Imageflow Dotnet Server

A super-fast image server for ASP.NET 6/7

.NET 6/7

Imageflow.NET Server is image processing and optimizing middleware for ASP.NET 6 & 7.

If you don't need an HTTP server, try Imageflow.NET. If you don't want to use .NET, try Imageflow, which has a server, command-line tool, and library with language bindings for Go, C, Rust, Node, Ruby and more. Imageflow is specifically designed for web servers and focuses on security, quality, and performance.

Serving optimized and correctly sized images is the fastest way to a quicker, more profitable site or app. 60% of website bytes are from images[1].

Imageflow.NET Server edits and optimizes images so quickly you can do it on-demand. No need to manually generate every size/format combination of every image.

Imageflow Server DiagramQuerystring animation

[1]According to the HTTP Archive, 60% of the data transferred to fetch a web page is images composed of JPEGs, PNGs and GIFs.

What else can it do?

  • Automatically crop away whitespace
  • Sharpen
  • Fix white balance
  • Apply watermarks
  • Adjust contrast/saturation/brightness
  • Rotate & Flip images
  • Crop
  • Resize & Constrain
  • Produce highly optimized jpeg images to reduce download times
  • More

All operations are designed to be fast enough for on-demand use.


  • Supports Windows, Mac, and Linux
  • Comes with Dockerfiles for cloud deployment.
  • Processes images located on disk, Azure Blob Storage or Amazon S3
  • Size-constrained Disk Caching with a write-ahead-log
  • Mapping arbitrary virtual paths to physical ones.
  • Imageflow Querystring API (compatible with ImageResizer)
  • Production-ready for trusted image files.


We offer commercial licenses at, or you can use Imageflow, Imageflow.NET and Imageflow.NET Server under the terms of the AGPLv3.

For users coming from ImageResizer

For help migrating from ImageResizer, see the migrating from ImageResizer section and open an issue or email [email protected] if you have any questions.

Basic Installation

You can look at examples/Imageflow.Server.ExampleMinimal to see the result.

These steps assume you want to serve and modify images from the wwwroot folder. You can call .SetMapWebRoot(false).MapPath("/", physicalPath) to map a different physical folder. For examples on serving files from S3 or Azure, see the full example after this.

  1. Create a new ASP.NET Core 3.1 project using the Empty template.
  2. Create a directory called "wwwroot" and add a file "image.jpg"
  3. Install Imageflow.Server
    dotnet add package Imageflow.Server
  4. Open Startup.cs and edit the Configure method. Add
    app.UseImageflow(new ImageflowMiddlewareOptions()
  5. Replace the endpoint with something that generates an image tag, like
    app.UseEndpoints(endpoints =>
        endpoints.MapGet("/", async context =>
            context.Response.ContentType = "text/html";
            await context.Response.WriteAsync("<img src=\"image.jpg?width=450\" />");
  6. Run your project and see the image be dynamically resized.
  7. If you're not open-sourcing your project, get a trial license key:
    app.UseImageflow(new ImageflowMiddlewareOptions()
        .SetLicenseKey(EnforceLicenseWith.RedDotWatermark, "License 50913...."));
    Then visit /imageflow.license to verify your license status. This page will be publicly available and show whether you are choosing AGPLv3 mode or using a license key.

Installing everything

See examples/Imageflow.Server.Example for this example.

dotnet add package Imageflow.Server
dotnet add package Imageflow.Server.HybridCache
dotnet add package Imageflow.Server.Storage.S3
dotnet add package Imageflow.Server.Storage.AzureBlob

Note: Older versions of Windows may not have the C Runtime installed (Install 32-bit or 64-bit).

using Amazon;
using Azure.Storage.Blobs;
using Imageflow.Fluent;
using Imageflow.Server.Storage.AzureBlob;
using Imageflow.Server.Storage.RemoteReader;
using Imageflow.Server.Storage.S3;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;
using System.IO;
using Imageflow.Server.HybridCache;

namespace Imageflow.Server.Example
    public class Startup
        public Startup(IConfiguration configuration, IWebHostEnvironment env)
            Configuration = configuration;
            Env = env;

        private IConfiguration Configuration { get; }
        private IWebHostEnvironment Env { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)

            // See the README in src/Imageflow.Server.Storage.RemoteReader/ for more advanced configuration
            // To add the RemoteReaderService, you need to all .AddHttpClient() first
            services.AddImageflowRemoteReaderService(new RemoteReaderServiceOptions
                    SigningKey = "ChangeMe"
            // Make S3 containers available at /ri/ and /imageflow-resources/
            // If you use credentials, do not check them into your repository
            // You can call AddImageflowS3Service multiple times for each unique access key
            services.AddImageflowS3Service(new S3ServiceOptions( null,null)
                .MapPrefix("/ri/", RegionEndpoint.USEast1, "resizer-images")
                .MapPrefix("/imageflow-resources/", RegionEndpoint.USWest2, "imageflow-resources")
                .MapPrefix("/custom-s3client/", () => new AmazonS3Client(), "custom-client", "", false, false)
            // Make Azure container available at /azure
            // You can call AddImageflowAzureBlobService multiple times for each connection string
                new AzureBlobServiceOptions(
                        new BlobClientOptions())
                    .MapPrefix("/azure", "imageflow-demo" ));

