gotosocial/internal/media/imaging.go

// GoToSocial
// Copyright (C) GoToSocial Authors admin@gotosocial.org
// SPDX-License-Identifier: AGPL-3.0-or-later
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package media

import (
	"image"
	"image/color"
	"math"

	"code.superseriousbusiness.org/gotosocial/internal/gtserror"
)

// NOTE:
// the following code is borrowed from
// github.com/disintegration/imaging
// and collapses in some places for our
// particular usecases and with parallel()
// function (spans work across goroutines)
// removed, instead working synchronously.
//
// at gotosocial we take particular
// care about where we spawn goroutines
// to ensure we're in control of the
// amount of concurrency in relation
// to the amount configured by user.

// resizeDownLinear resizes image to given width x height using linear resampling.
// This is specifically optimized for resizing down (i.e. smaller), else is noop.
func resizeDownLinear(img image.Image, width, height int) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	if srcW <= 0 || srcH <= 0 ||
		width < 0 || height < 0 {
		return &image.NRGBA{}
	}

	if width == 0 {
		// If no width is given, use aspect preserving width.
		tmp := float64(height) * float64(srcW) / float64(srcH)
		width = int(math.Max(1.0, math.Floor(tmp+0.5)))
	}

	if height == 0 {
		// If no height is given, use aspect preserving height.
		tmp := float64(width) * float64(srcH) / float64(srcW)
		height = int(math.Max(1.0, math.Floor(tmp+0.5)))
	}

	if width < srcW {
		// Width is smaller, resize horizontally.
		img = resizeHorizontalLinear(img, width)
	}

	if height < srcH {
		// Height is smaller, resize vertically.
		img = resizeVerticalLinear(img, height)
	}

	return img
}

// flipH flips the image horizontally (left to right).
func flipH(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcW
	dstH := srcH
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcY := y
		scanImage(img, 0, srcY, srcW, srcY+1, dst.Pix[i:i+rowSize])
		reverse(dst.Pix[i : i+rowSize])
	}
	return dst
}

// flipV flips the image vertically (from top to bottom).
func flipV(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcW
	dstH := srcH
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcY := dstH - y - 1
		scanImage(img, 0, srcY, srcW, srcY+1, dst.Pix[i:i+rowSize])
	}
	return dst
}

// rotate90 rotates the image 90 counter-clockwise.
func rotate90(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcH
	dstH := srcW
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcX := dstH - y - 1
		scanImage(img, srcX, 0, srcX+1, srcH, dst.Pix[i:i+rowSize])
	}
	return dst
}

// rotate180 rotates the image 180 counter-clockwise.
func rotate180(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcW
	dstH := srcH
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcY := dstH - y - 1
		scanImage(img, 0, srcY, srcW, srcY+1, dst.Pix[i:i+rowSize])
		reverse(dst.Pix[i : i+rowSize])
	}
	return dst
}

// rotate270 rotates the image 270 counter-clockwise.
func rotate270(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcH
	dstH := srcW
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcX := y
		scanImage(img, srcX, 0, srcX+1, srcH, dst.Pix[i:i+rowSize])
		reverse(dst.Pix[i : i+rowSize])
	}
	return dst
}

// transpose flips the image horizontally and rotates 90 counter-clockwise.
func transpose(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcH
	dstH := srcW
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcX := y
		scanImage(img, srcX, 0, srcX+1, srcH, dst.Pix[i:i+rowSize])
	}
	return dst
}

// transverse flips the image vertically and rotates 90 counter-clockwise.
func transverse(img image.Image) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dstW := srcH
	dstH := srcW
	rowSize := dstW * 4
	dst := image.NewNRGBA(image.Rect(0, 0, dstW, dstH))
	for y := 0; y < dstH; y++ {
		i := y * dst.Stride
		srcX := dstH - y - 1
		scanImage(img, srcX, 0, srcX+1, srcH, dst.Pix[i:i+rowSize])
		reverse(dst.Pix[i : i+rowSize])
	}
	return dst
}

