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Image Resizer

doc/devdocs/modules/imageresizer.md

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Image Resizer

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Image Resizer is a Windows shell extension that enables batch resizing of images directly from File Explorer.

Overview

Image Resizer provides a convenient way to resize images without opening a dedicated image editing application. It is accessible through the Windows context menu and allows users to:

  • Resize single or multiple images at once
  • Choose from predefined sizing presets or enter custom dimensions
  • Maintain or modify aspect ratios
  • Create copies or overwrite original files
  • Apply custom filename formats to resized images
  • Select different encoding quality settings

Architecture

Image Resizer consists of multiple components:

  • Shell Extension DLL (context menu integration)
  • WinUI 3 UI application
  • Core image processing library

Technology Stack

  • C++/WinRT
  • WPF (UI components)
  • Windows Imaging Component (WIC) for image processing
  • COM for shell integration

Context Menu Integration

Image Resizer integrates with the Windows context menu following the PowerToys Context Menu Handlers pattern. It uses a dual registration approach to ensure compatibility with both Windows 10 and Windows 11.

Registration Process

The context menu integration follows the same pattern as PowerRename, using:

  • A traditional shell extension for Windows 10
  • A sparse MSIX package for Windows 11 context menus

For more details on the implementation approach, see the Dual Registration section in the context menu documentation.

Context Menu Appearance Logic

Image Resizer dynamically determines when to show the context menu option:

  • AppxManifest.xml registers the extension for all file types (Type="*")
  • The shell extension checks if the selected files are images using AssocGetPerceivedType()
  • The menu appears only for image files (returns ECS_ENABLED); otherwise, it remains hidden (returns ECS_HIDDEN)

This approach provides flexibility to support additional file types by modifying only the detection logic without changing the system-level registration.

UI Implementation

Image Resizer uses WPF for its user interface, as evidenced by the App.xaml.cs file. The UI allows users to:

  • Select from predefined size presets or enter custom dimensions
  • Configure filename format for the resized images
  • Set encoding quality and format options
  • Choose whether to replace or create copies of the original files

From the App.xaml.cs file, we can see that the application:

  • Supports localization through LanguageHelper.LoadLanguage()
  • Processes command line arguments via ResizeBatch.FromCommandLine()
  • Uses a view model pattern with MainViewModel
  • Respects Group Policy settings via GPOWrapper.GetConfiguredImageResizerEnabledValue()

Debugging

Debugging the Context Menu

See the Debugging Context Menu Handlers section for general guidance on debugging PowerToys context menu extensions.

For Image Resizer specifically, there are several approaches:

Option 1: Manual Registration via Registry

  1. Create a registry file (e.g., register.reg) with the following content:
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\CLSID\{51B4D7E5-7568-4234-B4BB-47FB3C016A69}]
@="PowerToys Image Resizer Extension"

[HKEY_CLASSES_ROOT\CLSID\{51B4D7E5-7568-4234-B4BB-47FB3C016A69}\InprocServer32]
@="D:\\PowerToys\\x64\\Debug\\PowerToys.ImageResizerExt.dll"
"ThreadingModel"="Apartment"

[HKEY_CURRENT_USER\Software\Classes\SystemFileAssociations\.png\ShellEx\ContextMenuHandlers\ImageResizer]
@="{51B4D7E5-7568-4234-B4BB-47FB3C016A69}"
  1. Import the registry file:
reg import register.reg
  1. Restart Explorer to apply changes:
taskkill /f /im explorer.exe && start explorer.exe

  1. Attach the debugger to explorer.exe

  2. Add breakpoints to relevant code in the Image Resizer shell extension

Option 2: Using regsvr32

  1. Register the shell extension DLL:
regsvr32 "D:\PowerToys\x64\Debug\PowerToys.ImageResizerExt.dll"
  1. Restart Explorer and attach the debugger as in Option 1

Common Issues

  • Context menu not appearing:

    • Ensure the extension is properly registered
    • Verify you're right-clicking on supported image files
    • Restart Explorer to clear context menu cache

  • For Windows 11, check AppX package registration:

    • Use get-appxpackage -Name *imageresizer* to verify installation
    • Use Remove-AppxPackage to remove problematic registrations

  • Missing UI or processing failures:

    • Check Event Viewer for application errors
    • Verify file permissions for both source and destination folders