gen2d/backend/pkg/splitsprite/splitsprite.go

// Package splitsprite provides PNG sprite sheet splitting utilities.
//
// Pipeline: green screen removal → projection-based gap detection →
// split into tiles → filter out low-fill tiles → trim transparent edges.
package splitsprite

import (
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"math"
	"sort"
)

// Options configures the sprite sheet splitting pipeline.
type Options struct {
	// GreenScreen enables green background removal.
	GreenScreen bool
	// GreenTolerance controls how aggressively green pixels are removed (0–1, default 0.2).
	GreenTolerance float64

	// GapThreshold is the max fraction of non-transparent pixels a row/column
	// can have to be considered a gap (0–1, default 0.03).
	GapThreshold float64
	// MinGapWidth is the minimum width in pixels a gap must have.
	MinGapWidth int

	// MinFillRatio is the minimum fraction of non-transparent pixels a tile
	// must have to be kept (0–1, default 0.3).
	MinFillRatio float64

	// Trim removes transparent borders from output tiles.
	Trim bool
	// OutW / OutH specify the output tile size (0 = keep original).
	OutW, OutH int
}

// DefaultOptions returns sensible default splitting options.
func DefaultOptions() *Options {
	return &Options{
		GreenScreen:    true,
		GreenTolerance: 0.2,
		GapThreshold:   0.03,
		MinGapWidth:    2,
		MinFillRatio:   0.3,
		Trim:           true,
	}
}

// Process splits a sprite sheet image into individual cleaned tile images.
// It runs the full pipeline: green screen removal → split → trim → resize.
func Process(img image.Image, opts *Options) ([]image.Image, error) {
	if opts == nil {
		opts = DefaultOptions()
	}

	src := toRGBA(img)

	if opts.GreenScreen {
		src = removeGreenScreen(src, opts.GreenTolerance)
	}

	tiles := projectionSplit(src, opts.GapThreshold, opts.MinGapWidth, opts.MinFillRatio)
	if len(tiles) == 0 {
		return nil, fmt.Errorf("no tiles detected — try lowering GapThreshold or adjusting GreenTolerance")
	}

	results := make([]image.Image, len(tiles))
	for i, t := range tiles {
		sub := image.NewRGBA(image.Rect(0, 0, t.w, t.h))
		draw.Draw(sub, sub.Bounds(), src, image.Point{t.x, t.y}, draw.Src)

		if opts.Trim {
			sub = trimAlpha(sub)
		}
		if opts.OutW > 0 && opts.OutH > 0 {
			sub = resize(sub, opts.OutW, opts.OutH)
		}
		results[i] = sub
	}
	return results, nil
}

// RemoveGreenScreen removes green-dominant background pixels, making them transparent.
func RemoveGreenScreen(img image.Image, tol float64) image.Image {
	return removeGreenScreen(toRGBA(img), tol)
}

// TrimAlpha removes fully transparent borders from an image.
func TrimAlpha(img image.Image) image.Image {
	return trimAlpha(toRGBA(img))
}

// Resize resizes an image using nearest-neighbor interpolation.
func Resize(img image.Image, w, h int) image.Image {
	return resize(toRGBA(img), w, h)
}

// ============================
// internal
// ============================

type tile struct {
	x, y, w, h int
}

func removeGreenScreen(rgba *image.RGBA, tol float64) *image.RGBA {
	b := rgba.Bounds()
	dst := image.NewRGBA(b)
	absTol := tol * 255

	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			r16, g16, bl16, a16 := rgba.At(x, y).RGBA()
			if a16 == 0 {
				continue
			}
			r, g, bl := float64(r16>>8), float64(g16>>8), float64(bl16>>8)
			gDominance := g - (r+bl)/2
			if gDominance > absTol {
				alpha := 1.0 - math.Min(gDominance/(absTol*2), 1.0)
				dst.SetRGBA(x, y, color.RGBA{
					R: uint8(r), G: uint8(g), B: uint8(bl),
					A: uint8(alpha * 255),
				})
			} else {
				dst.Set(x, y, rgba.At(x, y))
			}
		}
	}
	return dst
}

func projectionSplit(rgba *image.RGBA, gapThreshold float64, minGap int, minFill float64) []tile {
	bounds := rgba.Bounds()
	W, H := bounds.Dx(), bounds.Dy()

