Image turns out all strange, and I can't find what's wrong

[minitech]

I'm making my own new image format where any pixel with an alpha less than a specific amount is dropped, and up to 255 colors are stored in a dictionary for additional compression. The format is like this:

Not one of the 255 colors in the dictionary: Argb representation
In dictionary: 0 for the alpha byte, number of repetitions in the R and G bytes, and index in the B byte.

Each group is stored like so:
0 x y colors.

I'm getting an IndexOutOfRangeException because the dictionary never has anything in it, and I can't figure out why. I have no idea what's going wrong! Here's my code:

Imports System.IO
Imports System.Runtime.InteropServices
Imports System.Drawing.Imaging
Public NotInheritable Class EACImage
 
    Public Shared Sub FromBitmap(ByVal b As Bitmap, ByVal output As String, Optional ByVal threshold As Integer = 0)
        Dim r As New Rectangle(0, 0, b.Width, b.Height)
        Using fs As New FileStream(output, FileMode.Create, FileAccess.Write)
            Using bw As New BinaryWriter(fs)
                Dim bd As BitmapData = b.LockBits(r, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb)
                Dim arr(bd.Width * bd.Height - 1) As Integer
                Marshal.Copy(bd.Scan0, arr, 0, arr.Length)
 
                'Prepare information
                Dim dictionary As New List(Of Integer)
                Dim data As New List(Of Integer)
                For i As Integer = 0 To arr.Length - 1
                    While GetAComp(arr(i)) <= threshold
                        i += 1
                        If i = arr.Length Then Exit For
                    End While
                    data.Add(0) 'Begin a new group
                    data.Add(i Mod bd.Width) 'Width
                    data.Add(i \ bd.Width) 'Height
                    While (i < arr.Length) AndAlso (GetAComp(arr(i)) > threshold)
                        data.Add(arr(i))
                        i += 1
                    End While
                Next
 
                'Compress the list
                CompressList(data, dictionary)
 
                'Write header information
                For Each item As Integer In dictionary
                    bw.Write(item)
                    MsgBox("Wrote " & item)
                Next
                bw.Write(0I)
                bw.Write(bd.Width)
                bw.Write(bd.Height)
 
                'Write data
                For Each item As Integer In data
                    bw.Write(item)
                Next
 
                'Unlock the bitmap
                b.UnlockBits(bd)
 
                'Close the streams.
                bw.Close()
                fs.Close()
            End Using
        End Using
    End Sub
 
    Public Shared Function FromFile(ByVal f As String) As Bitmap
        Dim r As Bitmap = Nothing
 
        Using fs As New FileStream(f, FileMode.Open, FileAccess.Read)
            Using br As New BinaryReader(fs)
                Dim dict As New List(Of Integer)
                Do
                    Dim itm As Integer = br.ReadInt32()
                    If itm = 0 Then Exit Do
                    dict.Add(itm)
                Loop
                r = New Bitmap(br.ReadInt32(), br.ReadInt32(), PixelFormat.Format32bppArgb)
                Dim bd As BitmapData = r.LockBits(New Rectangle(0, 0, r.Width, r.Height), ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb)
                Dim arr(bd.Width * bd.Height - 1) As Integer
                Dim x As Integer = 0, y As Integer = 0
                Do
                    If br.BaseStream.Position = br.BaseStream.Length Then Exit Do
                    Dim n_int As Integer = br.ReadInt32()
                    If n_int = 0 Then 'next group
                        x = br.ReadInt32()
                        y = br.ReadInt32()
                    End If
                    If GetAComp(n_int) = 0 Then 'It's compressed
                        For j As Integer = 1 To GetRepetition(n_int)
                            arr(y * bd.Width + x) = dict(GetIndex(n_int))
                            x += 1
                            If x >= bd.Width Then
                                y += 1
                                x = x Mod bd.Width
                            End If
                        Next
                    Else 'Not compressed
                        arr(y * bd.Width + x) = n_int
                    End If
                    x += 1
                    If x >= bd.Width Then
                        y += 1
                        x = x Mod bd.Width
                    End If
                Loop
 
                'Copy back and unlock
                Marshal.Copy(arr, 0, bd.Scan0, arr.Length)
                r.UnlockBits(bd)
 
                'Close the streams.
                br.Close()
                fs.Close()
            End Using
        End Using
 
        Return r
    End Function
 
#Region "GetXXX"
    Friend Shared Function GetAComp(ByVal i As Integer) As Integer
        Dim sh24 As Integer = 255I << 24
        Dim res As Integer = i And sh24
        Return res >> 24
    End Function
 
    Friend Shared Function GetRepetition(ByVal i As Integer) As Integer
        Dim sh16 As Integer = 255 << 16
        Dim sh8 As Integer = 255 << 8
        Dim res As Integer = i And (sh16 Or sh8)
        Return res >> 8
    End Function
 
    Friend Shared Function GetIndex(ByVal i As Integer) As Integer
        Return (i And 255)
    End Function
 
    Friend Shared Function GetInt(ByVal count As Integer, ByVal index As Integer) As Integer
        If count > Short.MaxValue OrElse index > 255 Then Throw New ArgumentException()
        Return index Or (count << 8)
    End Function
#End Region
 
 
    Private Shared Sub CompressList(ByRef l As List(Of Integer), ByRef dict As List(Of Integer), Optional ByVal minrep As Integer = 30)
        dict.Clear()
        Dim nl As New List(Of Integer)
        Dim l_itm As Integer = -1
        Dim count As Integer = 0
        For i As Integer = 0 To l.Count - 1
            Dim itm As Integer = l(i)
            If itm = 0 Then
                i += 2
                If i >= l.Count Then Exit For
                itm = l(i)
            End If
            If count = 0 Then
                count = 1
                l_itm = itm
            Else
                If l_itm = itm Then
                    count += 1
                    If (count < UInt16.MaxValue) AndAlso (dict.Count < 255) Then Continue For
                End If
                If count <= minrep Then
                    nl.Add(itm)
                Else
                    Dim idx As Integer = dict.Count
                    If Not dict.Contains(itm) Then
                        dict.Add(itm)
                    Else
                        idx = dict.IndexOf(itm)
                    End If
                    nl.Add(GetInt(count, idx))
                End If
                l_itm = itm
                count = 1
            End If
        Next
        l = nl
    End Sub
 
End Class