            // Custom blob services can do whatever you need. See CustomBlobService.cs in src/Imageflow.Service.Example
            services.AddImageflowCustomBlobService(new CustomBlobServiceOptions()
                Prefix = "/custom_blobs/",
                IgnorePrefixCase = true,
                ConnectionString = "UseDevelopmentStorage=true;",
                // Only allow 'my_container' to be accessed. /custom_blobs/my_container/key.jpg would be an example path.
                ContainerKeyFilterFunction = (container, key) =>
                    container == "my_container" ? Tuple.Create(container, key) : null

            var homeFolder = (Environment.OSVersion.Platform == PlatformID.Unix ||
                   Environment.OSVersion.Platform == PlatformID.MacOSX)
                    ? Environment.GetEnvironmentVariable("HOME")
                    : Environment.ExpandEnvironmentVariables("%HOMEDRIVE%%HOMEPATH%");

            // You can add a hybrid cache (in-memory persisted database for tracking filenames, but files used for bytes)
            // But remember to call ImageflowMiddlewareOptions.SetAllowCaching(true) for it to take effect
            // If you're deploying to azure, provide a disk cache folder *not* inside ContentRootPath
            // to prevent the app from recycling whenever folders are created.
                new HybridCacheOptions(Path.Combine(homeFolder, "imageflow_example_hybrid_cache"))
                    // How long after a file is created before it can be deleted
                    MinAgeToDelete = TimeSpan.FromSeconds(10),
                    // How much RAM to use for the write queue before switching to synchronous writes
                    QueueSizeLimitInBytes = 100 * 1000 * 1000,
                    // The maximum size of the cache (1GB)
                    CacheSizeLimitInBytes = 1024 * 1024 * 1024,


        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
            if (env.IsDevelopment())
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see

            // You have a lot of configuration options
            app.UseImageflow(new ImageflowMiddlewareOptions()
                // Maps / to WebRootPath
                // You can get a license key at
                .SetLicenseKey(EnforceLicenseWith.RedDotWatermark, "License 50913....")
                // Maps /folder to WebRootPath/folder
                .MapPath("/folder", Path.Combine(Env.ContentRootPath, "folder"))
                // Allow localhost to access the diagnostics page or remotely via /imageflow.debug?password=fuzzy_caterpillar
                .SetDiagnosticsPageAccess(env.IsDevelopment() ? AccessDiagnosticsFrom.AnyHost : AccessDiagnosticsFrom.LocalHost)
                // Allow HybridCache or other registered IStreamCache to run
                // Cache publicly (including on shared proxies and CDNs) for 30 days
                .SetDefaultCacheControlString("public, max-age=2592000")
                // Allows extensionless images to be served within the given directory(ies)
                .HandleExtensionlessRequestsUnder("/custom_blobs/", StringComparison.OrdinalIgnoreCase)
                // Force all paths under "/gallery" to be watermarked
                .AddRewriteHandler("/gallery", args =>
                    args.Query["watermark"] = "imazen";
                .AddCommandDefault("down.filter", "mitchell")
                .AddCommandDefault("f.sharpen", "15")
                .AddCommandDefault("webp.quality", "90")
                .AddCommandDefault("ignore_icc_errors", "true")
                //When set to true, this only allows ?preset=value URLs, returning 403 if you try to use any other commands. 
                .AddPreset(new PresetOptions("large", PresetPriority.DefaultValues)
                    .SetCommand("width", "1024")
                    .SetCommand("height", "1024")
                    .SetCommand("mode", "max"))
                // When set, this only allows urls with a &signature, returning 403 if missing/invalid. 
                // Use Imazen.Common.Helpers.Signatures.SignRequest(string pathAndQuery, string signingKey) to generate
                //.ForPrefix allows you to set less restrictive rules for subfolders. 
                // For example, you may want to allow unmodified requests through with SignatureRequired.ForQuerystringRequests
                // .SetRequestSignatureOptions(
                //     new RequestSignatureOptions(SignatureRequired.ForAllRequests, new []{"test key"})
                //         .ForPrefix("/logos/", StringComparison.Ordinal, 
                //             SignatureRequired.ForQuerystringRequests, new []{"test key"}))
                // It's a good idea to limit image sizes for security. Requests causing these to be exceeded will fail
                // The last argument to FrameSizeLimit() is the maximum number of megapixels
                .SetJobSecurityOptions(new SecurityOptions()
                    .SetMaxDecodeSize(new FrameSizeLimit(8000,8000, 40))
                    .SetMaxFrameSize(new FrameSizeLimit(8000,8000, 40))
                    .SetMaxEncodeSize(new FrameSizeLimit(8000,8000, 20)))
                // Register a named watermark that floats 10% from the bottom-right corner of the image
                // With 70% opacity and some sharpness applied. 
                    new NamedWatermark("imazen",
                        new WatermarkOptions()
                                new WatermarkFitBox(WatermarkAlign.Image, 10, 10, 90, 90),
                                new ConstraintGravity(100, 100))
                                new ResampleHints()
                                    .SetResampleFilters(InterpolationFilter.Robidoux_Sharp, null)
                                    .SetSharpen(7, SharpenWhen.Downscaling))
                            .SetMinCanvasSize(200, 150)))
                .AddWatermarkingHandler("/", args =>
                    if (args.Query.TryGetValue("water", out var value) && value == "mark")
                        args.AppliedWatermarks.Add(new NamedWatermark(null, "/images/imazen_400.png", new WatermarkOptions()));
            app.UseEndpoints(endpoints =>
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");