// resizeHorizontalLinear resizes image to given width using linear resampling.
func resizeHorizontalLinear(img image.Image, dstWidth int) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dst := image.NewRGBA(image.Rect(0, 0, dstWidth, srcH))
	weights := precomputeWeightsLinear(dstWidth, srcW)
	scanLine := make([]uint8, srcW*4)
	for y := 0; y < srcH; y++ {
		scanImage(img, 0, y, srcW, y+1, scanLine)
		j0 := y * dst.Stride
		for x := range weights {
			var r, g, b, a float64
			for _, w := range weights[x] {
				i := w.index * 4
				s := scanLine[i : i+4 : i+4]
				aw := float64(s[3]) * w.weight
				r += float64(s[0]) * aw
				g += float64(s[1]) * aw
				b += float64(s[2]) * aw
				a += aw
			}
			if a != 0 {
				j := j0 + x*4
				d := dst.Pix[j : j+4 : j+4]
				d[0] = clampFloatTo8(r / a)
				d[1] = clampFloatTo8(g / a)
				d[2] = clampFloatTo8(b / a)
				d[3] = clampFloatTo8(a)
			}
		}
	}
	return dst
}

// resizeVerticalLinear resizes image to given height using linear resampling.
func resizeVerticalLinear(img image.Image, height int) image.Image {
	srcW, srcH := img.Bounds().Dx(), img.Bounds().Dy()
	dst := image.NewNRGBA(image.Rect(0, 0, srcW, height))
	weights := precomputeWeightsLinear(height, srcH)
	scanLine := make([]uint8, srcH*4)
	for x := 0; x < srcW; x++ {
		scanImage(img, x, 0, x+1, srcH, scanLine)
		for y := range weights {
			var r, g, b, a float64
			for _, w := range weights[y] {
				i := w.index * 4
				s := scanLine[i : i+4 : i+4]
				aw := float64(s[3]) * w.weight
				r += float64(s[0]) * aw
				g += float64(s[1]) * aw
				b += float64(s[2]) * aw
				a += aw
			}
			if a != 0 {
				j := y*dst.Stride + x*4
				d := dst.Pix[j : j+4 : j+4]
				d[0] = clampFloatTo8(r / a)
				d[1] = clampFloatTo8(g / a)
				d[2] = clampFloatTo8(b / a)
				d[3] = clampFloatTo8(a)
			}
		}
	}
	return dst
}

type indexWeight struct {
	index  int
	weight float64
}

func precomputeWeightsLinear(dstSize, srcSize int) [][]indexWeight {
	du := float64(srcSize) / float64(dstSize)
	scale := du
	if scale < 1.0 {
		scale = 1.0
	}

	ru := math.Ceil(scale)
	out := make([][]indexWeight, dstSize)
	tmp := make([]indexWeight, 0, dstSize*int(ru+2)*2)

	for v := 0; v < len(out); v++ {
		fu := (float64(v)+0.5)*du - 0.5

		begin := int(math.Ceil(fu - ru))
		if begin < 0 {
			begin = 0
		}

		end := int(math.Floor(fu + ru))
		if end > srcSize-1 {
			end = srcSize - 1
		}

		var sum float64
		for u := begin; u <= end; u++ {
			w := resampleLinear((float64(u) - fu) / scale)
			if w != 0 {
				sum += w
				tmp = append(tmp, indexWeight{
					index:  u,
					weight: w,
				})
			}
		}

		if sum != 0 {
			for i := range tmp {
				tmp[i].weight /= sum
			}
		}

		out[v] = tmp
		tmp = tmp[len(tmp):]
	}

	return out
}

// resampleLinear is the resample kernel func for linear filtering.
func resampleLinear(x float64) float64 {
	x = math.Abs(x)
	if x < 1.0 {
		return 1.0 - x
	}
	return 0
}

