improving algorithm to be faster - scanning image from file

Question

My code here works but takes a couple seconds longer and with larger files it takes longer and i wanted to know if anyone can take a look over what i have and suggest any kind of improvement that will help make this faster.

Purpose:

This is to scan a pdf file and search for the bitmap image of a QR code and it will return the code for it (decode)

private void ScanQRPdf(string imagePath)
    {
        foreach (var item in Directory.GetFiles(imagePath))
        {
            if (Path.GetExtension(item).ToLower() == ".png")
            {
                Bitmap b = new Bitmap(imagePath);
                try
                {
                    QRCodeDecoder decoder = new QRCodeDecoder();
                    String decodedString = decoder.decode(new QRCodeBitmapImage(b));
                    rtbpdfData.Text += decodedString + "\n";
                }
                catch (Exception ex)
                {
                }
            }
        }
    }

 static void AddQRTag(PdfSharp.Drawing.XGraphics gfx, int xPosition, int yPosition, string QRdata, string HRdata)
    {
        gfx.DrawRectangle(XBrushes.White, xPosition, yPosition, xPosition + 55, yPosition + 85);

        PdfSharp.Drawing.XImage xImage =
            PdfSharp.Drawing.XImage.FromGdiPlusImage(BuildQR(QRdata.ToUpper(), 3,
                                            QRCodeEncoder.ENCODE_MODE.ALPHA_NUMERIC, 2, QRCodeEncoder.ERROR_CORRECTION.M));
        gfx.DrawImage(xImage, xPosition + 5, yPosition + 5, xImage.PixelWidth * .8, xImage.PixelWidth * .8);


        XFont font = new XFont("OCR B", 6);
        XTextFormatter tf = new XTextFormatter(gfx);
        tf.Alignment = XParagraphAlignment.Left;


        XRect layout = new XRect(xPosition + 5, yPosition + 55, 55, 30);
        tf.DrawString(HRdata.ToUpper(), font, XBrushes.Black, layout, XStringFormats.TopLeft);
    }

Answer 1

In the code, which you past everything is ok. The problem must be in QRCodeDecoder.decode function. If you are scanning image pixel by pixel, via Bitmap.GetPixel function, it will waste a lot of time. Better way will be to use unsafe code and convert bitmap to BitmapData.