在 .NET 中校正文档图像的透视变形

目前，文档图像通常使用手机摄像头拍摄。显然，很难将摄像头理想地放置在被拍摄文档的正上方，因此几乎所有拍摄的图像都存在一定程度的透视变形。

以下是现代智能手机摄像头拍摄的旋转文档图像示例：




几乎所有用于处理电子文档的系统都要求图像没有旋转或变形。因此，校正（矫正）旋转或变形的图像就成了一项任务。
VintaSoft Document Cleanup .NET Plug-in 7.3 (VintaSoft Imaging .NET SDK 12.3) 添加了一个用于校正文档图像透视变形的命令。

DocumentPerspectiveCorrectionCommand 可用于解决以下问题：

• 校正从手机摄像头、网络摄像头或照相机接收到的文档图像的透视变形、倾斜校正和裁剪边缘
• 校正从图像扫描仪接收到的文档图像的倾斜校正和裁剪边缘

注意： DocumentPerspectiveCorrectionCommand 如果文档图像边框对比度良好，即人眼可以轻松辨认文档边框，则会返回良好的结果。

以下是经DocumentPerspectiveCorrectionCommand 处理的已校正文档图像示例：





以下 C# 代码演示了如何校正文档图像的透视变形：

/// <summary>
/// Corrects perspective distortion of document image.
/// </summary>
/// <param name="sourceFile">Source image file.</param>
/// <param name="resultFile">Result image file.</param>
public void ApplyPerspectiveCorrection(string sourceFile, string resultFile)
{
    using (Vintasoft.Imaging.VintasoftImage image =
        new Vintasoft.Imaging.VintasoftImage(sourceFile))
    {
        // create the perspective correction command
        Vintasoft.Imaging.ImageProcessing.Document.DocumentPerspectiveCorrectionCommand command =
            new Vintasoft.Imaging.ImageProcessing.Document.DocumentPerspectiveCorrectionCommand();

        // apply the perspective correction to a document image
        command.ExecuteInPlace(image);

        // save the result image to a file
        image.Save(resultFile);
    }
}

如果文档图像的宽高比已知，则必须使用 DocumentPerspectiveCorrectionCommand.DocumentAspectRatio 属性。
以下 C# 代码演示了如何校正 A4 文档图像的透视变形：

/// <summary>
/// Corrects perspective distortion of A4 document image.
/// </summary>
/// <param name="sourceFile">Source file.</param>
/// <param name="resultFile">Result file.</param>
public void ApplyPerspectiveCorrectionA4Paper(string sourceFile, string resultFile)
{
    using (Vintasoft.Imaging.VintasoftImage image =
        new Vintasoft.Imaging.VintasoftImage(sourceFile))
    {
        // create the perspective correction command
        Vintasoft.Imaging.ImageProcessing.Document.DocumentPerspectiveCorrectionCommand command =
            new Vintasoft.Imaging.ImageProcessing.Document.DocumentPerspectiveCorrectionCommand();

        // set the document aspect ratio to the aspect ratio of the A4 paper
        Vintasoft.Imaging.ImageSize paperSizeA4 = Vintasoft.Imaging.ImageSize.FromPaperKind(Vintasoft.Imaging.PaperSizeKind.A4);
        command.DocumentAspectRatio = paperSizeA4.WidthInDip / paperSizeA4.HeightInDip;

        // apply the perspective correction to a document image
        command.ExecuteInPlace(image);

        // save the result image to a file
        image.Save(resultFile);
    }
}

DocumentPerspectiveCorrectionCommand 使用 QuadrilateralWarpCommand 来执行透视校正变换。如果你知道扭曲图像的角点，那么你可以使用QuadrilateralWarpCommand来执行透视的逆变换。
以下 C# 代码演示了如何在已知扭曲图像的角点的情况下校正文档图像的透视变形：

/// <summary>
/// Corrects perspective distortion of document image.
/// </summary>
/// <param name="sourceFile">Source image file.</param>
/// <param name="resultFile">Result image file.</param>
/// <param name="documentImagePoints">An array of four points, which define the corner points of document image.
/// Points should be set in the following order: 0 - top-left, 1 - top-right,
/// 2 - bottom-left, 3 - bottom-right.</param>
public void ApplyQuadrilateralUnwarp(string sourceFile, string resultFile, System.Drawing.PointF[] documentImagePoints)
{
    using (Vintasoft.Imaging.VintasoftImage image =
        new Vintasoft.Imaging.VintasoftImage(sourceFile))
    {
        // create the perspective correction command
        Vintasoft.Imaging.ImageProcessing.Transforms.QuadrilateralWarpCommand command =
            new Vintasoft.Imaging.ImageProcessing.Transforms.QuadrilateralWarpCommand();

        // specify that command must use invert transform (command must work as unwarp command)
        command.IsInverseTransform = true;

        // set the corner points of document image
        command.DestinationPoints = documentImagePoints;

        // apply perspective correction to a document image
        command.ExecuteInPlace(image);

        // save the result image to a file
        image.Save(resultFile);
    }
}