// scan scans the given rectangular region of the image into dst.
func scanImage(img image.Image, x1, y1, x2, y2 int, dst []uint8) {
	switch img := img.(type) {
	case *image.NRGBA:
		scanNRGBA(img, x1, y1, x2, y2, dst)
	case *image.NRGBA64:
		scanNRGBA64(img, x1, y1, x2, y2, dst)
	case *image.RGBA:
		scanRGBA(img, x1, y1, x2, y2, dst)
	case *image.RGBA64:
		scanRGBA64(img, x1, y1, x2, y2, dst)
	case *image.Gray:
		scanGray(img, x1, y1, x2, y2, dst)
	case *image.Gray16:
		scanGray16(img, x1, y1, x2, y2, dst)
	case *image.YCbCr:
		scanYCbCr(img, x1, y1, x2, y2, dst)
	case *image.Paletted:
		scanPaletted(img, x1, y1, x2, y2, dst)
	default:
		scanAny(img, x1, y1, x2, y2, dst)
	}
}

func scanNRGBA(img *image.NRGBA, x1, y1, x2, y2 int, dst []uint8) {
	size := (x2 - x1) * 4
	j := 0
	i := y1*img.Stride + x1*4
	if size == 4 {
		for y := y1; y < y2; y++ {
			d := dst[j : j+4 : j+4]
			s := img.Pix[i : i+4 : i+4]
			d[0] = s[0]
			d[1] = s[1]
			d[2] = s[2]
			d[3] = s[3]
			j += size
			i += img.Stride
		}
	} else {
		for y := y1; y < y2; y++ {
			copy(dst[j:j+size], img.Pix[i:i+size])
			j += size
			i += img.Stride
		}
	}
}

func scanNRGBA64(img *image.NRGBA64, x1, y1, x2, y2 int, dst []uint8) {
	if img == nil {
		panic(gtserror.New("nil check elimination"))
	}
	j := 0
	for y := y1; y < y2; y++ {
		i := y*img.Stride + x1*8
		for x := x1; x < x2; x++ {
			s := img.Pix[i : i+8 : i+8]
			d := dst[j : j+4 : j+4]
			d[0] = s[0]
			d[1] = s[2]
			d[2] = s[4]
			d[3] = s[6]
			j += 4
			i += 8
		}
	}
}

func scanRGBA(img *image.RGBA, x1, y1, x2, y2 int, dst []uint8) {
	if img == nil {
		panic(gtserror.New("nil check elimination"))
	}
	j := 0
	for y := y1; y < y2; y++ {
		i := y*img.Stride + x1*4
		for x := x1; x < x2; x++ {
			d := dst[j : j+4 : j+4]
			a := img.Pix[i+3]
			switch a {
			case 0:
				d[0] = 0
				d[1] = 0
				d[2] = 0
				d[3] = a
			case 0xff:
				s := img.Pix[i : i+4 : i+4]
				d[0] = s[0]
				d[1] = s[1]
				d[2] = s[2]
				d[3] = a
			default:
				s := img.Pix[i : i+4 : i+4]
				r16 := uint16(s[0])
				g16 := uint16(s[1])
				b16 := uint16(s[2])
				a16 := uint16(a)
				d[0] = uint8(r16 * 0xff / a16) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g16 * 0xff / a16) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b16 * 0xff / a16) // #nosec G115 -- Overflow desired.
				d[3] = a
			}
			j += 4
			i += 4
		}
	}
}

func scanRGBA64(img *image.RGBA64, x1, y1, x2, y2 int, dst []uint8) {
	if img == nil {
		panic(gtserror.New("nil check elimination"))
	}
	j := 0
	for y := y1; y < y2; y++ {
		i := y*img.Stride + x1*8
		for x := x1; x < x2; x++ {
			s := img.Pix[i : i+8 : i+8]
			d := dst[j : j+4 : j+4]
			a := s[6]
			switch a {
			case 0:
				d[0] = 0
				d[1] = 0
				d[2] = 0
			case 0xff:
				d[0] = s[0]
				d[1] = s[2]
				d[2] = s[4]
			default:
				r32 := uint32(s[0])<<8 | uint32(s[1])
				g32 := uint32(s[2])<<8 | uint32(s[3])
				b32 := uint32(s[4])<<8 | uint32(s[5])
				a32 := uint32(s[6])<<8 | uint32(s[7])
				d[0] = uint8((r32 * 0xffff / a32) >> 8) // #nosec G115 -- Overflow desired.
				d[1] = uint8((g32 * 0xffff / a32) >> 8) // #nosec G115 -- Overflow desired.
				d[2] = uint8((b32 * 0xffff / a32) >> 8) // #nosec G115 -- Overflow desired.
			}
			d[3] = a
			j += 4
			i += 8
		}
	}
}

