Drawing and scaling rectangle using a ratio

Question

I have a program where I can, with the mouse, draw a rectangle in any of four directions.

These rectangles are used on a pictureBox to crop parts of an image.

These rectangles must be drawn while maintaining the ratio of a given dimension for example 320 x 200.

I want this tool to behave pretty much exactly like the crop tool in Photoshop, or like in the crop example found here: https://imageresize.org/

I have most elements working correctly I'm just struggling on a few geometric calculations.

See the "Bottom right" example in my code. This works perfectly and basically I just want to apply this exact formula to the other directions.

I have been playing with different calculations for hours and I just can't seem to work it out.

Here is the working code:

 Protected Overrides Sub OnMouseMove(ByVal e As MouseEventArgs)
    'Draw rectangle keeping aspect ratio
    If e.Button = Windows.Forms.MouseButtons.Left Then
        If e.X > startPos.X And e.Y > startPos.Y Then
            'Bottom right
            mRect = New Rectangle(mRect.Left, mRect.Top, e.X - mRect.Left, e.Y - mRect.Top)
            mRect.Size = New Size(mRect.Width, mRect.Width / Ratio.Text)
            If e.Y < mRect.Bottom Then
                mRect = Rectangle.FromLTRB(startPos.X, startPos.Y, e.X, e.Y)
                mRect.Size = New Size(mRect.Height * Ratio.Text, mRect.Height)
            End If
            Me.Invalidate()
        ElseIf e.X < startPos.X And e.Y > startPos.Y Then
            'Bottom left
            mRect = New Rectangle(e.X, startPos.Y, startPos.X - e.X, e.Y - startPos.Y)
            mRect.Size = New Size(mRect.Width, mRect.Width / Ratio.Text)
            Me.Invalidate()
        ElseIf e.X > startPos.X And e.Y < startPos.Y Then
            'Top right
            mRect = New Rectangle(startPos.X, e.Y, e.X - startPos.X, startPos.Y - e.Y)
            mRect.Size = New Size(mRect.Height * 1.6, mRect.Height)
            Me.Invalidate()
        ElseIf e.X < startPos.X And e.Y < startPos.Y Then
            'Top left
            mRect = New Rectangle(e.X, e.Y, startPos.X - e.X, startPos.Y - e.Y)
            mRect.Size = New Size(mRect.Width, mRect.Width / Ratio.Text)
            Me.Invalidate()
        End If
    End If
End Sub

Any help would be hugely appreciated. Thanks!

Below is how things currently work, you can see things go funky when drawing in the north west region. I need to get the same behavior as the south east (or bottom right per the code) for all quadrants.

Answer 1

You have sample rectangle S with given ratio and dimensions sw, sh (320x200 in your example)

Mouse positions form new rectangle N with dimensions nw, nh (absolute values!)

Your task is, as far as I understand, to inscribe rectangle with the same ratio as S into rectangle N, getting rectangle R with base point (rx0, ry0) and dimensions (rw, rh)

   nx0 = min(e.x, startpos.x) 
   ny0 = min(e.y, startpos.y) 
   nw = abs(e.x - startpos.x)
   nh = abs(e.y - startpos.y) 

   if nw * sh >= nh * sw then   //    if N is "too wide"
        rh = nh
        rw = rh * sw / sh
        ry0 = ny0                   
        rx0 = nx0 + (nw - rw) / 2
   else                        //      N is "too slim"
        rw = nw
        rh = rw * sh / sw
        rx0 = nx0
        ry0 = ny0 + (nh - rh) / 2

then

  mRect = New Rectangle(rx0,  ry0, rx0 + rw, ry0 + rh)

Answer 2

I propose a slightly different method to calculate the current position of the cursor, when a Ratio is applied to the Rectangle dimensions.

You need of course to save the starting position of the Rectangle, using the MouseDown event of a Control, then track the Mouse movements, using the MouseMove event.

The current position of the Cursor is calculated as usual (swapping the current Cursor Location the and Starting Position when the Offsets are negative).

