Generating Thumbnail C# from base64string

Question

I am trying to generate a thumbnail from the base64string. I am storing the image in a table and am trying to generate a thumbnail from the base64string being stored.

I am able to generate the thumbnail if I provide a path to the image, but that will not work in my case.

This is the working solution of generating a thumbnail from an image path:

protected void GenerateThumbnail(object sender, EventArgs e)
{
        string path = Server.MapPath("../src/img/myImage.png");
        System.Drawing.Image image = System.Drawing.Image.FromFile(path);
        using (System.Drawing.Image thumbnail = image.GetThumbnailImage(100, 100, 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;
            }
        }
}

Can anyone provide any advice on how to use the base64string instead of the image path the generate thumbnail?

If you want to thumbnail the bitmap, do it before you convert it to base64. Starting with the base64 string means undoing the last few steps — Ňɏssa Pøngjǣrdenlarp, Jan 26 '16 at 20:50
Sorry, the post confused me. You **have** a base64 string and want to convert it back to bitmap and thumbnail it? — Ňɏssa Pøngjǣrdenlarp, Jan 26 '16 at 21:14
Yes, I am able to convert the FromFile(path) to a thumbnail, but am unable how to figure converting the base64 to a thumbnail — Code, Jan 26 '16 at 21:21
`byte[] bytes = Convert.FromBase64String(b64);` will convert the b64 string to byte[] which you can use in the memstream ctor — Ňɏssa Pøngjǣrdenlarp, Jan 26 '16 at 21:27

Ňɏssa Pøngjǣrdenlarp · Accepted Answer · 2016-01-27T01:24:48.163

5

Assuming, b64 is the base64 string, you can convert it to a byte array and use that to construct the starting image.

byte[] bytes = Convert.FromBase64String(b64);
using (MemoryStream ms = new MemoryStream(bytes))
{
    Bitmap thumb = new Bitmap(100, 100);
    using (Image bmp = Image.FromStream(ms))
    {
        using (Graphics g = Graphics.FromImage(thumb))
        { 
            g.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g.CompositingQuality = CompositingQuality.HighQuality;
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.DrawImage(bmp, 0, 0, 100, 100);
        }
    }
    // a picturebox to show/test the result
    picOut.Image = thumb;
}

Be sure to dispose of the thumb when you are done with it.

edited Jan 27 '16 at 01:24

answered Jan 26 '16 at 21:36

Ňɏssa Pøngjǣrdenlarp

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Forgot to declare `picOut` – Rodolfo Jan 26 '16 at 23:36
Hmm... I am getting `Parameter is not valid` on the line where we are declaring the `bmp` variable.. Any ideas? – Code Jan 27 '16 at 14:52
It looks like it is not actually saving in base64.. That was the problem – Code Jan 27 '16 at 15:10
It is your data - if the B64 doesnt represent image data it would fail on that line. If it was not really Base64, it should fail on the Convert.From line – Ňɏssa Pøngjǣrdenlarp Jan 27 '16 at 15:12

Arun Prasad E S · Answer 2 · 2022-02-15T04:16:40.043

Mixed few tricks found online. (one from @Plutonix)

string ThumbNailBase64 = ResizeBase64Image(YourBase64String,200, 300);

base64 Input => resize => base64 Output

You get the desired thumbnail with auto aspect ratio.

public static string ResizeBase64Image(string Base64String, int desiredWidth, int desiredHeight)
{
    Base64String = Base64String.Replace("data:image/png;base64,", "");

    // Convert Base64 String to byte[]
    byte[] imageBytes = Convert.FromBase64String(Base64String);

    using (MemoryStream ms = new MemoryStream(imageBytes))
    {                
        // Convert byte[] to Image
        ms.Write(imageBytes, 0, imageBytes.Length);
        Image image = Image.FromStream(ms, true);

        var imag = ScaleImage(image, desiredWidth, desiredHeight);

        using (MemoryStream ms1 = new MemoryStream())
        {
            //First Convert Image to byte[]
            imag.Save(ms1, imag.RawFormat);
            byte[] imageBytes1 = ms1.ToArray();

            //Then Convert byte[] to Base64 String
            string base64String = Convert.ToBase64String(imageBytes1);
            return "data:image/png;base64,"+base64String;
        }
    }
}

public static Image ScaleImage(Image image, int maxWidth, int maxHeight)
{
    var ratioX = (double)maxWidth / image.Width;
    var ratioY = (double)maxHeight / image.Height;
    var ratio = Math.Min(ratioX, ratioY);

    var newWidth = (int)(image.Width * ratio);
    var newHeight = (int)(image.Height * ratio);

    var newImage = new Bitmap(newWidth, newHeight);

    using (var graphics = Graphics.FromImage(newImage))
        graphics.DrawImage(image, 0, 0, newWidth, newHeight);

    return newImage;
}

Getting a null exception when it tries to access scaledImage.RawFormat. Tried using JPEG format and it doesn't load the images correctly. — Nick Gallimore, Nov 05 '19 at 21:52

Generating Thumbnail C# from base64string

2 Answers2