Maintain apsect ratio while resizing Image in ASP.Net using C# an VB.Net

Hi southern38spe...,

 First you need to calculate the image height and width.

Then based on the height and width you want, calculate the aspect ratio.

Finally, using the aspect ratio calculate the final height and width of the image to be generated.

Refer below sample.

HTML

<asp:Image ID="Image1" runat="server" ImageUrl="~/Jellyfish.jpg" Height="200px" Width="200px" />
<br />
<br />
<asp:Button ID="btnGenerate" OnClick="GenerateThumbnail" runat="server" Text="Generate Thumbnail" />
<hr />
<asp:Image ID="Image2" runat="server" Visible="false" />

Namespaces

C#

using System.IO;
using System.Drawing.Imaging;

VB.Net

Imports System.IO
Imports System.Drawing.Imaging

Code

C#

protected void GenerateThumbnail(object sender, EventArgs e)
{
    string path = Server.MapPath("~/Jellyfish.jpg");
    System.Drawing.Image image = System.Drawing.Image.FromFile(path);
    int height = image.Height;
    int width = image.Width;

    int maxHeight = 150;
    int maxWidth = 150;

    double x = maxHeight / height;
    double y = maxWidth / width;
    double ratio = Math.Min(x, y);

    int newHeight = (int)(height * ratio);
    int newWidth = (int)(width * ratio);

    using (System.Drawing.Image thumbnail = image.GetThumbnailImage(newWidth, newHeight,
        new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback), IntPtr.Zero))
    {
        using (MemoryStream memoryStream = new MemoryStream())
        {
            thumbnail.Save(memoryStream, ImageFormat.Png);
            Byte[] bytes = new Byte[memoryStream.Length];
            memoryStream.Position = 0;
            memoryStream.Read(bytes, 0, (int)bytes.Length);
            string base64String = Convert.ToBase64String(bytes, 0, bytes.Length);
            Image2.ImageUrl = "data:image/png;base64," + base64String;
            Image2.Visible = true;
        }
    }
}

public bool ThumbnailCallback()
{
    return false;
}

VB.Net

Protected Sub GenerateThumbnail(sender As Object, e As EventArgs)
    Dim path As String = Server.MapPath("~/Jellyfish.jpg")
    Dim image As System.Drawing.Image = System.Drawing.Image.FromFile(path)
    Dim height As Integer = image.Height
    Dim width As Integer = image.Width

    Dim maxHeight As Integer = 150
    Dim maxWidth As Integer = 150

    Dim x As Double = maxHeight / height
    Dim y As Double = maxWidth / width
    Dim ratio As Double = Math.Min(x, y)

    Dim newHeight As Integer = CInt((height * ratio))
    Dim newWidth As Integer = CInt((width * ratio))

    Using thumbnail As System.Drawing.Image = image.GetThumbnailImage(newWidth, newHeight,
                                                New System.Drawing.Image.GetThumbnailImageAbort(AddressOf ThumbnailCallback), IntPtr.Zero)
        Using memoryStream As New MemoryStream()
            thumbnail.Save(memoryStream, ImageFormat.Png)
            Dim bytes As [Byte]() = New [Byte](memoryStream.Length - 1) {}
            memoryStream.Position = 0
            memoryStream.Read(bytes, 0, CInt(bytes.Length))
            Dim base64String As String = Convert.ToBase64String(bytes, 0, bytes.Length)
            Image2.ImageUrl = "data:image/png;base64," & base64String
            Image2.Visible = True
        End Using
    End Using
End Sub

Public Function ThumbnailCallback() As Boolean
    Return False
End Function

Screenshot