Rotate with offset/anchor/origin about imaging HOT 6 CLOSED

package main

import (
	"fmt"
	"image"
	"log"
	"math"

	"github.com/disintegration/imaging"
	"golang.org/x/image/draw"
	"golang.org/x/image/math/f64"
)

func main() {
	src, err := imaging.Open("test.png")
	if err != nil {
		log.Fatal(err)
	}
	for a := float64(0); a < 360; a += 10 {
		dst := rotateAround(src, a, 20, 20)
		err := imaging.Save(dst, fmt.Sprintf("rotated-%03d.png", int(a)))
		if err != nil {
			log.Fatal(err)
		}
	}
}

func rotateAround(src image.Image, angle float64, x, y int) image.Image {
	a := math.Pi * angle / 180
	xf, yf := float64(x), float64(y)
	sin := math.Sin(a)
	cos := math.Cos(a)
	matrix := f64.Aff3{
		cos, -sin, xf - xf*cos + yf*sin,
		sin, cos, yf - xf*sin - yf*cos,
	}
	dst := image.NewRGBA(src.Bounds())
	draw.BiLinear.Transform(dst, matrix, src, src.Bounds(), draw.Src, nil)
	return dst
}

package main

import (
	"fmt"
	"image"
	"log"
	"math"

	"github.com/disintegration/imaging"
	"golang.org/x/image/draw"
	"golang.org/x/image/math/f64"
)

func main() {
	src, err := imaging.Open("test.png")
	if err != nil {
		log.Fatal(err)
	}
	for a := float64(0); a < 360; a += 10 {
		dst := rotateAround(src, a, 20, 20)
		fmt.Printf("angle=%03.0f, bounds=%v\n", a, dst.Bounds())
		err := imaging.Save(dst, fmt.Sprintf("rotated-%03.0f.png", a))
		if err != nil {
			log.Fatal(err)
		}
	}
}

func rotateAround(src image.Image, angle float64, x, y int) image.Image {
	xf, yf := float64(x), float64(y)
	sin, cos := math.Sincos(math.Pi * angle / 180)
	aff := f64.Aff3{
		cos, -sin, xf - xf*cos + yf*sin,
		sin, cos, yf - xf*sin - yf*cos,
	}
	dst := image.NewRGBA(calcBounds(aff, src.Bounds()))
	draw.CatmullRom.Transform(dst, aff, src, src.Bounds(), draw.Src, nil)
	return dst
}

func calcBounds(aff f64.Aff3, b image.Rectangle) image.Rectangle {
	points := [...]image.Point{
		{b.Min.X, b.Min.Y},
		{b.Max.X - 1, b.Min.Y},
		{b.Min.X, b.Max.Y - 1},
		{b.Max.X - 1, b.Max.Y - 1},
	}
	var min, max image.Point
	for i, p := range points {
		x0 := float64(p.X) + 0.5
		y0 := float64(p.Y) + 0.5
		x := aff[0]*x0 + aff[1]*y0 + aff[2]
		y := aff[3]*x0 + aff[4]*y0 + aff[5]
		pmin := image.Point{int(math.Floor(x)), int(math.Floor(y))}
		pmax := image.Point{int(math.Ceil(x)), int(math.Ceil(y))}
		if i == 0 {
			min = pmin
			max = pmax
			continue
		}
		if min.X > pmin.X {
			min.X = pmin.X
		}
		if min.Y > pmin.Y {
			min.Y = pmin.Y
		}
		if max.X < pmax.X {
			max.X = pmax.X
		}
		if max.Y < pmax.Y {
			max.Y = pmax.Y
		}

	}
	return image.Rectangle{Min: min, Max: max}
}