Question

I’ve got a list of System.Drawing.RectangleF objects that all overlap the same RectangleF object. In my picture below, the 3 overlapping rectangles would be the pink, yellow, and red rectangles. My main rectangle in question is the light blue rectangle.

Second Image:

I know that with RectangleF objects I can use the Intersect() method that will return me another RectangleF object representing the overlap. But as far as I can tell, this only really works when comparing two rectangles.

My question is: How could I determine the TOTAL area/percentage (i.e. the combined total overlap of the red, yellow, and pink rectangles when compared to the light blue rectangle – but it would need to be smart enough to not count the area in which the red and yellow overlaps twice, and same for the pink and yellow)?

NOTE: The green lines represent the area I’m looking for, just the total area of the blue rectangle that is not visible.

UPDATE: I’ve added a 2nd image to further demonstrate what I’m looking for. In the second image, the presence of the burgundy rectangle should have no affect on the total percent covered because that area is already covered by the yellow and green rectangles.

Answer 1

OK I think I found a solution using a Region that seems to be working for both of my example images above:

Private Function TotalCoveredAreaPercent(ByVal oRectToCheck As RectangleF, ByVal oOverlappingRects As List(Of RectangleF)) As Double

    Dim oRegion As New Region(oRectToCheck)
    Dim dTotalVisibleArea As Double = 0
    Dim dTotalCoveredArea As Double = 0

    'now we need to exclude the intersection of our 
    'overlapping rectangles with our main rectangle:
    For Each oOverlappingRect As RectangleF In oOverlappingRects

        oRegion.Exclude(RectangleF.Intersect(oRectToCheck, oOverlappingRect))

    Next

    'now we have access to the non-overlapping 
    'rectangles that make up the visible area of our main rectangle:
    Dim oVisibleRects As RectangleF()

    oVisibleRects = oRegion.GetRegionScans(New Drawing2D.Matrix())

    'add the area of the visible rectangles together 
    'to find the total visible area of our main rectangle:
    For Each oVisibleRect As RectangleF In oVisibleRects
        dTotalVisibleArea += AreaOf(oVisibleRect)
    Next

    Dim dPercentVisible As Double = dTotalVisibleArea / AreaOf(oRectToCheck) * 100

    'percent covered is 100 - the visible percentage:
    Return (100 - dPercentVisible)

End Function

This seems to be working pretty well, and is quite simple.