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WebRSize Configuration Guide


WebRSize currently only works with ASP.NET hosted with IIS Integrated Pipeline Mode. A version for ASP.NET Core will be released later.

In order to reduce overhead and improve security, WebRSize requires explicit opt-in for the image folders you want processed. You must register a WebRSize ConfigSection to store this configuration.

  <section name="webrsize" type="PhotoSauce.WebRSize.WebRSizeSection" />

A minimal configuration must specify the disk cache location and at least one image folder.

  <diskCache path="/webrsizecache" />
    <add name="images" path="/images/" />

The folder specified in the diskCache element must exist, and the App Pool identity must have write access to that folder. Requests matching paths specified in the imageFolders element are eligible for processing by WebRSize. All other requests pass through WebRSize unchanged.


WebRSize will not perform any processing at all unless at least one image folder is configured.

Processing and caching are managed by an IHttpModule implementation called WebRSizeModule. This module examines inbound requests matching the configured image folder paths and either serves a cached already-processed image or processes the image on the fly and schedules a copy to be saved to the disk cache.

The WebRSizeModule must receive event notifications for static image requests in your configured folders in order to do its job. There are multiple ways to accomplish this.

  1. The module will automatically register itself by using the assembly PreApplicationStartMethodAttribute. For some configurations, this may be adequate. However, modules registered programmatically are set up with the ManagedHandler PreCondition, which means the module will only receive even notifications if a managed IHttpHandler is associated with the file extension being requested. For most IIS configurations, image file extensions will be mapped to the unmanaged (and very efficient) IIS static file handler. In these cases, the self-registered module will not receive event notifications and won't work.

  2. You can register the module explicitly in the system.webServer section.

        <add name="WebRSize" type="PhotoSauce.WebRSize.WebRSizeModule" />

    Omit the preCondition attribute on the module entry to ensure it will see all requests, managed or not. The WebRSizeModule detects when it has been manually registered and will skip self-registration. This option will always work and is the safe way to go if you are in doubt.

  3. You can set up an explicit handler mapping for image file extension you want to process that maps to a managed IHttpHandler.

        <add name="ManagedJpeg" path="*.jpg" verb="GET" type="System.Web.StaticFileHandler" />

    This is common in scenarios where images will be served from a VirtualPathProvider that uses a database or cloud storage. The built-in StaticFileHandler will request files from the registered VirtualPathProvider, and because the handler is managed, the self-registered WebRSizeModule instance will be able to intercept image processing requests.


Configuration Reference

Disk Cache

The diskCache element configures local disk caching for WebRSize. Disk caching is mandatory, so this configuration element is required.

Cache Root

The path attribute specifies the site-relative path to the local root cache folder. The folder must exist, and the App Pool Identity must have write access to the folder.

If you perform regular maintenance on the disk cache folder, such as deleting subfolders, it is recommended that you create the cache folder outside your web application root and create an NTFS junction within the app that points to the external cache folder. This prevents unwanted App Pool recycling when subfolders are deleted.

Cache File Naming

The namingStrategy attribute specifies a Type name of a class that inherits from CacheFileNamingStrategyBase. There are three preconfigured naming strategies included with WebRSize:

  • PhotoSauce.WebRSize.CacheFileNamingStrategyMirror mirrors the subfolder and file structure from the source imageFolder. If your master images are well-organized, with only a few images per folder, this makes it easy to see what's in the cache and to clean up cache files that belong to master images when they are deleted.
  • PhotoSauce.WebRSize.CacheFileNamingStrategyDistribute1K distributes cache files across up to 1024 folders within the cache root based on a hash of the input path. All cache files belonging to a single master image will be placed in the same cache folder, but the folders are not easy to guess. Use this strategy if the folder structure for your master images is unwieldy or if you have so many files per folder that storing all their cached variants in one folder would result in too many files.
  • PhotoSauce.WebRSize.CacheFileNamingStrategyDistribute1M distributes cache files across up to 1024 folders within the cache root and then up to another 1024 files within each of those folders, giving you up to 1 MegaFolders (I just made that up). Choose this one for the same reasons as above but if your image library is so large that 1024 folders would still have too many files in each.

The file names generated by each of the included naming strategies are the same and look something like this: H3BD4R4N.test1.121x150.jpg

The pattern used is as follows: [40-Bit Settings Hash Base32 Encoded].[Original File Name].[Width]x[Height].[Extension]

The hash is used as the first part of the file name to prevent performance issues associated with 8.3-compatible name collisions. If you have different preferences, you may create your own naming strategy by inheriting the CacheFileNamingStrategyBase class and implementing the abstract GetCacheFilePath method.

Note that WebRSize uses a normalized version of the settings when creating the Settings Hash so that parameter combinations that yield the same file net results all share the same cache file. For example, imagine you have a master JPEG image that is 3000x2000 pixels. The following query string parameter sets would all generate the same image, so they would all serve the same file from the cache.

... and many, many more

Image Folders

The imageFolders element defines the collection of source image folders WebRSize can process. Use the add element to define an ImageFolder and configure its settings.


