Convert Hex string to Image

Question

I am using FormsConnect an App for the iPad to export signatures to XML. The output from the App is as follows:

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

Now I'm trying to convert this Hex string back into an image using this code:

 Bitmap newImage = new Bitmap(new MemoryStream(StringToByteArray(hex)));
 
 public static byte[] StringToByteArray(string hex)
    {
        hex = hex.Substring(2, hex.Length - 2);
        if (hex.Length % 2 != 0)
        {
            throw new ArgumentException(String.Format(CultureInfo.InvariantCulture, "The binary key cannot have an odd number of digits: {0}", hex));
        }

        byte[] HexAsBytes = new byte[hex.Length / 2];
       
        for (int index = 0; index < HexAsBytes.Length; index++)
        {
            string byteValue = hex.Substring(index * 2, 2);
            HexAsBytes[index] = byte.Parse(byteValue, NumberStyles.HexNumber, CultureInfo.InvariantCulture);
        }

        return HexAsBytes;
    }

But I keep getting Parameter is not valid, when attempting to Load the stream into a Bitmap. I posed the question to the developer of the App, and here is his response:

Dear Alex

The data is stored in binary, hex format. Here are the instructions on how to read the binary data and write it back out as a jpg.

byteData[i] = (digit [i * 2] shl 4) + (digit [i * 2 + 1])

The above assumes that the digits are already converted to numbers from 0 to 15. If you’re trying to optimize your script by writing to memory in larger chunks (integers, for instance) you need to take endianness into account. All major CPU architectures are little-endian which means the numbers are stored with their bytes arranged in the opposite order from their actual value (ie, 0x1234 is stored as 0x3412 in memory).

Below are two different scripts for extracting blob data from the form:

 1. Python Script:
iterate over each photo attached to image field in FormConnect xml file
data = R[i]

to strip '0x' off front of each blob
a = data[2:]
iterate over each byte, write as binary int to array
for i in range(len(a)/2):
                b = (int(bin(int(a[i*2],16)),2)<<4) + int(bin(int(a[i*2+1],16)),2)
                data.append(b)
 write array to binary file

F = file(outFile,'wb')
data.tofile(F)
F.close()


PHP Script:
$pic = "";
$field = substr($field,2);
header("Content-Type: image/jpeg"); 
$pic = pack("H*", $field);
echo $pic;

If anyone can help me on this, I would be very grateful. Kind regards, Alex

Answer 1

Your problem isn't the hex conversion routine, it's that the original data isn't encoded properly. Based on all the zeroes within it, I'd say it's just a raw bitmap! My guess is that the author of the program that generated that data forgot to encode the bitmap into a specific file format before outputting it.