Get DPI definition of image extracted from PDF

Question

I’m using the itextSharp to extract images from PDF files.

I used this code as a basis: https://psycodedeveloper.wordpress.com/2013/01/10/how-to-extract-images-from-pdf-files-using-c-and-itextsharp/

Here is my version modified to support files in memory instead of having to work with files on disk:

    /// <summary>Helper class to extract images from a PDF file. Works with the most
    /// common image types embedded in PDF files, as far as I can tell.</summary>
    /// <example>
    /// Usage example:
    /// <code>
    /// foreach (var filename in Directory.GetFiles(searchPath, "*.pdf", SearchOption.TopDirectoryOnly))
    /// {
    ///    var images = ImageExtractor.ExtractImages(filename);
    ///    var directory = Path.GetDirectoryName(filename);
    ///
    ///    foreach (var image in images)
    ///    {
    ///       image.Save(Path.Combine(directory, name));
    ///    }
    ///  }
    /// </code></example>
    public static class PdfImageExtractor
    {
        #region Methods
        #region Public Methods

        /// <summary>Checks whether a specified page of a PDF file contains images.</summary>
        /// <returns>True if the page contains at least one image; false otherwise.</returns>
        public static bool PageContainsImages(byte[] pdfFile, int pageNumber)
        {
            using (var reader = new PdfReader(pdfFile))
            {
                var parser = new PdfReaderContentParser(reader);
                ImageRenderListener listener = null;
                parser.ProcessContent(pageNumber, (listener = new ImageRenderListener()));
                return listener.Images.Count > 0;
            }
        }

        /// <summary>Extracts all images (of types that iTextSharp knows how to decode) from a PDF file.</summary>
        public static List<System.Drawing.Image> ExtractImages(byte[] pdfFile)
        {
            var images = new List<System.Drawing.Image>();

            using (var reader = new PdfReader(pdfFile))
            {
                var parser = new PdfReaderContentParser(reader);
                ImageRenderListener listener = null;
                for (var i = 1; i <= reader.NumberOfPages; i++)
                {
                    parser.ProcessContent(i, (listener = new ImageRenderListener()));
                    var index = 1;
                    if (listener.Images.Count > 0)
                    {
                        Console.WriteLine("Found {0} images on page {1}.", listener.Images.Count, i);
                        foreach (var pair in listener.Images)
                        {
                            images.Add(pair);
                            index++;
                        }
                    }
                }
                return images;
            }
        }

        /// <summary>Extracts all images (of types that iTextSharp knows how to decode)
        /// from a specified page of a PDF file.</summary>
        /// <returns>Returns a generic <see cref="List&lt;System.Drawing.Image&gt;"/>,
        /// where the key is a suggested file name, in the format: PDF filename without extension,
        /// page number and image index in the page.</returns>
        public static List<System.Drawing.Image> ExtractImages(byte[] pdfFile, int pageNumber)
        {
            var images = new List<System.Drawing.Image>();
            using (var reader = new PdfReader(pdfFile))
            {
                var parser = new PdfReaderContentParser(reader);
                ImageRenderListener listener = null;

                parser.ProcessContent(pageNumber, (listener = new ImageRenderListener()));
                int index = 1;
                if (listener.Images.Count > 0)
                {
                    Console.WriteLine("Found {0} images on page {1}.", listener.Images.Count, pageNumber);
                    foreach (System.Drawing.Image image in listener.Images)
                    {
                        images.Add(image);
                        index++;
                    }
                }

            }


            return images;
        }
        #endregion Public Methods
        #endregion Methods
    }
    internal class ImageRenderListener : IRenderListener
    {
        #region Fields
        List<System.Drawing.Image> images = new List<System.Drawing.Image>();
        #endregion Fields

        #region Properties
        public List<System.Drawing.Image> Images
        {
            get { return images; }
        }
        #endregion Properties

        #region Methods

        #region Public Methods
        public void BeginTextBlock() { }
        public void EndTextBlock() { }
        public void RenderImage(ImageRenderInfo renderInfo)
        {
            PdfImageObject image = renderInfo.GetImage();

            var imageBytes = image.GetImageAsBytes();
            var bytesType = image.GetImageBytesType();

            var fileExtension = bytesType.FileExtension;

            using (var memoryStream = new MemoryStream(imageBytes))
            {
                var drawingImage = System.Drawing.Image.FromStream(memoryStream);

                var dpiX = drawingImage.HorizontalResolution;

                this.Images.Add(drawingImage);
            }
        }
        public void RenderText(TextRenderInfo renderInfo) { }
        #endregion Public Methods
        #endregion Methods
    }

The problem is that in the method RenderImage the value of the variable dpiX always is 96. I’d like to get the original DPI image resolution, there is some way to do this?

To use the method is enough:

var pageImages = PdfImageExtractor.ExtractImages(fileBytes, pageNumber);

For each image, I tried to convert to Bitmap too, but got the same result:

var bitmapImage = new Bitmap(pageImage);
var dpiX = bitmapImage.HorizontalResolution;
var dpiY = bitmapImage.VerticalResolution;

I do not want to define a resolution on my own, I would like to get the original image resolution.