func scanGray(img *image.Gray, x1, y1, x2, y2 int, dst []uint8) {
	if img == nil {
		panic(gtserror.New("nil check elimination"))
	}
	j := 0
	for y := y1; y < y2; y++ {
		i := y*img.Stride + x1
		for x := x1; x < x2; x++ {
			c := img.Pix[i]
			d := dst[j : j+4 : j+4]
			d[0] = c
			d[1] = c
			d[2] = c
			d[3] = 0xff
			j += 4
			i++
		}
	}
}

func scanGray16(img *image.Gray16, x1, y1, x2, y2 int, dst []uint8) {
	if img == nil {
		panic(gtserror.New("nil check elimination"))
	}
	j := 0
	for y := y1; y < y2; y++ {
		i := y*img.Stride + x1*2
		for x := x1; x < x2; x++ {
			c := img.Pix[i]
			d := dst[j : j+4 : j+4]
			d[0] = c
			d[1] = c
			d[2] = c
			d[3] = 0xff
			j += 4
			i += 2
		}
	}
}

func scanYCbCr(img *image.YCbCr, x1, y1, x2, y2 int, dst []uint8) {
	j := 0

	x1 += img.Rect.Min.X
	x2 += img.Rect.Min.X
	y1 += img.Rect.Min.Y
	y2 += img.Rect.Min.Y

	hy := img.Rect.Min.Y / 2
	hx := img.Rect.Min.X / 2

	switch img.SubsampleRatio {
	case image.YCbCrSubsampleRatio420:
		for y := y1; y < y2; y++ {
			iy := (y-img.Rect.Min.Y)*img.YStride + (x1 - img.Rect.Min.X)

			yBase := (y/2 - hy) * img.CStride

			for x := x1; x < x2; x++ {
				ic := yBase + (x/2 - hx)

				yy1 := int32(img.Y[iy]) * 0x10101
				cb1 := int32(img.Cb[ic]) - 128
				cr1 := int32(img.Cr[ic]) - 128

				r := yy1 + 91881*cr1
				if uint32(r)&0xff000000 == 0 { //nolint:gosec
					r >>= 16
				} else {
					r = ^(r >> 31)
				}

				g := yy1 - 22554*cb1 - 46802*cr1
				if uint32(g)&0xff000000 == 0 { //nolint:gosec
					g >>= 16
				} else {
					g = ^(g >> 31)
				}

				b := yy1 + 116130*cb1
				if uint32(b)&0xff000000 == 0 { //nolint:gosec
					b >>= 16
				} else {
					b = ^(b >> 31)
				}

				d := dst[j : j+4 : j+4]
				d[0] = uint8(r) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b) // #nosec G115 -- Overflow desired.
				d[3] = 0xff

				iy++
				j += 4
			}
		}

	case image.YCbCrSubsampleRatio422:
		for y := y1; y < y2; y++ {
			iy := (y-img.Rect.Min.Y)*img.YStride + (x1 - img.Rect.Min.X)

			yBase := (y - img.Rect.Min.Y) * img.CStride

			for x := x1; x < x2; x++ {
				ic := yBase + (x/2 - hx)

				yy1 := int32(img.Y[iy]) * 0x10101
				cb1 := int32(img.Cb[ic]) - 128
				cr1 := int32(img.Cr[ic]) - 128

				r := yy1 + 91881*cr1
				if uint32(r)&0xff000000 == 0 { //nolint:gosec
					r >>= 16
				} else {
					r = ^(r >> 31)
				}

				g := yy1 - 22554*cb1 - 46802*cr1
				if uint32(g)&0xff000000 == 0 { //nolint:gosec
					g >>= 16
				} else {
					g = ^(g >> 31)
				}

				b := yy1 + 116130*cb1
				if uint32(b)&0xff000000 == 0 { //nolint:gosec
					b >>= 16
				} else {
					b = ^(b >> 31)
				}

