Here is an example playing with BooFCV's Java API

It'll pop up a little window displaying an image

    (add-libs {'org.boofcv/boofcv-all {:mvn/version "0.35"}})
    (import 'boofcv.alg.color.ColorRgb
            'boofcv.core.image.ConvertImage
            'boofcv.gui.ListDisplayPanel
            'boofcv.gui.image.ShowImages
            'boofcv.io.UtilIO
            'boofcv.io.image.ConvertBufferedImage
            'boofcv.io.image.UtilImageIO
            'boofcv.struct.image.GrayU8
            'boofcv.struct.image.ImageType
            'boofcv.struct.image.Planar
            'java.awt.image.BufferedImage)
    (let [image-url  "https://kxygk.github.io/web/chengdu.jpg"
          color (ConvertBufferedImage/convertFrom (UtilImageIO/loadImage image-url)
                                                  true,
                                                  (ImageType/pl 3 GrayU8))
          weighted (GrayU8. (.width color)
                            (.height color))
          gui (ListDisplayPanel.)]
      (ColorRgb/rgbToGray_Weighted color weighted)
      (.addImage gui
                 weighted
                 (str (.width color)
                      " "
                      (.height color)))
      (ShowImages/showWindow gui
                             "RGB222"
                             true))