Arint System.Drawing.Graphics suppose to write/modify the image its attached to? - vb.net

Question

As you use a graphics object shouldn't the changes occur on the bitmap (source image) at some point? Running the code below I get 5 images that are all identical to the source. 1.bmp, 2.bmp, 3.bmp, 4.bmp, and 5.bmp are identical to "scaleCharacter" except 4 & 5 have higher compression (smaller file size)
Private Function DrawCharacterMenu() As Boolean
Try
'Background
Dim rect As Rectangle = New Rectangle(100, 100, 128, 128)
Graphics.FromImage(Render).FillRectangle(Brushes.Black, rect)
'Scale up sprite
Dim scaleCharacter As Bitmap = ActiveCharacter.img.Clone
Using grDest = Graphics.FromImage(scaleCharacter)
scaleCharacter.Save("1.bmp")
grDest.ScaleTransform(4.0F, 4.0F)
scaleCharacter.Save("2.bmp")
grDest.InterpolationMode = Drawing2D.InterpolationMode.NearestNeighbor
scaleCharacter.Save("3.bmp")
grDest.DrawImage(scaleCharacter, 0, 0)
scaleCharacter.Save("4.bmp")
End Using
scaleCharacter.Save("5.bmp")
'Draw scaled up sprite to rendering
Graphics.FromImage(Render).DrawImage(scaleCharacter, 100, 100)
Catch ex As Exception
addDebugMessage("Error: Mainmenu.DrawCharacterMenu: " & ex.Message)
Return False
End Try
Return True
End Function
I would Expect 1 to be the same as 'scaleCharacter'
2 and beyond to be 4 times larger (32x32 to 128x128)
3 and beyond to have less interpolation (not looked stretched)
The finished 'scaleCharacter' drawn onto the rendering also is identical to the original image...

Answer 1

All your images are the same because technically you never change them.
Graphics.ScaleTransform() changes only the internal "world" matrix used when drawing primitives. ScaleTransform(4.0F, 4.0F) makes the drawing grid 4x wider and 4x taller, but it doesn't change the image itself until you draw something on it. For instance, if you were to draw a 20 x 10 rectangle on your image now it would result in a rectangle 80 x 40 in size.
To resize the actual image you have to create a new bitmap with the scaled size, then draw the old image scaled onto it.
Changing Graphics.InterpolationMode affects only newly drawn objects. Again it doesn't change your image until you draw something on it.
Finally, while grDest.DrawImage(scaleCharacter, 0, 0) does change your image, it draws the same image in the top-left corner (0, 0) of itself, so there is no visible change.
Here's how you can make it work:
Scaling your image:
'Scale factor.
Dim scaleFactor As Single = 4.0F
'Create a new bitmap of the scaled size.
Using scaledBmp As New Bitmap(scaleCharacter.Width * scaleFactor, scaleCharacter.Height * scaleFactor)
Using g As Graphics = Graphics.FromImage(scaledBmp)
'Draw the old image, scaled, onto the new one.
'srcRect: The rectangle specifying which portion of the source image (scaleCharacter) to draw.
' We want the full image so we specify (0, 0, source width, source height).
'destRect: The rectangle specifying where on the destination image (scaledBmp) to draw the source image.
' Since we want to scale it we specify the full destination image (0, 0, dest width, dest height).
Dim srcRect As New Rectangle(0, 0, scaleCharacter.Width, scaleCharacter.Height)
Dim destRect As New Rectangle(0, 0, scaledBmp.Width, scaledBmp.Height)
g.DrawImage(scaleCharacter, destRect, srcRect, GraphicsUnit.Pixel)
'Save the image.
scaledBmp.Save("2.bmp")
End Using
End Using
Scaling your image using Nearest Neighbour interpolation:
'Create a new bitmap of the scaled size.
Using scaledBmp As New Bitmap(scaleCharacter.Width * scaleFactor, scaleCharacter.Height * scaleFactor)
Using g As Graphics = Graphics.FromImage(scaledBmp)
'Set the interpolation mode before drawing.
g.InterpolationMode = Drawing2D.InterpolationMode.NearestNeighbor
'Draw the old image, scaled, onto the new one.
'srcRect: The rectangle specifying which portion of the source image (scaleCharacter) to draw.
' We want the full image so we specify (0, 0, source width, source height).
'destRect: The rectangle specifying where on the destination image (scaledBmp) to draw the source image.
' Since we want to scale it we specify the full destination image (0, 0, dest width, dest height).
Dim srcRect As New Rectangle(0, 0, scaleCharacter.Width, scaleCharacter.Height)
Dim destRect As New Rectangle(0, 0, scaledBmp.Width, scaledBmp.Height)
g.DrawImage(scaleCharacter, destRect, srcRect, GraphicsUnit.Pixel)
'Save the image.
scaledBmp.Save("3.bmp")
End Using
End Using