Cropping image

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Most of image type in image package having SubImage(r Rectangle) Image method, except image.Uniform. Based on this fact, We can implement a function to crop an arbitrary image as follows

func CropImage(img image.Image, cropRect image.Rectangle) (cropImg image.Image, newImg bool) {
    //Interface for asserting whether `img`
    //implements SubImage or not.
    //This can be defined globally.
    type CropableImage interface {
        SubImage(r image.Rectangle) image.Image

    if p, ok := img.(CropableImage); ok {
        // Call SubImage. This should be fast,
        // since SubImage (usually) shares underlying pixel.
        cropImg = p.SubImage(cropRect)
    } else if cropRect = cropRect.Intersect(img.Bounds()); !cropRect.Empty() {
        // If `img` does not implement `SubImage`,
        // copy (and silently convert) the image portion to RGBA image.
        rgbaImg := image.NewRGBA(cropRect)
        for y := cropRect.Min.Y; y < cropRect.Max.Y; y++ {
            for x := cropRect.Min.X; x < cropRect.Max.X; x++ {
                rgbaImg.Set(x, y, img.At(x, y))
        cropImg = rgbaImg
        newImg = true
    } else {
        // Return an empty RGBA image
        cropImg = &image.RGBA{}
        newImg = true

    return cropImg, newImg

Note that the cropped image may shared its underlying pixels with the original image. If this is the case, any modification to the cropped image will affect the original image.

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