How to implement Custom cropping using golang - image

Question

Am looking to do custom cropping on a set of images, Instead of cropping normally, using height and width i want the flexibility of getting an output image that is cropped like a polygon or an hexagon for example, Am using the library github.com/fogleman/gg, and the built in module "image", and github.com/disintegration/imaging, but I didn't find a way to customize the cropping, i also looked for an online SaaS to do this, like imgix or imageresizer.io, but they don't seem to offer that, i know golang is the right language for this maybe i didn't look hard enough, please Help
my sample Code looks like:
var image image.Image
dc := NewContext(1000, 1000)
image = imaging.Fill(profile, 800, 750, imaging.Center, imaging.Lanczos)
// Cropping needs to happen here
dc.DrawImage(image, 123, 250)

Answer 1

A bit longer than expected but here you have PNG image cropping with transparent background to a rectangle. You can modify the code for different shapes by changing the getPixAlpha function.
Just add the package name and it should include the imports, then add an image test.png and it should create a test-output.png
Note: You may want to make some minor modifications for using it as a service.
type Pixel struct {
R int
G int
B int
A int
}
func LogPanic(err error, msg string) {
if err != nil {
log.Printf("ERROR: %v %s", err, msg)
panic(err)
}
}
func getPixAlpha(x, y, halfWidth int) int {
if x < halfWidth-y || x > halfWidth+y {
return 0
}
if y > halfWidth+x {
return 0
}
if x > halfWidth*3-y && y > halfWidth*3-x {
return 0
}
return int(255)
}
func getPixels(file io.Reader) ([][]Pixel, error) {
img, _, err := image.Decode(file)
LogPanic(err, "error reading image")
bounds := img.Bounds()
width, height := bounds.Max.X, bounds.Max.Y
var pixels [][]Pixel
for x := 0; x < width; x++ {
var row []Pixel
for y := 0; y < height; y++ {
row = append(row, rgbaToPixel(img.At(x, y).RGBA()))
}
pixels = append(pixels, row)
}
return pixels, nil
}
func rgbaToPixel(r uint32, g uint32, b uint32, a uint32) Pixel {
return Pixel{int(r / 257), int(g / 257), int(b / 257), int(a / 257)}
}
func getRgbaPic(pixels [][]Pixel) [][]Pixel {
dx := len(pixels)
dy := len(pixels[0])
for x := 0; x < dx; x++ {
for y := 0; y < dy; y++ {
pixels[x][y].A = getPixAlpha(x, y, len(pixels)/2)
}
}
return pixels
}
func main() {
file, err := os.Open("./test.png")
LogPanic(err, "Error opening file")
defer file.Close()
pixels, err := getPixels(file)
LogPanic(err, "Error reading image")
pixels = getRgbaPic(pixels)
img := image.NewRGBA(image.Rect(0, 0, len(pixels), len(pixels[0])))
for x := 0; x < len(pixels); x++ {
for y := 0; y < len(pixels[0]); y++ {
img.Set(x, y, color.RGBA{
uint8(pixels[x][y].R),
uint8(pixels[x][y].G),
uint8(pixels[x][y].B),
uint8(pixels[x][y].A),
})
}
}
buf := &bytes.Buffer{}
err = png.Encode(buf, img)
LogPanic(err, "Error encoding")
err = ioutil.WriteFile("test-output.png", buf.Bytes(), 0666)
LogPanic(err, "Error writing file")
}