Faster Scala NDArray to BufferedImage function (apache#16219)

Author: zachgk

scala-package/core/src/main/scala/org/apache/mxnet/Image.scala

@@ -174,16 +174,18 @@ object Image {
   def toImage(src: NDArray): BufferedImage = {
     require(src.dtype == DType.UInt8, "The input NDArray must be bytes")
     require(src.shape.length == 3, "The input should contains height, width and channel")
+    require(src.shape(2) == 3, "There should be three channels: RGB")
     val height = src.shape.get(0)
     val width = src.shape.get(1)
     val img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB)
+    val arr = src.toArray
     (0 until height).par.foreach(r => {
       (0 until width).par.foreach(c => {
-        val arr = src.at(r).at(c).toArray
         // NDArray in RGB
-        val red = arr(0).toByte & 0xFF
-        val green = arr(1).toByte & 0xFF
-        val blue = arr(2).toByte & 0xFF
+        val cellIndex = r * width * 3 + c * 3
+        val red = arr(cellIndex).toByte & 0xFF
+        val green = arr(cellIndex + 1).toByte & 0xFF
+        val blue = arr(cellIndex + 2).toByte & 0xFF
         val rgb = (red << 16) | (green << 8) | blue
         img.setRGB(c, r, rgb)
       })