	rowRatio := make([]float64, H)
	colRatio := make([]float64, W)
	for y := 0; y < H; y++ {
		n := 0
		for x := 0; x < W; x++ {
			if alphaAt(rgba, x, y) > 0 {
				n++
			}
		}
		rowRatio[y] = float64(n) / float64(W)
	}
	for x := 0; x < W; x++ {
		n := 0
		for y := 0; y < H; y++ {
			if alphaAt(rgba, x, y) > 0 {
				n++
			}
		}
		colRatio[x] = float64(n) / float64(H)
	}

	rowCuts := findCuts(rowRatio, gapThreshold, minGap)
	colCuts := findCuts(colRatio, gapThreshold, minGap)

	if len(rowCuts) < 2 || len(colCuts) < 2 {
		return nil
	}

	var tiles []tile
	for ri := 0; ri < len(rowCuts)-1; ri++ {
		for ci := 0; ci < len(colCuts)-1; ci++ {
			x := colCuts[ci]
			y := rowCuts[ri]
			w := colCuts[ci+1] - x
			h := rowCuts[ri+1] - y
			if tileFillRatio(rgba, x, y, w, h) >= minFill {
				tiles = append(tiles, tile{x: x, y: y, w: w, h: h})
			}
		}
	}
	return tiles
}

func tileFillRatio(rgba *image.RGBA, x0, y0, w, h int) float64 {
	total := w * h
	if total == 0 {
		return 0
	}
	n := 0
	for y := y0; y < y0+h; y++ {
		for x := x0; x < x0+w; x++ {
			if alphaAt(rgba, x, y) > 0 {
				n++
			}
		}
	}
	return float64(n) / float64(total)
}

func findCuts(ratios []float64, threshold float64, minGap int) []int {
	n := len(ratios)
	isGap := make([]bool, n)
	for i, r := range ratios {
		isGap[i] = r < threshold
	}

	type segment struct{ start, end int }
	var gaps []segment
	i := 0
	for i < n {
		if isGap[i] {
			start := i
			for i < n && isGap[i] {
				i++
			}
			if i-start >= minGap {
				gaps = append(gaps, segment{start, i})
			}
		} else {
			i++
		}
	}

	if len(gaps) == 0 {
		return []int{0, n}
	}

	cuts := []int{0}
	for _, seg := range gaps {
		cuts = append(cuts, seg.start+(seg.end-seg.start)/2)
	}
	cuts = append(cuts, n)
	sort.Ints(cuts)

	dedup := cuts[:1]
	for j := 1; j < len(cuts); j++ {
		if cuts[j] != dedup[len(dedup)-1] {
			dedup = append(dedup, cuts[j])
		}
	}
	return dedup
}

func alphaAt(rgba *image.RGBA, x, y int) uint8 {
	return rgba.Pix[rgba.PixOffset(x, y)+3]
}

func trimAlpha(rgba *image.RGBA) *image.RGBA {
	b := rgba.Bounds()
	minX, minY := b.Max.X, b.Max.Y
	maxX, maxY := b.Min.X, b.Min.Y
	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			if alphaAt(rgba, x-b.Min.X, y-b.Min.Y) > 0 {
				if x < minX {
					minX = x
				}
				if x > maxX {
					maxX = x
				}
				if y < minY {
					minY = y
				}
				if y > maxY {
					maxY = y
				}
			}
		}
	}
	w := maxX - minX + 1
	h := maxY - minY + 1
	if w <= 0 || h <= 0 {
		return rgba
	}
	dst := image.NewRGBA(image.Rect(0, 0, w, h))
	draw.Draw(dst, dst.Bounds(), rgba, image.Point{minX, minY}, draw.Src)
	return dst
}

func resize(rgba *image.RGBA, w, h int) *image.RGBA {
	dst := image.NewRGBA(image.Rect(0, 0, w, h))
	sw, sh := rgba.Bounds().Dx(), rgba.Bounds().Dy()
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			sx := x * sw / w
			sy := y * sh / h
			dst.Set(x, y, rgba.At(sx, sy))
		}
	}
	return dst
}

func toRGBA(src image.Image) *image.RGBA {
	if rgba, ok := src.(*image.RGBA); ok {
		return rgba
	}
	b := src.Bounds()
	rgba := image.NewRGBA(b)
	draw.Draw(rgba, b, src, b.Min, draw.Src)
	return rgba
}