Question

I have an operation that will be performed MANY times and so I need it to be as fast as possible, thus I think keeping validations to a minimum is important here.

I’m given an image, an offset and a size; and the idea is to take the color that appears the most within the area determined by the size and offset, so far I have this:

Private Shared Function someFunction(image As Bitmap, offset As Point, sampleSize As Size) As Color
    Dim pixelsColors As New List(Of Color)

    For i As Integer = offset.X To offset.X + sampleSize.Width
        For j As Integer = offset.Y To offset.Y + sampleSize.Height
            pixelsColors.Add(image.GetPixel(i, j))
        Next
    Next
End Function

What I now need is to know what is the most repeating item within the list and choose a in the case more than one have the top count, choose a random item of those that won.

I don’t even mind getting rid of the List(Of Color) and using some other (more appropriate) object.

UPDATE AFTER ACCEPTING ANSWER

The idea of the answer seems good but since there are a few minor error I’ll post this code for future reference for other.

Private Shared Function someFunction(image As Bitmap, offset As Point, sampleSize As Size) As Color
    Dim pixelsColors As New Collections.Generic.Dictionary(Of Color, Integer)

    For i As Integer = offset.X To offset.X + sampleSize.Width
        For j As Integer = offset.Y To offset.Y + sampleSize.Height
            Dim color As Color = image.GetPixel(i, j)
            Dim count As Integer
            If Not pixelsColors.TryGetValue(color, count) Then count = 0
            pixelsColors(color) = count + 1
        Next
    Next
    Return pixelsColors.OrderByDescending(Function(colorCount) colorCount.Value).First.Key
End Function

Answer 1

Instead of storing a linear List(Of Color) you should be using a hash table which allows you to associate a Color with a count. Then you con do a simple traversal at the end to discover what the highest count is

Dim map As New Dictionary(Of Color, Integer)

For i As Integer = offset.X To offset.X + sampleSize.Width
  For j As Integer = offset.Y To offset.Y + sampleSize.Height
    Dim color = image.GetPixel(i, j)
    Dim count As Integer
    If Not map.TryGetValue(color, count) Then
      count = 0
    End If
    map(color) = count + 1
  Next
Next

Dim mostColor = map.Max(Function(x) x.Count).Key