Add .NET-specific OpenCvSharp guides (#2092)

* Add .NET-specific OpenCvSharp guides

* Address documentation review feedback

Author: shimat

docs/docfx/articles/guides/aspnet-image-processing.md

@@ -0,0 +1,251 @@
+# ASP.NET Core Image Uploads and Streams
+
+An uploaded PNG or JPEG is compressed file data, while an OpenCvSharp `Mat` contains decoded pixels. A typical ASP.NET Core request therefore crosses two explicit conversion boundaries:
+
+1. Read and limit the uploaded bytes.
+2. Decode the bytes with `Cv2.ImDecode`.
+3. Process caller-owned `Mat` objects.
+4. Encode the result with `Cv2.ImEncode`.
+5. Return the encoded managed byte array.
+
+OpenCV does not decode an arbitrary .NET `Stream` incrementally. Even when ASP.NET Core receives the request as a stream, the encoded image must be collected into a contiguous buffer before calling `ImDecode`.
+
+## Process an IFormFile in a controller
+
+The following controller accepts one buffered multipart upload, converts it to grayscale, and returns a PNG:
+
+```csharp
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using OpenCvSharp;
+
+namespace ImageApi.Controllers;
+
+[ApiController]
+[Route("images")]
+public sealed class ImagesController : ControllerBase
+{
+    private const long MaxFileBytes = 10 * 1024 * 1024;
+    private const long MaxRequestBytes = 11 * 1024 * 1024;
+    private const long MaxDecodedPixels = 40_000_000;
+
+    [HttpPost("grayscale")]
+    [RequestSizeLimit(MaxRequestBytes)]
+    public async Task<IActionResult> Grayscale(
+        IFormFile file,
+        CancellationToken cancellationToken)
+    {
+        if (file.Length is <= 0 or > MaxFileBytes)
+        {
+            return BadRequest("Upload one non-empty image up to 10 MiB.");
+        }
+
+        using var encoded = new MemoryStream(
+            capacity: checked((int)file.Length));
+        await file.CopyToAsync(encoded, cancellationToken);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        Mat source;
+        try
+        {
+            source = Cv2.ImDecode(
+                encoded.GetBuffer().AsSpan(
+                    start: 0,
+                    length: checked((int)encoded.Length)),
+                ImreadModes.Color);
+        }
+        catch (OpenCVException)
+        {
+            return BadRequest("The upload could not be decoded.");
+        }
+
+        using (source)
+        {
+            if (source.Empty())
+            {
+                return BadRequest("The upload is not a supported image.");
+            }
+
+            if (source.Total() > MaxDecodedPixels)
+            {
+                return BadRequest("The decoded image is too large.");
+            }
+
+            cancellationToken.ThrowIfCancellationRequested();
+
+            using var grayscale = new Mat();
+            Cv2.CvtColor(
+                source,
+                grayscale,
+                ColorConversionCodes.BGR2GRAY);
+
+            if (!Cv2.ImEncode(".png", grayscale, out byte[] png))
+            {
+                return StatusCode(
+                    StatusCodes.Status500InternalServerError,
+                    "Could not encode the result.");
+            }
+
+            return File(png, "image/png");
+        }
+    }
+}
+```
+
+`IFormFile` is already buffered by ASP.NET Core, potentially in memory or a temporary file. Copying it into a `MemoryStream` creates a contiguous buffer for `ImDecode`. `GetBuffer()` avoids the additional array copy that `ToArray()` would make.
+
+The request limit is slightly larger than the file limit because a multipart request also contains boundaries and headers. Hosting layers such as IIS or a reverse proxy may impose their own limits.
+
+Do not trust `IFormFile.FileName` or `ContentType` to select a decoder or response type. Decode the content, choose the output format on the server, and return the media type that matches that encoder.
+
+## Decode an arbitrary Stream with a byte limit
+
+For `HttpRequest.Body`, blob storage, or another stream source, enforce a byte limit while buffering. This helper returns a caller-owned `Mat`:
+
+```csharp
+using OpenCvSharp;
+
+static async Task<Mat> DecodeImageAsync(
+    Stream source,
+    int maxEncodedBytes,
+    ImreadModes mode,
+    CancellationToken cancellationToken)
+{
+    ArgumentNullException.ThrowIfNull(source);
+    ArgumentOutOfRangeException.ThrowIfNegativeOrZero(maxEncodedBytes);
+
+    using var encoded = new MemoryStream(
+        capacity: Math.Min(maxEncodedBytes, 64 * 1024));
+    byte[] chunk = new byte[64 * 1024];
+
+    while (true)
+    {
+        int remaining = maxEncodedBytes - checked((int)encoded.Length);
+        int readSize = remaining >= chunk.Length
+            ? chunk.Length
+            : remaining + 1;