The path attribute defines a base folder that contains source images to be processed. The path is specified as a site-relative folder, like /images/. Any requests matching this path are examined for processing by WebRSize. It is best if this path contains only images, as WebRSize does not examine the file extension of the request to before trying to decode the requested file as an image. Due to the fact that WIC allows codec plugins, there is no good way of knowing in advance whether or not a file is an image.

This is the only attribute that is required for an ImageFolder definition.

Force Processing

The forceProcessing attribute accepts a boolean value to enable or disable mandatory processing of images in the ImageFolder. If, for example, a source image is requested with no query string parameters, you can force WebRSize to process and re-encode the image using its default settings or your own configured defaults. This allows you to store high-quality master images but makes sure a more web-friendly copy is served, even at full resolution.

Default Value: false

Allow Enlarge

The allowEnlarge attribute accepts a boolean value indicating whether or not WebRSize is allowed to serve a processed image that is larger than the source. Most web browsers do a fine job of enlarging images, so enlarging them on the server is generally a waste of server resources and bandwidth. When allowEnlarge is false, if a request specifies dimensions greater than those of the source image, WebRSize will change the settings to return the image in its original size or a cropped version thereof.

Default Value: false

Max Pixels

The maxPixels attribute allows you to limit the size of images served by WebRSize. The limit is defined as total pixels rather than a hard limit on width or height. A request whose width*height exceeds the maxPixels value will generate a 400 (Bad Request) response.

Default Value 10,000,000

Default Settings

The defaultSettings element allows you to define a collection of key/value pairs that specify settings you would like applied to all WebRSize requests in the absence of an override on the query string. For example, if you wish to disable sharpening on an entire image folder, you need not append the sharpen=false value to all request query strings. You can configure it as a folder default.

  <diskCache path="/webrsizecache" />
    <add name="images" path="/images/" forceProcessing="true">
        <add key="width" value="300"/>
        <add key="sharpen" value="false"/>

Any parameter value that would be valid on the query string of a request can be specified as a default for an ImageFolder. If a value has a default set but is also specified on the query string of a request, the default is overridden.

Query String Parameters

The following MagicScaler ProcessImageSettings values can be set from the query string when processing an image through a configured ImageFolder. See the MagicScaler documentation for more details.

FrameIndex frame
Width width
Height height
Crop crop [left(int)],[top(int)],[width(int)],[height(int)]
CropBasis cropbasis [width(int)],[height(int)]
CropAnchor anchor [vertical(center, top, bottom)]|[horizontal(center, left, right)]
ResizeMode mode crop, max, stretch
Sharpen sharpen bool
HybridMode hybrid favorquality, favorspeed, turbo, off
SaveFormat format jpg, jpeg, png, png8, gif, bmp, tiff
JpegQuality quality
JpegSubsampleMode subsample 420, 422, 444
MatteColor bgcolor
[CSS3 named color], [rgba hex values]
Interpolation filter nearestneighbor, average, linear, quadratic, catrom, cubic, lanczos, spline36

Additionally, WebRSize supports a devicepixelratio (or dpr) query string parameter. This setting will automatically adjust both the size and quality settings for the target client. Using srcset, you can configure images with the same logical (CSS) size and have WebRSize take care of the rest. In this example, a retina device will receive a 200px wide image with a lower jpeg quality/subsampling to compensate and keep file sizes reasonable.

<img src="photo.jpg?w=100" srcset="photo.jpg?w=100 1x, photo.jpg?w=100&dpr=2 2x" />

Advanced Topics

Output Caching

When WebRSize determines that an image processing request can be served from the disk cache, it rewrites the request to the cache location so that IIS can map its unmanaged static file handler to serve the image. This is done as quickly and efficiently as possible, but it misses out on one performance advantage static images normally have.

Normally static images can be cached and served by the http.sys kernel cache. Unfortunately, a couple of properties of processed image requests make http.sys consider those requests ineligible for output caching. In our case, it is both the presence of a query string and the fact that the request URL is rewritten that cause it to be ineligible. In order to force those requests to be eligible for kernel caching, you must explicitly configure a kernelCachePolicy.

This sample policy ensures that any requests for .jpg files can be kernel-cached as long as the URL (including query string) matches. This will allow your processed images to be served with performance equal to that of static image files once they've been cached to disk.

      <add extension=".jpg" kernelCachePolicy="CacheUntilChange" location="Any" />

You can verify kernel caching is working for processed image requests with the netsh http show cachestate command. The output looks something like this when items are cached:

PS C:\> netsh http show cachestate

Snapshot of HTTP response cache:

URL: http://localhost/images/test1.jpg?w=150&h=150&mode=max
    Status code: 200
    HTTP verb: GET
    Cache policy type: User invalidates
    Creation time: 2015.5.4:23.9.11:0
    Request queue name: webrsize
    Content type: image/jpeg
    Content encoding: (null)
    Headers length: 212
    Content length: 21335
    Hit count: 3
    Force disconnect after serving: FALSE

There are several other restrictions that affect caching, so you may not see many (or any) responses cached, even if you have a policy configured. By default, a single URL has to be requested more than twice within a 10-second window to be considered cache-worthy, and the response must be smaller than 256KB.

The kernel cache is configurable in the caching element, the serverRuntime element and the registry if you wish to tweak its default settings.