Images

Means for converting images, and scaling their quality are readily available in the .NET Framework version of the RadPdfProcessing library. In contrast, the .NET Standard one does not provide such functionality and requires some manual settings to achieve this. The FixedExtensibilityManager class is exposed specifically to address this need. More information on how to configure it can be found in the code samples later in this article.

Exporting Images

To reduce file size, PDF supports only a number of compression filters like Jpeg and Jpeg2000 compression of color and grayscale images. So to allow the library to export images different than Jpeg and Jpeg2000, these images should be additionally processed. The .NET Standard specification does not define APIs for converting/processing images or scaling their quality. That is why, to export images different than Jpeg and Jpeg2000 or ImageQuality different than High, PdfProcessing comes with two extensibility points exposed by the static FixedExtensibilityManager class - ImagePropertiesResolver and JpegImageConverter.

If neither ImagePropertiesResolver and JpegImageConverter are set, an InvalidOperationException is thrown during export of document.

ImagePropertiesResolver

This property enables you to set a resolver implementation that can parse the image raw data to separate its colors and alpha channel. While this implementation can be used for any type of supported image, it is required when working with PNG images so their transparency can be preserved in the generated PDF document.

Default Implementation

PdfProcessing comes with a default implementation for such resolver called ImagePropertiesResolver. The built-in logic depends on the ImageSharp library to parse the image data. To use the default functionality, you should add a reference to the Telerik.Documents.ImageUtils.dll assembly and set an instance of the ImagePropertiesResolver class it to the FixedExtensibilityManager.ImagePropertiesResolver property.

Telerik.Documents.ImageUtils.dll is not available for Xamarin. If targeting this platform, you will need to implement a custom resolver.

Example 1: Set the default implementation of the ImagePropertiesResolver class

    Telerik.Documents.ImageUtils.ImagePropertiesResolver defaultImagePropertiesResolver = new Telerik.Documents.ImageUtils.ImagePropertiesResolver(); 
    Telerik.Windows.Documents.Extensibility.FixedExtensibilityManager.ImagePropertiesResolver = defaultImagePropertiesResolver; 

Custom Implementation

In case you have specific requirements and the default ImagePropertiesResolver doesn't fit them, you can implement custom logic that can handle them. To achieve that, you should: 1. inherit the Telerik.Windows.Documents.Core.Imaging.ImagePropertiesResolverBase class 1. implement its members 1. assign an instance of the custom implementation to the FixedExtensibilityManager.ImagePropertiesResolver property

JpegImageConverter

The JpegImageConverter property uses an implementation of the JpegImageConverterBase abstract class to convert an image to Jpeg. This implementation should be passed to the JpegImageConverter property of the of FixedExtensibilityManager.

If you have both the ImagePropertiesResolver and JpegImageConverter properties set, the ImagePropertiesResolver is prioritized and used to parse the image.

Default Implementation

The Telerik.Documents.ImageUtils assembly provides a default implementation of the JpegImageConverter class that could be used when exporting a document. The default implementation depends on the ImageSharp library to convert images to Jpeg format.

Telerik.Documents.ImageUtils.dll is not available for Xamarin. A custom converter can be implemented by inheriting Telerik.Windows.Documents.Extensibility.JpegImageConverterBase.

Example 2: Set the default implementation of the JpegImageConverter class

   Telerik.Windows.Documents.Extensibility.JpegImageConverterBase defaultJpegImageConverter = new Telerik.Documents.ImageUtils.JpegImageConverter(); 
   Telerik.Windows.Documents.Extensibility.FixedExtensibilityManager.JpegImageConverter = defaultJpegImageConverter; 

The following example depends on the Magick.NET library to convert images to Jpeg format.

Example 3: Create a custom implementation inheriting the JpegImageConverterBase abstract class

    internal class CustomJpegImageConverter : Telerik.Windows.Documents.Extensibility.JpegImageConverterBase 
    { 
        public override bool TryConvertToJpegImageData(byte[] imageData, ImageQuality imageQuality, out byte[] jpegImageData) 
        { 
            string[] magickImageFormats = Enum.GetNames(typeof(MagickFormat)).Select(x => x.ToLower()).ToArray(); 
            string imageFormat; 
            if (this.TryGetImageFormat(imageData, out imageFormat) && magickImageFormats.Contains(imageFormat.ToLower())) 
            { 
                using (MagickImage magickImage = new MagickImage(imageData)) 
                { 
                    magickImage.Format = MagickFormat.Jpeg; 
                    magickImage.Alpha(AlphaOption.Remove); 
                    magickImage.Quality = (int)imageQuality; 
 
                    jpegImageData = magickImage.ToByteArray(); 
                } 
 
                return true; 
            } 
 
            jpegImageData = null; 
            return false; 
        } 
    } 

Example 4: Set the custom implementation to the JpegImageConverter property of the FixedExtensibilityManager

    JpegImageConverterBase customJpegImageConverter = new CustomJpegImageConverter(); 
    Telerik.Windows.Documents.Extensibility.FixedExtensibilityManager.JpegImageConverter = customJpegImageConverter; 

A complete SDK example of a custom implementation JpegImageConverterBase is available on our GitHub repository.