Question

I’m having trouble creating an Image/Bitmap object in C# from a base64 encoded byte array.

Here’s what I’m dealing with:

---

I have a frontend where a user can crop an image. When the user selects an image via an input[type=file], my javascript code uses HTML5’s FileReader to save the DataUrl (base64 string) to a hidden field, that is posted along with the crop coordinates and dimensions, and everything else in that form.

The essence:

base64 data, if you want to test yourself:

http://kristianbak.com/test_image.txt

1. base64 string is posted to the action, and received as the parameter imageData
2. the action converts the string to a base64 byte array, like this:
3. byte[] imageBytes = Convert.FromBase64String(imageData.EncodeTo64());

EncodeTo64 extension method:

public static string EncodeTo64(this String toEncode)
{
    var toEncodeAsBytes = Encoding.ASCII.GetBytes(toEncode);
    var returnValue = Convert.ToBase64String(toEncodeAsBytes);
    return returnValue;
}

After the base64 string is converted to a byte array, I read the bytes into the memory using a MemoryStream:

using (var imageStream = new MemoryStream(imageBytes, false))
{
    Image image = Image.FromStream(imageStream); //ArgumentException: Parameter is not valid.
}

---

I have also tried the following variations:

a)

using (var imageStream = new MemoryStream(imageBytes))
{
    Bitmap image = new Bitmap(imageStream); //ArgumentException: Parameter is not valid.
}

b)

using (var imageStream = new MemoryStream(imageBytes))
{
    imageStream.Position = 0;
    Image image = Image.FromStream(imageStream); //ArgumentException: Parameter is not valid.
}

c)

TypeConverter typeConverter = TypeDescriptor.GetConverter(typeof(Bitmap));
Bitmap image = (Bitmap)typeConverter.ConvertFrom(imageBytes);

d)

Using this method:

private Bitmap GetBitmap(byte[] buf)
{
    Int16 width = BitConverter.ToInt16(buf, 18);
    Int16 height = BitConverter.ToInt16(buf, 22);

    Bitmap bitmap = new Bitmap(width, height); //ArgumentException: Parameter is not valid.

    int imageSize = width * height * 4;
    int headerSize = BitConverter.ToInt16(buf, 10);

    System.Diagnostics.Debug.Assert(imageSize == buf.Length - headerSize);

    int offset = headerSize;
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
            bitmap.SetPixel(x, height - y - 1, Color.FromArgb(buf[offset + 3], buf[offset], buf[offset + 1], buf[offset + 2]));
            offset += 4;
        }
    }
    return bitmap;
}

Conclusion:

I feel that there’s something else wrong, and I hope you can answer to this question.

Edit:

Sample of the frontend code:

<script>
    $(function() {
        var reader = new window.FileReader();

        function readImage(file, callBack) {
            reader.onload = function (e) {
                var image = new Image();
                image.onload = function (imageEvt) {
                    if (typeof callBack == "function") {
                        callBack(e.target.result);
                    }
                };
                image.src = e.target.result;
            };
            reader.readAsDataURL(file);
        }

        $j('#file').change(function (e) {
            var file = e.target.files[0];

            readImage(file, function(imageData) {
                $('#imageData').val(imageData);
            });
        });
    });
</script>

@using (Html.BeginForm("UploadImage", "Images", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
    @Html.ValidationSummary(true)
    <input name="PostedImage.ImageData" type="hidden" id="imageData" value="" />

    @* REST OF THE HTML CODE HERE *@

    <p>Choose an image:</p>
    <input type="file" name="file" id="file" />

    <input type="submit" value="Upload" />
}

Sample of the Controller / Action:

[HttpPost]
public ActionResult Opret(PostedImage postedImage)
{
    String imageData = PostedImage.ImageData;

    byte[] imageBytes = Convert.FromBase64String(imageData.EncodeTo64());

    using (var imageStream = new MemoryStream(imageBytes, false))
    {
        Image image = Image.FromStream(imageStream);
    }
}

Answer 1

I think your problem is that you are taking a base64 string that was posted to your controller , treating it like ASCII, and then converting it to base64 again.

I think you need to change this line

byte[] imageBytes = Convert.FromBase64String(imageData.EncodeTo64());

to

byte[] imageBytes = Convert.FromBase64String(imageData);

from there your bytes should be correct and you should be able to create your image

–Edit–

I took the sample data you provided in the text document and parsed it before loading it into a Bitmap. I was able to save the image to my hard drive and was greeted by a Spartan (Go Green!!)

Give this code a try and see what happens.

Please note imageData is exactly what’s exposed on http://kristianbak.com/test_image.txt. I would have provided the initialization, but it’s a pretty big string and would probably break things.

string imageDataParsed = imageData.Substring( imageData.IndexOf( ',' ) + 1 );
byte[] imageBytes = Convert.FromBase64String( imageDataParsed );
using ( var imageStream = new MemoryStream( imageBytes, false ) )
{
   Bitmap image = new Bitmap( imageStream );
}