				d := dst[j : j+4 : j+4]
				d[0] = uint8(r) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b) // #nosec G115 -- Overflow desired.
				d[3] = 0xff

				iy++
				j += 4
			}
		}

	case image.YCbCrSubsampleRatio440:
		for y := y1; y < y2; y++ {
			iy := (y-img.Rect.Min.Y)*img.YStride + (x1 - img.Rect.Min.X)

			yBase := (y/2 - hy) * img.CStride

			for x := x1; x < x2; x++ {
				ic := yBase + (x - img.Rect.Min.X)

				yy1 := int32(img.Y[iy]) * 0x10101
				cb1 := int32(img.Cb[ic]) - 128
				cr1 := int32(img.Cr[ic]) - 128

				r := yy1 + 91881*cr1
				if uint32(r)&0xff000000 == 0 { //nolint:gosec
					r >>= 16
				} else {
					r = ^(r >> 31)
				}

				g := yy1 - 22554*cb1 - 46802*cr1
				if uint32(g)&0xff000000 == 0 { //nolint:gosec
					g >>= 16
				} else {
					g = ^(g >> 31)
				}

				b := yy1 + 116130*cb1
				if uint32(b)&0xff000000 == 0 { //nolint:gosec
					b >>= 16
				} else {
					b = ^(b >> 31)
				}

				d := dst[j : j+4 : j+4]
				d[0] = uint8(r) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b) // #nosec G115 -- Overflow desired.
				d[3] = 0xff

				iy++
				j += 4
			}
		}

	case image.YCbCrSubsampleRatio444:
		for y := y1; y < y2; y++ {
			iy := (y-img.Rect.Min.Y)*img.YStride + (x1 - img.Rect.Min.X)

			yBase := (y - img.Rect.Min.Y) * img.CStride

			for x := x1; x < x2; x++ {
				ic := yBase + (x - img.Rect.Min.X)

				yy1 := int32(img.Y[iy]) * 0x10101
				cb1 := int32(img.Cb[ic]) - 128
				cr1 := int32(img.Cr[ic]) - 128

				r := yy1 + 91881*cr1
				if uint32(r)&0xff000000 == 0 { //nolint:gosec
					r >>= 16
				} else {
					r = ^(r >> 31)
				}

				g := yy1 - 22554*cb1 - 46802*cr1
				if uint32(g)&0xff000000 == 0 { //nolint:gosec
					g >>= 16
				} else {
					g = ^(g >> 31)
				}

				b := yy1 + 116130*cb1
				if uint32(b)&0xff000000 == 0 { //nolint:gosec
					b >>= 16
				} else {
					b = ^(b >> 31)
				}

				d := dst[j : j+4 : j+4]
				d[0] = uint8(r) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b) // #nosec G115 -- Overflow desired.
				d[3] = 0xff

				iy++
				j += 4
			}
		}

	default:
		for y := y1; y < y2; y++ {
			iy := (y-img.Rect.Min.Y)*img.YStride + (x1 - img.Rect.Min.X)
			for x := x1; x < x2; x++ {
				ic := img.COffset(x, y)

				yy1 := int32(img.Y[iy]) * 0x10101
				cb1 := int32(img.Cb[ic]) - 128
				cr1 := int32(img.Cr[ic]) - 128

				r := yy1 + 91881*cr1
				if uint32(r)&0xff000000 == 0 { //nolint:gosec
					r >>= 16
				} else {
					r = ^(r >> 31)
				}

				g := yy1 - 22554*cb1 - 46802*cr1
				if uint32(g)&0xff000000 == 0 { //nolint:gosec
					g >>= 16
				} else {
					g = ^(g >> 31)
				}

				b := yy1 + 116130*cb1
				if uint32(b)&0xff000000 == 0 { //nolint:gosec
					b >>= 16
				} else {
					b = ^(b >> 31)
				}

				d := dst[j : j+4 : j+4]
				d[0] = uint8(r) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b) // #nosec G115 -- Overflow desired.
				d[3] = 0xff

