Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/20168#discussion_r161662177
  
    --- Diff: mllib/src/main/scala/org/apache/spark/ml/image/ImageSchema.scala 
---
    @@ -37,20 +37,67 @@ import org.apache.spark.sql.types._
     @Since("2.3.0")
     object ImageSchema {
     
    -  val undefinedImageType = "Undefined"
    +  /**
    +   * OpenCv type representation
    +   *
    +   * @param mode ordinal for the type
    +   * @param dataType open cv data type
    +   * @param nChannels number of color channels
    +   */
    +  case class OpenCvType(mode: Int, dataType: String, nChannels: Int) {
    +    def name: String = if (mode == -1) { "Undefined" } else { 
s"CV_$dataType" + s"C$nChannels" }
    +    override def toString: String = s"OpenCvType(mode = $mode, name = 
$name)"
    +  }
     
       /**
    -   * (Scala-specific) OpenCV type mapping supported
    +   * Return the supported OpenCvType with matching name or raise error if 
there is no matching type.
    +   *
    +   * @param name: name of existing OpenCvType
    +   * @return OpenCvType that matches the given name
        */
    -  val ocvTypes: Map[String, Int] = Map(
    -    undefinedImageType -> -1,
    -    "CV_8U" -> 0, "CV_8UC1" -> 0, "CV_8UC3" -> 16, "CV_8UC4" -> 24
    -  )
    +  def ocvTypeByName(name: String): OpenCvType = {
    +    ocvTypes.find(x => x.name == name).getOrElse(
    +      throw new IllegalArgumentException("Unknown open cv type " + name))
    +  }
    +
    +  /**
    +   * Return the supported OpenCvType with matching mode or raise error if 
there is no matching type.
    +   *
    +   * @param mode: mode of existing OpenCvType
    +   * @return OpenCvType that matches the given mode
    +   */
    +  def ocvTypeByMode(mode: Int): OpenCvType = {
    +    ocvTypes.find(x => x.mode == mode).getOrElse(
    +      throw new IllegalArgumentException("Unknown open cv mode " + mode))
    +  }
    +
    +  val undefinedImageType = OpenCvType(-1, "N/A", -1)
    +
    +  /**
    +   * A Mapping of Type to Numbers in OpenCV
    +   *
    +   *        C1 C2  C3  C4
    +   * CV_8U   0  8  16  24
    +   * CV_8S   1  9  17  25
    +   * CV_16U  2 10  18  26
    +   * CV_16S  3 11  19  27
    +   * CV_32S  4 12  20  28
    +   * CV_32F  5 13  21  29
    +   * CV_64F  6 14  22  30
    +   */
    +  val ocvTypes: IndexedSeq[OpenCvType] = {
    +    val types =
    +      for (nc <- Array(1, 2, 3, 4);
    --- End diff --
    
    `numChannel`


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