Migrating from ImageResizer

General Notes

  • Imageflow does not let shadows overwhelm image highlights (it resizes images in linear RGB instead of averaging compressed sRGB values). This is correct behavior, but can lower the visual impact of certain images while improving most. To restore the old behavior add down.colorspace=srgb. To do this site-wide, use .AddCommandDefault("down.colorspace", "srgb")
  • Imageflow focuses on correctness, so it does not over-sharpen images by default. For some image types, additional sharpening is appropriate. You can make images slightly sharper by using the Mitchell resampling filter with .AddCommandDefault("down.filter", "mitchell"). You can add stronger sharpening with .AddCommandDefault("f.sharpen", "15")
  • Unlike ImageResizer, Imageflow does not support .TIFF files. Please convert them to .png or .jpg before migrating to Imageflow.NET Server. There is no secure open-source codec for .TIFF files, so we chose to not support the format.
  • Nearly all querystring commands are supported, with few infrequently exceptions:
    • We no longer support adding borders to images as that can be done better in CSS.
    • We no longer support adding drop shadows; this can also be done in CSS.
    • Rotation must be in intervals of 90 degrees.
    • Blurring and noise removal is not yet supported.
  • Blob storage providers now expect blobs to be treated as immutable, as there is too much latency to check the modified date.
  • Most popular plugins are now built-in, including WebP, AnimatedGifs, PrettyGifs, SimpleFilters, FastScaling, Watermark, VirtualFolder, ClientCache, AutoRotate, and WhitespaceTrimmer.
  • The following plugins are not available: DropShadow, Gradient, Image404, RedEye, Faces, SeamCarving, WIC, TinyCache, PsdReader, PsdComposer, MongoReader, FreeImage, FFMpeg, AdvancedFilters, CopyMetadata.
  • SqlReader functionality can be replicated by implementing Imazen.Common.Storage.IBlobProvider.
  • Blob Providers now only need to implement Imazen.Common.Storage.IBlobProvider, which greatly simplifies plugging in new storage.

Querystring Command Migration Details

Nearly all features are supported

  • The following commands are supported: mode, anchor, flip, sflip, quality, zoom, dpr, crop, cropxunits, cropyunits, w, h, width, height, maxwidth, maxheight, format, srotate, rotate, stretch, webp.lossless, webp.quality, f.sharpen, f.sharpen_when, down.colorspace, bgcolor, jpeg_idct_downscale_linear, watermark, s.invert, s.sepia, s.grayscale, s.alpha, s.brightness, s.contrast, s.saturation, trim.threshold, trim.percentpadding, a.balancewhite,, decoder.min_precise_scaling_ratio, scale, preset, s.roundcorners, 'ignoreicc'
  • TIFF files are not supported, so page=x is not supported.
  • Animated GIFs are fully supported, so frame=x is ignored.
  • Images are always auto-rotated based on Exif information, so autorotate is ignored. This is a breaking change from ImageResizer 4.x where images are not autorotated by default.
  • Images can only be rotated in 90 degree intervals, so rotate is partially supported.
  • PNG encoding adapts the palette size as needed, so colors is ignored.
  • PNG and GIFs are always dithered, so dither is ignored.
  • Jpeg subsampling is auto-selected by chroma evaluation, so subsampling is ignored.
  • Adding margins, padding, and borders to images is obsolete, so paddingwidth, paddingheight, margin borderwidth, bordercolor and paddingcolor are now ignored.
  • Caching, processing, and encoders/builders/decoders are not configurable via the querystring, so cache, process, encoder, decoder, and builder are ignored.
  • Sharpening is now done with f.sharpen, not a.sharpen, and a.sharpen is ignored.
  • Noise removal is not yet supported, so a.removenoise is ignored.
  • Blurring is not yet supported, so a.blur is ignored.
  • 404 redirects are not implemented, so 404 is ignored.

Integrations into Other Systems

The following platforms / solutions have a direct integration

  1. Oqtane Blazor platform (website / docs / git): Use the ToSic.ImageFlow.Oqtane (git / nuget) middleware. The package will automatically appear in Oqtane as an installable extension.

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