The only difference is the Height of the Rectangle, when the overall size is subject to a Ratio constraint.
In this case, the Rectangle.Location.Y is determined by the Rectangle.Width / Ratio measure. This becomes visible if the Cursor.Location.Y is above the starting location (Cursor.Location.Y <= StartingPosition.Y). Just like in the code you posted.

For the example, I'm using a custom Rectangle class that holds all the informations need to draw a shape, with or without a specific Ratio applied to its dimensions.

Note that the Ratio is hardcoded to 1.6: it's just to for testing, of course it can be set to anything else.

Visual sample of the results:

Private DrawingRects As List(Of DrawingRectangle) = New List(Of DrawingRectangle)()


Private Sub PicureBox1_MouseDown(sender As Object, e As MouseEventArgs) Handles PicureBox1.MouseDown

    If e.Button = MouseButtons.Left Then
        DrawingRects.Add(New DrawingRectangle() With {
            .DrawingcColor = Color.LightGreen,
            .Location = e.Location,
            .Owner = CType(sender, Control),
            .Ratio = 1.6,
            .Size = Size.Empty,
            .StartPosition = e.Location
        })
    End If
End Sub

Private Sub PicureBox1_MouseMove(sender As Object, e As MouseEventArgs) Handles PicureBox1.MouseMove

    If e.Button = MouseButtons.Left Then
        Dim rect As DrawingRectangle = DrawingRects.Last()
        If e.X < rect.StartPosition.X Then rect.Location = New Point(e.X, rect.Location.Y)
        If e.Y < rect.StartPosition.Y Then rect.Location = New Point(rect.Location.X, e.Y)

        Dim currentWidth As Integer = Math.Abs(rect.StartPosition.X - e.X)

        If rect.Ratio = 1.0F Then
            rect.Size = New Size(currentWidth, Math.Abs(rect.StartPosition.Y - e.Y))
        Else
            If rect.StartPosition.Y <= rect.Location.Y Then
                rect.Size = New Size(currentWidth, CType(Math.Abs(rect.StartPosition.X - e.X) / rect.Ratio, Integer))
            Else
                Dim currentHeight As Integer = CType(currentWidth / rect.Ratio, Integer)
                rect.Location = New Point(rect.Location.X, rect.StartPosition.Y - currentHeight)
                rect.Size = New Size(currentWidth, currentHeight)
            End If
        End If
        DrawingRects(DrawingRects.Count - 1) = rect

        DirectCast(sender, Control).Invalidate()
    End If
End Sub

Private Sub PicureBox1_Paint(sender As Object, e As PaintEventArgs) Handles PicureBox1.Paint

    Dim canvas As Control = DirectCast(sender, Control)

    If DrawingRects.Count > 0 Then
        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias
        For Each rect As DrawingRectangle In DrawingRects
            If canvas IsNot rect.Owner Then Continue For
            Using pen As New Pen(rect.DrawingcColor, rect.PenSize)
                e.Graphics.DrawRectangle(pen, New Rectangle(rect.Location, rect.Size))
            End Using
        Next
    End If
End Sub

The DrawingRectangle class:

Note: the class has an Owner property, referencing the current Control where the shape is drawn: this allows to use a List(Of DrawingRectangle) with different controls at the same time.

Public Class DrawingRectangle
    Private rectAspect As SizeF = SizeF.Empty
    Private rectRatio As Single = 0F

    Public Property Owner As Control
    Public Property Location As Point
    Public Property Size As Size
    Public Property StartPosition As Point
    Public Property DrawingcColor As Color
    Public Property PenSize As Single

    Public Property Aspect() As SizeF
        Get
            Return rectAspect
        End Get
        Set(ByVal value As SizeF)
            Me.rectAspect = value
            SetAspectRatio(value)
        End Set
    End Property

    Public Property Ratio As Single
        Get
            Return rectRatio
        End Get
        Set(ByVal value As Single)
            rectRatio = value
            SetAspectRatio(value)
        End Set
    End Property

    Private Sub SetAspectRatio(aspect As SizeF)
        Me.rectRatio = aspect.Width / aspect.Height
    End Sub
    Private Sub SetAspectRatio(ratio As Single)
        Me.rectAspect = New SizeF(100, 100 / ratio)
    End Sub
End Class