c# - 如何使用 PDFSharp 从 PDF 中提取 FlateDecoded 图像

Question

如何使用 PDFSharp 从 PDF 文档中提取经过 FlateDecoded(例如 PNG)的图像？

我在 PDFSharp 示例中发现了该评论:

// TODO: You can put the code here that converts vom PDF internal image format to a
// Windows bitmap
// and use GDI+ to save it in PNG format.
// [...]
// Take a look at the file
// PdfSharp.Pdf.Advanced/PdfImage.cs to see how we create the PDF image formats.

有人能解决这个问题吗？

感谢您的回复。

编辑:因为我无法在 8 小时内回答我自己的问题，所以我这样做了:

感谢您的快速回复。

我在“ExportAsPngImage”方法中添加了一些代码，但没有得到想要的结果。它只是提取了一些更多的图像 (png)，它们没有正确的颜色并且被扭曲了。

这是我的实际代码:

PdfSharp.Pdf.Filters.FlateDecode flate = new PdfSharp.Pdf.Filters.FlateDecode();
        byte[] decodedBytes = flate.Decode(bytes);

        System.Drawing.Imaging.PixelFormat pixelFormat;

        switch (bitsPerComponent)
        {
            case 1:
                pixelFormat = PixelFormat.Format1bppIndexed;
                break;
            case 8:
                pixelFormat = PixelFormat.Format8bppIndexed;
                break;
            case 24:
                pixelFormat = PixelFormat.Format24bppRgb;
                break;
            default:
                throw new Exception("Unknown pixel format " + bitsPerComponent);
        }

        Bitmap bmp = new Bitmap(width, height, pixelFormat);
        var bmpData = bmp.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.WriteOnly, pixelFormat);
        int length = (int)Math.Ceiling(width * bitsPerComponent / 8.0);
        for (int i = 0; i < height; i++)
        {
            int offset = i * length;
            int scanOffset = i * bmpData.Stride;
            Marshal.Copy(decodedBytes, offset, new IntPtr(bmpData.Scan0.ToInt32() + scanOffset), length);
        }
        bmp.UnlockBits(bmpData);
        using (FileStream fs = new FileStream(@"C:\Export\PdfSharp\" + String.Format("Image{0}.png", count), FileMode.Create, FileAccess.Write))
        {
            bmp.Save(fs, System.Drawing.Imaging.ImageFormat.Png);
        }

这是正确的方法吗？还是我应该选择另一种方式？非常感谢!

Answer 1

我知道这个答案可能要晚几年，但也许会对其他人有所帮助。

在我的例子中出现了这种困惑，因为 image.Elements.GetInteger(PdfImage.Keys.BitsPerComponent) 似乎没有返回正确的值。作为Vive la déraison根据您的问题指出，您将获得使用 Marshal.Copy 的 BGR 格式。因此，在执行 Marshal.Copy 后反转字节并旋转位图将完成这项工作。

生成的代码如下所示:

private static void ExportAsPngImage(PdfDictionary image, ref int count)
    {
        int width = image.Elements.GetInteger(PdfImage.Keys.Width);
        int height = image.Elements.GetInteger(PdfImage.Keys.Height);

        var canUnfilter = image.Stream.TryUnfilter();
        byte[] decodedBytes;

        if (canUnfilter)
        {
            decodedBytes = image.Stream.Value;
        }
        else
        {
            PdfSharp.Pdf.Filters.FlateDecode flate = new PdfSharp.Pdf.Filters.FlateDecode();
            decodedBytes = flate.Decode(image.Stream.Value);
        }

        int bitsPerComponent = 0;
        while (decodedBytes.Length - ((width * height) * bitsPerComponent / 8) != 0)
        {
            bitsPerComponent++;
        }

        System.Drawing.Imaging.PixelFormat pixelFormat;
        switch (bitsPerComponent)
        {
            case 1:
                pixelFormat = System.Drawing.Imaging.PixelFormat.Format1bppIndexed;
                break;
            case 8:
                pixelFormat = System.Drawing.Imaging.PixelFormat.Format8bppIndexed;
                break;
            case 16:
                pixelFormat = System.Drawing.Imaging.PixelFormat.Format16bppArgb1555;
                break;
            case 24:
                pixelFormat = System.Drawing.Imaging.PixelFormat.Format24bppRgb;
                break;
            case 32:
                pixelFormat = System.Drawing.Imaging.PixelFormat.Format32bppArgb;
                break;
            case 64:
                pixelFormat = System.Drawing.Imaging.PixelFormat.Format64bppArgb;
                break;
            default:
                throw new Exception("Unknown pixel format " + bitsPerComponent);
        }

        decodedBytes = decodedBytes.Reverse().ToArray();

        Bitmap bmp = new Bitmap(width, height, pixelFormat);
        BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.WriteOnly, bmp.PixelFormat);
        int length = (int)Math.Ceiling(width * (bitsPerComponent / 8.0));
        for (int i = 0; i < height; i++)
        {
            int offset = i * length;
            int scanOffset = i * bmpData.Stride;
            Marshal.Copy(decodedBytes, offset, new IntPtr(bmpData.Scan0.ToInt32() + scanOffset), length);
        }
        bmp.UnlockBits(bmpData);
        bmp.RotateFlip(RotateFlipType.Rotate180FlipNone);
        bmp.Save(String.Format("exported_Images\\Image{0}.png", count++), System.Drawing.Imaging.ImageFormat.Png);
    }

代码可能需要一些优化，但在我的例子中它确实正确地导出了 FlateDecoded Images。