entry lookup table from the control points of a
// curve. The first and last points extend to the edges of the range, and the
// values in between are interpolated linearly.
func curveLUT(points []CurvePoint) [256]uint8 {
	var lut [256]uint8
	pts := normalizeCurve(points)
	if len(pts) < 2 {
		for i := range lut {
			lut[i] = uint8(i)
		}
		return lut
	}
	for i := 0; i < 256; i++ {
		x := float64(i) / 255
		lut[i] = uint8(math.Round(clamp01(curveEval(pts, x)) * 255))
	}
	return lut
}

// curveEval evaluates the curve at x using linear interpolation between the
// surrounding control points.
func curveEval(points []CurvePoint, x float64) float64 {
	if x <= points[0].X {
		return points[0].Y
	}
	last := points[len(points)-1]
	if x >= last.X {
		return last.Y
	}
	for i := 1; i < len(points); i++ {
		if x > points[i].X {
			continue
		}
		x0, y0 := points[i-1].X, points[i-1].Y
		x1, y1 := points[i].X, points[i].Y
		if x1-x0 < 1e-9 {
			return y1
		}
		t := (x - x0) / (x1 - x0)
		return y0 + t*(y1-y0)
	}
	return last.Y
}

// lightAdjuster holds the precomputed factors of the light adjustments. The
// factors are calculated once per render, so the per-pixel loop stays cheap.
type lightAdjuster struct {
	exposure   float64
	contrast   float64
	highlights float64
	shadows    float64
	whites     float64
	blacks     float64
}

func newLightAdjuster(p LightParams) lightAdjuster {
	return lightAdjuster{
		exposure:   math.Pow(2, p.Exposure),
		contrast:   1 + p.Contrast/100,
		highlights: p.Highlights / 100 * 80,
		shadows:    p.Shadows / 100 * 80,
		whites:     p.Whites / 100 * 80,
		blacks:     p.Blacks / 100 * 80,
	}
}

// apply maps a single channel value through the adjustments and the tone
// curves: the master curve first, the channel curve second. The order of the
// operations is fixed and mirrored by the client-side preview, so the live
// preview and the rendered result match.
func (a lightAdjuster) apply(v float64, master, channel *[256]uint8) float64 {
	// Экспозиция (стопы).
	v *= a.exposure
	// Контраст вокруг средней точки.
	v = 128 + (v-128)*a.contrast
	// Света: верхняя часть диапазона.
	w := clamp01((v - 128) / 127)
	v += a.highlights * w * w
	// Тени: нижняя часть диапазона.
	w = clamp01((128 - v) / 128)
	v += a.shadows * w * w
	// Белые: самый верх диапазона.
	w = clamp01((v - 192) / 63)
	v += a.whites * w
	// Чёрные: самый низ диапазона.
	w = clamp01((64 - v) / 64)
	v += a.blacks * w
	// Тоновая кривая: общая, затем канальная.
	idx := clampInt(int(v+0.5), 0, 255)
	v = float64(master[idx])
	idx = clampInt(int(v+0.5), 0, 255)
	return float64(channel[idx])
}

// adjustChannel applies the light adjustments to a single channel value. It
// is the per-pixel helper of the renderer.
func adjustChannel(v float64, p LightParams, master, channel *[256]uint8) float64 {
	return newLightAdjuster(p).apply(v, master, channel)
}