osdb-1-claude-opusplan/backend/internal/imaging/crop.go

package imaging

import (
	"bytes"
	"image"
	"image/color"
	"image/draw"
	"image/jpeg"
	_ "image/png"
)

const (
	bgThreshold = 10
	padding     = 5
	maxDim      = 300
)

// CropToContent auto-crops src to the bounding box of non-background pixels,
// adds padding, scales to fit within maxDim×maxDim, and re-encodes as JPEG.
// Returns the original bytes unchanged if decoding fails.
func CropToContent(src []byte) ([]byte, error) {
	img, _, err := image.Decode(bytes.NewReader(src))
	if err != nil {
		return src, nil
	}

	bounds := img.Bounds()
	if bounds.Dx() == 0 || bounds.Dy() == 0 {
		return src, nil
	}

	bgColor := img.At(bounds.Min.X, bounds.Min.Y)

	minX, minY, maxX, maxY := findContentBounds(img, bgColor, bounds)
	if minX > maxX || minY > maxY {
		return src, nil
	}

	minX = max(bounds.Min.X, minX-padding)
	minY = max(bounds.Min.Y, minY-padding)
	maxX = min(bounds.Max.X-1, maxX+padding)
	maxY = min(bounds.Max.Y-1, maxY+padding)

	cropRect := image.Rect(minX, minY, maxX+1, maxY+1)
	cropped := cropImage(img, cropRect)
	scaled := scaleDown(cropped)

	var buf bytes.Buffer
	if err := jpeg.Encode(&buf, scaled, &jpeg.Options{Quality: 85}); err != nil {
		return src, nil
	}
	return buf.Bytes(), nil
}

func findContentBounds(img image.Image, bg color.Color, bounds image.Rectangle) (minX, minY, maxX, maxY int) {
	minX = bounds.Max.X
	minY = bounds.Max.Y
	maxX = bounds.Min.X - 1
	maxY = bounds.Min.Y - 1

	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
		for x := bounds.Min.X; x < bounds.Max.X; x++ {
			if channelDiff(img.At(x, y), bg) > bgThreshold {
				if x < minX {
					minX = x
				}
				if y < minY {
					minY = y
				}
				if x > maxX {
					maxX = x
				}
				if y > maxY {
					maxY = y
				}
			}
		}
	}
	return
}

func channelDiff(c1, c2 color.Color) int {
	r1, g1, b1, _ := c1.RGBA()
	r2, g2, b2, _ := c2.RGBA()
	dr := abs(int(r1>>8) - int(r2>>8))
	dg := abs(int(g1>>8) - int(g2>>8))
	db := abs(int(b1>>8) - int(b2>>8))
	return max(dr, max(dg, db))
}

func abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}

type subImager interface {
	SubImage(r image.Rectangle) image.Image
}

func cropImage(img image.Image, rect image.Rectangle) image.Image {
	if si, ok := img.(subImager); ok {
		return si.SubImage(rect)
	}
	dst := image.NewRGBA(image.Rect(0, 0, rect.Dx(), rect.Dy()))
	draw.Draw(dst, dst.Bounds(), img, rect.Min, draw.Src)
	return dst
}

func scaleDown(img image.Image) image.Image {
	bounds := img.Bounds()
	w, h := bounds.Dx(), bounds.Dy()
	if w <= maxDim && h <= maxDim {
		return img
	}

	scaleW := float64(maxDim) / float64(w)
	scaleH := float64(maxDim) / float64(h)
	scale := scaleW
	if scaleH < scale {
		scale = scaleH
	}

	newW := max(1, int(float64(w)*scale))
	newH := max(1, int(float64(h)*scale))

	dst := image.NewRGBA(image.Rect(0, 0, newW, newH))
	for y := 0; y < newH; y++ {
		for x := 0; x < newW; x++ {
			srcX := bounds.Min.X + int(float64(x)/scale)
			srcY := bounds.Min.Y + int(float64(y)/scale)
			dst.Set(x, y, img.At(srcX, srcY))
		}
	}
	return dst
}