Drawing Rectangles in Picture box Coordinate flip? - vb.net

Question

So I have a picture box with an image. I have coordinates to draw certain boxes (as in multiple rectangles) on the image (from a device).
Dim rectPoint_Start As Point =
New Point((newStartPoint_X * pbZoneImage.Width / resWidth),
newStartPoint_Y * pbZoneImage.Height / resHeight)
Dim rectPoint_End As Point =
New Point((newEndPoint_X * pbZoneImage.Width / resWidth),
newEndPoint_Y * pbZoneImage.Height / resHeight)
Dim rectangleHeight As Integer = (rectPoint_End.Y - rectPoint_Start.Y)
Dim rectangleWidth As Integer = (rectPoint_End.X - rectPoint_Start.X)
'localize
camRect1 = New Rectangle(rectPoint_Start.X, rectPoint_Start.Y, _
rectangleWidth, rectangleHeight)
If camRect1 <> Nothing Then g.DrawRectangle(Pens.Blue, camRect1)
The rect coordinates are coming in scaled with a resolution, so you see me there changing it to a relative point value for the pbZoneImage picture box. The problem I am having is that the coordinates I'm receiving (from the device) are assuming the (0,0) point is in the bottom left, where the PB draws these boxes starting from the top left. Is there a way to change the orientation of the starting point being drawn in the g.drawRectangle()? Or another method.
What I have tried so far is to add (pbzoneImage.bottom) - ((newEndPoint_X * pbZoneImage.Width ....) in the rectPoint_End and rectPoint_Start points. It doesn't seem to work and gives me negative values.
Update:
So this is what I ended up doing to fix it. I ended up drawing out a coordinate system and doing some old school algebra based on some of the answers I received. (Reminder, pbZoneImage is a picture box, startpoint x and y along with endpoint x and y are the values from the device)
Dim rectangleHeight As Integer = Math.Floor((newEndPoint_Y - newStartPoint_Y) * pbZoneImage.Height)
Dim rectangleWidth As Integer = Math.Floor((newEndPoint_X - newStartPoint_X) * pbZoneImage.Height)
'flip rectangle
Dim rectPoint_Start As New Point((newStartPoint_X * pbZoneImage.Width), _
pbZoneImage.Height - (newStartPoint_Y * pbZoneImage.Height) - rectangleHeight)
Dim rectPoint_End As Point = New Point((newEndPoint_X * pbZoneImage.Width), _
pbZoneImage.Height - (newEndPoint_Y * pbZoneImage.Height) - rectangleHeight)
'localize
camRect1 = New Rectangle(rectPoint_Start.X, rectPoint_Start.Y, _
rectangleWidth, rectangleHeight)

Answer 1

Upon receiving newStartPoint_Y and newEndPoint_Y values from the device, do the following :
newStartPoint_Y = resHeight - newStartPoint_Y
newEndPoint_Y = resHeight - newEndPoint_Y
assuming your device gives you coordinates on a zero-based reference (where the point at the lower left corner has the coordinate (0,0) which would fit with the zero-based coordinates of the top left corner of a PictureBox)
If I understand well, newStartPoint_X/Y and newEndPoint_X/Y are coordinates directly from the device ?
I suggest you to take the habit to use either CInt() when using Point with integer X and Y values, or better : CSng() with PointF, since you're scaling coordinates anyway.

Answer 2

I believe you are forgetting to also adjust for the rectangle height. If the rectangle origin is the lower left and its position was mapped to the resized image coordinate system based upon image lower left origin then you must adjust for the resized image height and the mapped rectangle height. Sketch it out on paper to get the relationships right.