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Create a bitmap from another bitmap

Posted on 2006-03-20
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Last Modified: 2010-04-23

What's the easiest way to create a bitmap object that contains an offset rectangle within an existing bitmap?  I thought I had a (long) method using BitBlt, but turns out if didn't work...

Thanks!
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Question by:riceman0
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Mike Tomlinson earned 500 total points
ID: 16237038
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        Dim sourceBmp As Bitmap = Image.FromFile("c:\someImage.bmp")
        Dim sourceRect As New Rectangle(100, 50, 75, 25)

        Dim destBmp As New Bitmap(sourceRect.Width, sourceRect.Height)
        Dim g As Graphics = Graphics.FromImage(destBmp)
        g.DrawImage(sourceBmp, New Rectangle(0, 0, destBmp.Width, destBmp.Height), sourceRect, GraphicsUnit.Pixel)
        g.Dispose()

        ' do something with destBmp...
        destBmp.Save("c:\someOtherImage.bmp")
        PictureBox1.Image = destBmp

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by:riceman0
ID: 16237154

You rock.
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Expert Comment

by:Mike Tomlinson
ID: 16237232
Just FYI, when you load an image using Image.FromFile(), the actual image file remains LOCKED until you dispose of the Bitmap class (or Image class) that opened it.

So "c:\someImage.bmp" will be locked until "sourceBmp" is explicitly disposed of or goes out of scope.

You can load an image without locking the file by first using Image.FromFile(), and then creating another Bitmap instance from the first Bitmap instance and then Disposing of the first Bitmap:

    Dim bmpA As Bitmap = Image.FromFile("c:\someImage.bmp") ' file is locked
    Dim bmpB As Bitmap = New Bitmap(bmpA) ' create a copy in memory
    bmpA.Dispose() ' file is unlocked now

Now you can work with bmpB and "c:\someImage.bmp" will NOT be locked because we disposed of bmpA.
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