The cropping area has an x and y offset in the new bitmap, but only if the original has been scaled - vb.net

Question

a strange phenomenon occurs.
With my edge detection program, I can transfer the inside of the GraphicsPath to a new image.
It always works great – except when I scale the original image with GIMP and Word (aspect ratio remains, only the dimensions are changed). Then the area is shifted. To the left and up. See attachement. In line 68, I looked what is in rectCutout. Everything OK.
Does this have anything to do with GIMP? The dots per inch are the same (72). The compression quality of the JPEG also (100%).
I just realized: if I scale an image larger, the result is completely black.
The strange thing is: I'm not saying: the picture that is drawn on is larger than the picture that is saved. Then it would be logical that the path is not in the same position. It's about the fact that the loaded image is just smaller.
I would be happy if someone could tell me why. 😄
this is the scaled image which is loaded
Here you see the GUI, ready to save
cropped image, area has x and y offset
#Disable Warning CA1707 ' Bezeichner dürfen keine Unterstriche enthalten
Imports System.Drawing.Drawing2D
Imports Microsoft.WindowsAPICodePack.Dialogs
Public NotInheritable Class AllesGrafische
Public Shared Sub Paint_the_Rectangle(ByVal g As Graphics, ByVal recta As Rectangle)
If g IsNot Nothing Then
g.SmoothingMode = SmoothingMode.AntiAlias
g.CompositingQuality = CompositingQuality.HighQuality
g.PixelOffsetMode = PixelOffsetMode.HighQuality
g.InterpolationMode = InterpolationMode.HighQualityBilinear
Using Pen_Hellblau As Pen = New Pen(Color.FromArgb(0, 200, 255), 1.0F)
g.DrawRectangle(Pen_Hellblau, recta)
End Using
End If
End Sub
Public Shared Sub Draw_Curve(ByVal g As Graphics, ByVal theList As List(Of Point))
If theList IsNot Nothing AndAlso theList.Count > 0 AndAlso g IsNot Nothing Then
g.SmoothingMode = SmoothingMode.AntiAlias
g.CompositingQuality = CompositingQuality.HighQuality
g.PixelOffsetMode = PixelOffsetMode.HighQuality
g.InterpolationMode = InterpolationMode.HighQualityBilinear
Dim theList_neu As New List(Of Point)
Using gp As New GraphicsPath
For i As Integer = 1 To theList.Count - 1 Step 1
Dim a As Integer = theList(i).X
Dim b As Integer = theList(i).Y
Dim c As Integer = theList(i - 1).X
Dim d As Integer = theList(i - 1).Y
Dim Entfernungsbetrag As Double = Math.Sqrt(Math.Pow(a, 2) + Math.Pow(b, 2) + Math.Pow(c, 2) + Math.Pow(d, 2) - 2 * a * c - 2 * b * d)
If Entfernungsbetrag < Form1.erlaubte_Entfernung Then
theList_neu.Add(theList(i))
End If
Next
If theList_neu.Count = 0 Then Return
gp.AddLines(theList_neu.ToArray())
Using Pen_hellrosa As Pen = New Pen(Color.FromArgb(255, 64, 239), 1.0F)
g.DrawPath(Pen_hellrosa, gp)
End Using
If Form1.ClosePath Then
gp.CloseFigure()
End If
If Form1.CheckBox1.Checked Then
Dim Speicherpfad As String
Using SFD1 As New CommonSaveFileDialog
SFD1.Title = "Wo soll das Bild gespeichert werden?"
SFD1.Filters.Add(New CommonFileDialogFilter("PNG", ".png"))
If System.IO.Directory.Exists("C:\Users\...\source\repos\VB.NET\Get mouse position and draw rectangle on screen") Then
SFD1.InitialDirectory = "C:\Users\...\source\repos\VB.NET\Get mouse position and draw rectangle on screen"
Else
SFD1.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.Desktop)
End If
If SFD1.ShowDialog = CommonFileDialogResult.Ok Then
Speicherpfad = SFD1.FileName & ".png"
Else
Return
End If
End Using
Using bmpSource As Bitmap = New Bitmap(Form1.Pfad_Bild)
Dim rectCutout As RectangleF = gp.GetBounds()
Using m As Matrix = New Matrix()
m.Translate(-rectCutout.Left, -rectCutout.Top)
gp.Transform(m)
End Using
Using bmpCutout As Bitmap = New Bitmap(CInt(Math.Round(rectCutout.Width, 0)), CInt(Math.Round(rectCutout.Height, 0)))
Using graphicsCutout As Graphics = Graphics.FromImage(bmpCutout)
graphicsCutout.Clip = New Region(gp)
graphicsCutout.DrawImage(bmpSource, CInt(-rectCutout.Left), CInt(-rectCutout.Top))
bmpCutout.Save(Speicherpfad, Imaging.ImageFormat.Png)
Form1.CheckBox1.Checked = False
End Using
End Using
End Using
End If
End Using
End If
End Sub
End Class
#Enable Warning CA1707 ' Bezeichner dürfen keine Unterstriche enthalten

Answer 1

The solution is to use .SetResolution()
Using Original As Bitmap = New Bitmap(Form1.Pfad_Bild)
Dim rectCutout As RectangleF = gp.GetBounds()
Using m As System.Drawing.Drawing2D.Matrix = New System.Drawing.Drawing2D.Matrix()
m.Translate(-rectCutout.Left, -rectCutout.Top)
gp.Transform(m)
End Using
Using bmpCutout As Bitmap = New Bitmap(CInt(Math.Round(rectCutout.Width, 0)), CInt(Math.Round(rectCutout.Height, 0)))
bmpCutout.SetResolution(Original.HorizontalResolution, Original.VerticalResolution)
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