resolution (dpi) info

I'm trying to retrive resolution info from a jpg file. I was able to obtain width and height info but could not find code to access resolution (dpi) info.

' size info
Dim img = Image.FromFile(path)
MsgBox(img.Size.Width)
MsgBox(img.Size.Height)

Do you know the code to change the resolution and size? Shrinking a file from 300dpi to 72dpi.

dpi is not a property of the image, it's a property of the screen on which the image is shown. The width and height of an image are measured in pixels, pure and simple. dpi merely tells you how many pixels it takes to fill an inch of screen.

Image resolution (dpi) and image size (width and height in pixels) are independent of one another. Most methods in GDI+ ignore the resolution. For example, if you show an image in a PictureBox the displayed size depends only on its Width and Height properties. But the Graphics.DrawImageUnscaled method does respect the image resolution as well as the pixel size, as do various printing functions.

You can get the image resolution (dpi) from the Image.HorizontalResolution and Image.VerticalResolution properties. You can set the resolution using the Bitmap.SetResolution method. The following example sets the resolution to 72,72 and the pixel size to 600*400:

Code:

---

Dim bmp As New Bitmap(img, 600, 400)
bmp.SetResolution(72, 72)
img = bmp

---

BB

When you edit an image say in photoshop, there's a command that allows adjusting image size. The width, height and resolution is adjustable. How do I replicate that same operation using VB 2008? If the image size is unchanged and the resolution if dropped down (ie 300dpi to 72 dpi), the file size changes because information is loss. I'm writing a script that will change all the high resolution photos I put in a folder to a lower resolution web quality image.

The dpi resolution is just a pair of numbers stored in the file. Changing them doesn't have a direct effect on the image display size or quality. But there is another factor which does affect quality. That is the jpeg compression quality (%). Changing that compression affects both the clarity of the image and the size of the file. It depends a bit on the image, but decreasing the jpeg quality can often lead to a big saving in file size without much visible deterioration.

In your program, you probably you need to use compression quality in combination with changing the pixel size in order to optimize the file size. Here's a method to save an image as a jpeg file with a chosen compression quality:

Code:

---

Imports System.Drawing.Imaging
'....

        Dim jpegCodec As ImageCodecInfo

        Public Sub Save(ByVal path As String, ByVal img As Image, ByVal quality As Integer)
                If quality <= 0 Then quality = 0 : If quality >= 100 Then quality = 100
                Dim qParam As New EncoderParameter(Encoder.Quality, quality)
                If jpegCodec Is Nothing Then
                        jpegCodec = ImageCodecInfo.GetImageEncoders.Where _
                          (Function(e As ImageCodecInfo) e.MimeType = "image/jpeg")(0)
                End If
                Dim encoderParams As New EncoderParameters(1)
                encoderParams.Param(0) = qParam
                Try
                        img.Save(path, jpegCodec, encoderParams)
                Catch
                        MessageBox.Show("Unable to save PNG image to path " & path)
                End Try
        End Sub

---

I think you can afford to ignore the dpi resolution for your present purposes. Or at most, use it to calculate a desired print size in inches instead of a pixel size.

BB

boops boops,

Thank you for your compression code. I'd like to use your code and incorporate it to the sizing code I'm using below. I'm not sure how to integrate the two.

**********************************
Dim i As Integer
i = DataGridView1.CurrentRow.Index
If DataGridView1.Item(1, i).Value <> DataGridView1.Item(8, i).Value Then
DataGridView1.Item(7, i).Value = DataGridView1.Item(5, i).Value * (DataGridView1.Item(6, i).Value) / DataGridView1.Item(4, i).Value
Dim img = Image.FromFile(ListBox1.Text & "\" & DataGridView1.Item(1, i).Value)
Dim w, h As Integer
w = DataGridView1.Item(6, i).Value
h = DataGridView1.Item(7, i).Value
Dim bmp As Bitmap = New Bitmap(w, h)
Using g As Graphics = Graphics.FromImage(bmp)
g.DrawImage(img, 0, 0, bmp.Width, bmp.Height)
End Using

bmp.Save(ListBox1.Text & "\" & DataGridView1.Item(8, i).Value, System.Drawing.Imaging.ImageFormat.Jpeg)

I don't know how PhotoShop does it, but programs I have used allow the user to choose the jpeg quality in order to keep the file size down. Is that what you have in mind? BB

The code you provided varies the jpeg quality by adjusting the compression. The code I'm using below allows resizing the image. How do I resize an image, compress the jpeg and save it to a new file?

Dim bmp As Bitmap = New Bitmap(w, h)
Using g As Graphics = Graphics.FromImage(bmp)
g.DrawImage(img, 0, 0, bmp.Width, bmp.Height)
End Using
bmp.Save(path, System.Drawing.Imaging.ImageFormat.Jpeg)

Suppose you have a trackbar (TrackBar1) with a range from 1 to 100 for the user to choose the compression %. Replace your bmp.Save line by a call to the Save sub I gave you:

Code:

---

Save(path, bmp, TrackBar1.Value)

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BB