Change of color on the selected part of the image

Option Base 0

' A Graphics object which draws the polygon
Private objGraphics As Graphics

' An array of points which represents the polygon
Private arrPolygon() As Long

' A number of point which is currently being dragged. If no current point selected, it equals to the NotInitialized
Private currentPoint As Long

' A value which specifies an "epsilon neighborhood" of the anchor point (i.e. the distance around the point
' which is considered as this point when hit test is applied)
Const Epsilon As Long = 6

' A value which specifies that the numeric value is not initialized (e.g. the index of the
' current point when no point selected). It should be negative (invalid) so that it would
' not be confused with valid values.
Const NotInitialized = -100

Private Sub DrawPolygon()

    ' Since our polygon is stored in coordinates relatively the image, we need to recalculate these coordinates
    ' relatively the BitmapViewer control.
    Dim tempPolygon() As Long
    ReDim tempPolygon(UBound(arrPolygon))
    For I = 0 To UBound(arrPolygon) Step 2
        tempPolygon(I) = BitmapViewer1.BitmapToControlXCoord(arrPolygon(I))
        tempPolygon(I + 1) = BitmapViewer1.BitmapToControlYCoord(arrPolygon(I + 1))
    Next I


    ' Polygon requires at least four points, that's why if user clicked less points, skip this line.
    If UBound(tempPolygon) - LBound(tempPolygon) + 1 >= 8 Then
        ' Set drawing settings for the polygon
        objGraphics.Brush.PrimaryColor = ColorBlack
        objGraphics.Brush.IsBackgroundTransparent = True
        objGraphics.Brush.Type = BrushTypeHatch
        objGraphics.Brush.HatchStyle = HatchStyleBackwardDiagonal
        
        ' Draw a filled and outlined polygon
        objGraphics.DrawPolygon tempPolygon, True, True
    End If

    ' Draw anchor points
    For I = LBound(tempPolygon) To UBound(tempPolygon) Step 2
        ' Set drawing settins for the anchor point.
        objGraphics.Brush.PrimaryColor = ColorRed
        objGraphics.Brush.Type = BrushTypeSolid
        
        ' Draw an anchor point
        objGraphics.DrawRectangle tempPolygon(I) - Epsilon / 2, tempPolygon(I + 1) - Epsilon / 2, Epsilon, Epsilon, True
    Next
    
End Sub

' This function tests whether a point with specified coordinates is already
' placed to the polygon. If yes, it returns the index of this point (i.e. the index
' of X coordinate of this point). Otherwise NotInitialized value is used.
Private Function HitTest(x As Long, y As Long) As Long
    For I = LBound(arrPolygon) To UBound(arrPolygon) Step 2
    
        ' Since it is very hard to hit the exact pixel which specifies the coordinate
        ' of the polygon vertex, we define an "epsilon neighborhood" for this point.
        ' In other words if the selected coordinates will be as close to this point as
        ' a number of pixels specified as Epsilon, the function will say "OK".
        If Abs(arrPolygon(I) - x) < Epsilon And Abs(arrPolygon(I + 1) - y) < Epsilon Then
            HitTest = I
            Exit Function

        End If
    Next
    
    ' Not found
    HitTest = NotInitialized
End Function

Private Sub BitmapViewer1_DoubleBufferPaint(ByVal Hdc As stdole.OLE_HANDLE)
    If UBound(arrPolygon) > 0 Then
        objGraphics.Hdc = Hdc
        DrawPolygon
    End If
End Sub

Private Sub BitmapViewer1_ImageMouseDown(ByVal button As Long, ByVal shift As Long, ByVal x As Long, ByVal y As Long)
    
    ' Determine if some point has been selected.
    currentPoint = HitTest(x, y)
    
    ' If not, it means that the user wants to add new one.
    If currentPoint = NotInitialized Then
        
        ' A number of elements in the arrPolygon array. The number of points is count/2.
        Dim count As Long
        count = UBound(arrPolygon) - LBound(arrPolygon) + 1
        
        ' I did not find an easy way to handle the situation when no elements are available
        ' in array. That's why I had to do a trick - in Form_Load I create a single point and
        ' set their coordinates to NotInitialized. When the user add the first point, I just
        ' rewrite these coordinates, and next points are added in a common way (using Redim statement).
        If count = 2 And arrPolygon(0) = NotInitialized And arrPolygon(1) = NotInitialized Then
            arrPolygon(0) = x
            arrPolygon(1) = y
        Else
            ReDim Preserve arrPolygon(count + 1)
            arrPolygon(count) = x
            arrPolygon(count + 1) = y
        End If
        
        ' Redraw the polygon at the control.
        BitmapViewer1.FastRefresh
    End If
End Sub

Private Sub BitmapViewer1_ImageMouseMove(ByVal button As Long, ByVal shift As Long, ByVal x As Long, ByVal y As Long)
    
    ' If the current point is selected, move this point.
    If currentPoint <> NotInitialized Then
        arrPolygon(currentPoint) = x
        arrPolygon(currentPoint + 1) = y
        BitmapViewer1.FastRefresh
    End If
    
    ' Display current coordinates
    Label1.Caption = "X :" & x & " Y: " & y
End Sub

Private Sub BitmapViewer1_ImageMouseUp(ByVal button As Long, ByVal shift As Long, ByVal x As Long, ByVal y As Long)
    
    ' When the mouse button is released, we need to "detach" from the current point.
    currentPoint = NotInitialized
End Sub

' Initialization is made there.
Private Sub Form_Load()

    ' Here you can load the bitmap from file, etc. I am just creating 640x480 image for brevity.
    BitmapViewer1.Bitmap.CreateNew 640, 480
    
    ' Create and initialize a Graphics object. Note, I am using GDI engine (rather than GDI+)
    ' to provide higher performance. But if you want to draw semitransparent polygon, etc., you
    ' should use GDI+.
    Set objGraphics = New Graphics
    objGraphics.Engine = DrawingEngineGdi
    
    ' Initialize polygon. See comments in BitmapViewer1_ImageMouseDown function
    ' for more details.
    ReDim arrPolygon(1)
    arrPolygon(0) = NotInitialized
    arrPolygon(1) = NotInitialized
    
    ' No current point selected.
    currentPoint = NotInitialized
End Sub

    ' Initialize polygon. See comments in BitmapViewer1_ImageMouseDown function
    ' for more details.
    ReDim arrPolygon(1)
    arrPolygon(0) = NotInitialized
    arrPolygon(1) = NotInitialized