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.
Requirements
To export images different than Jpeg and Jpeg2000 or ImageQuality different than High you will need to add references to the following .Net Standard assemblies:
| Assembly/ NuGet package | Description |
|---|---|
| Telerik.Documents.ImageUtils | This assembly is currently not available in UI for Xamarin. |
| SkiaSharp.NativeAssets.* (version 2.88.8) | May differ according to the used platform. For Linux use SkiaSharp.NativeAssets.Linux.NoDependencies |
| SkiaSharp.Views.Blazor and wasm-tools | For Blazor Web Assembly. |
With the R2 2023 changes SkiaSharp replaced ImageSharp as the required dependency.
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. The resolver gets the image bytes as they are and does not take into consideration the Image Quality set through the Export Settings.
Default Implementation for ImagePropertiesResolver
PdfProcessing comes with a default implementation for such resolver called ImagePropertiesResolver. The built-in logic depends on the SkiaSharp library to parse the image data. To use the default functionality, you should set an instance of the ImagePropertiesResolver class to the FixedExtensibilityManager.ImagePropertiesResolver property.
View Implementation Requirements.
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 for ImagePropertiesResolver
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
2. Implement its members
3. 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
ImagePropertiesResolverandJpegImageConverterproperties set, theImagePropertiesResolveris prioritized and used to parse the image.
Default Implementation for JpegImageConverter
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 SkiaSharp library to convert images to Jpeg format.
View Implementation Requirements.
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;
Custom Implementation for JpegImageConverter
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)
{
IMagickFormatInfo? formatInfo = MagickFormatInfo.Create(imageData);
if (formatInfo != null && formatInfo.SupportsReading)
{
using (MagickImage magickImage = new MagickImage(imageData))
{
magickImage.Alpha(AlphaOption.Remove);
magickImage.Quality = (int)imageQuality;
jpegImageData = magickImage.ToByteArray(MagickFormat.Jpeg);
}
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.