				iy++
				j += 4
			}
		}
	}
}

func scanPaletted(img *image.Paletted, x1, y1, x2, y2 int, dst []uint8) {
	var palette [256]color.NRGBA
	if len(palette) < len(img.Palette) {
		panic(gtserror.New("bound check elimination"))
	}
	for i := 0; i < len(img.Palette); i++ {
		palette[i] = colorToNRGBA(img.Palette[i])
	}
	j := 0
	for y := y1; y < y2; y++ {
		i := y*img.Stride + x1
		for x := x1; x < x2; x++ {
			c := palette[img.Pix[i]]
			d := dst[j : j+4 : j+4]
			d[0] = c.R
			d[1] = c.G
			d[2] = c.B
			d[3] = c.A
			j += 4
			i++
		}
	}
}

// inlined from: image/color.NRGBAModel.Convert()
func colorToNRGBA(c color.Color) color.NRGBA {
	if c, ok := c.(color.NRGBA); ok {
		return c
	}
	r, g, b, a := c.RGBA()
	if a == 0xffff {
		return color.NRGBA{
			uint8(r >> 8), // #nosec G115 -- from stdlib
			uint8(g >> 8), // #nosec G115 -- from stdlib
			uint8(b >> 8), // #nosec G115 -- from stdlib
			0xff,
		}
	}
	if a == 0 {
		return color.NRGBA{
			0,
			0,
			0,
			0,
		}
	}
	// Since Color.RGBA returns an alpha-premultiplied color,
	// we should have r <= a && g <= a && b <= a.
	r = (r * 0xffff) / a
	g = (g * 0xffff) / a
	b = (b * 0xffff) / a
	return color.NRGBA{
		uint8(r >> 8), // #nosec G115 -- from stdlib
		uint8(g >> 8), // #nosec G115 -- from stdlib
		uint8(b >> 8), // #nosec G115 -- from stdlib
		uint8(a >> 8), // #nosec G115 -- from stdlib
	}
}

func scanAny(img image.Image, x1, y1, x2, y2 int, dst []uint8) {
	j := 0
	b := img.Bounds()
	x1 += b.Min.X
	x2 += b.Min.X
	y1 += b.Min.Y
	y2 += b.Min.Y
	for y := y1; y < y2; y++ {
		for x := x1; x < x2; x++ {
			r16, g16, b16, a16 := img.At(x, y).RGBA()
			d := dst[j : j+4 : j+4]
			switch a16 {
			case 0xffff:
				d[0] = uint8(r16 >> 8) // #nosec G115 -- Overflow desired.
				d[1] = uint8(g16 >> 8) // #nosec G115 -- Overflow desired.
				d[2] = uint8(b16 >> 8) // #nosec G115 -- Overflow desired.
				d[3] = 0xff
			case 0:
				d[0] = 0
				d[1] = 0
				d[2] = 0
				d[3] = 0
			default:
				d[0] = uint8(((r16 * 0xffff) / a16) >> 8) // #nosec G115 -- Overflow desired.
				d[1] = uint8(((g16 * 0xffff) / a16) >> 8) // #nosec G115 -- Overflow desired.
				d[2] = uint8(((b16 * 0xffff) / a16) >> 8) // #nosec G115 -- Overflow desired.
				d[3] = uint8(a16 >> 8)                    // #nosec G115 -- Overflow desired.
			}
			j += 4
		}
	}
}

// reverse reverses the data
// in contained pixel slice.
func reverse(pix8 []uint8) {
	if len(pix8) <= 4 || len(pix8)%4 != 0 {
		return
	}
	for i, j := 0, len(pix8)-4; i < j; i, j = i+4, j-4 {
		di := pix8[i : i+4 : i+4]
		dj := pix8[j : j+4 : j+4]
		di[0], dj[0] = dj[0], di[0]
		di[1], dj[1] = dj[1], di[1]
		di[2], dj[2] = dj[2], di[2]
		di[3], dj[3] = dj[3], di[3]
	}
}

// clampFloatTo8 rounds and clamps
// float64 value to fit into uint8.
func clampFloatTo8(x float64) uint8 {
	v := int64(x + 0.5)
	if v > 255 {
		return 255
	}
	if v > 0 {
		return uint8(v) // #nosec G115 -- Just checked.
	}
	return 0
}