+        int read = await source.ReadAsync(
+            chunk.AsMemory(0, readSize),
+            cancellationToken);
+
+        if (read == 0)
+        {
+            break;
+        }
+
+        if (read > remaining)
+        {
+            throw new InvalidDataException(
+                "The encoded image exceeds the configured limit.");
+        }
+
+        await encoded.WriteAsync(
+            chunk.AsMemory(0, read),
+            cancellationToken);
+    }
+
+    if (encoded.Length == 0)
+    {
+        throw new InvalidDataException("The encoded image is empty.");
+    }
+
+    cancellationToken.ThrowIfCancellationRequested();
+
+    Mat image = Cv2.ImDecode(
+        encoded.GetBuffer().AsSpan(
+            start: 0,
+            length: checked((int)encoded.Length)),
+        mode);
+
+    if (!image.Empty())
+    {
+        return image;
+    }
+
+    image.Dispose();
+    throw new InvalidDataException(
+        "The stream does not contain a supported image.");
+}
+```
+
+The caller owns the returned matrix:
+
+```csharp
+using Mat image = await DecodeImageAsync(
+    Request.Body,
+    maxEncodedBytes: 10 * 1024 * 1024,
+    ImreadModes.Color,
+    HttpContext.RequestAborted);
+```
+
+This helper applies an application limit even when the stream does not report a length. Configure the server or endpoint request-body limit as the first line of defense so the application does not need to receive an oversized body before rejecting it.
+
+For very frequent requests, an `ArrayPool<byte>` or a recyclable stream can reduce managed buffer allocation. Return pooled storage only after `ImDecode` completes because the decoder reads the buffer synchronously during the call.
+
+## Validate encoded and decoded sizes
+
+An encoded file can expand to a much larger pixel buffer. Validate at multiple layers:
+
+- Request-body size at Kestrel, IIS, or the reverse proxy.
+- Multipart and per-file size before copying an `IFormFile`.
+- A byte limit while reading an arbitrary stream.
+- `Empty()`, dimensions, channel count, and decoded pixel count after `ImDecode`.
+- Application-specific limits before expensive processing or encoding.
+
+Checking decoded dimensions happens after the decoder has allocated the image. If hostile image formats or decompression bombs are in scope, use an image-header parser with strict dimension limits, isolate decoding in a constrained worker, or apply operating-system memory limits. A post-decode check alone cannot prevent the decoder's initial allocation.
+
+## Manage cancellation and CPU work
+
+`CopyToAsync` and `ReadAsync` observe a `CancellationToken`. Most `Cv2` calls are synchronous native operations and cannot be interrupted by an ASP.NET Core cancellation token after they start.
+
+Check cancellation before expensive stages and avoid starting new work after `HttpContext.RequestAborted` is signaled. For long-running or untrusted workloads, move processing to a bounded queue or worker process that has an explicit timeout and resource limits.
+
+Wrapping every `Cv2` call in `Task.Run` does not reduce its CPU or memory cost. Under load it can add ThreadPool contention. Use bounded concurrency, and benchmark it together with OpenCV's own native thread usage. `Cv2.SetNumThreads` changes process-wide OpenCV behavior, so configure it once only when measurements justify doing so.
+
+Each request should own its mutable matrices. Do not share a writable `Mat` between concurrent requests without external synchronization and a clear ownership model.
+
+## Return encoded data, not a Mat
+
+`Mat` is a native resource and is not an HTTP response representation. Encode it before leaving the request scope:
+
+```csharp
+if (!Cv2.ImEncode(".jpg", result, out byte[] jpeg, [
+    new ImageEncodingParam(
+        ImwriteFlags.JpegQuality,
+        90),
+]))
+{
+    throw new InvalidOperationException(
+        "Could not encode the JPEG response.");
+}
+
+return Results.File(jpeg, "image/jpeg");
+```
+
+The returned `byte[]` is managed and remains valid after the source and result matrices are disposed. Encoding necessarily creates a compressed output buffer; it is not a zero-copy view of the `Mat`.
+
+## Deploy without a desktop GUI
+
+ASP.NET Core services normally do not use `Cv2.ImShow`, WPF, or another desktop UI. Choose the headless runtime package when the service needs the full non-GUI OpenCV module set, or the slim package only when its reduced modules are sufficient.
+
+Native runtime assets must match the deployment runtime identifier and architecture. Containers may also require native dependencies for the selected image codecs. Log `Cv2.GetVersionString()` and `Cv2.GetBuildInformation()` when production codec behavior differs from development.
+
+## Related guides
+
+- [Image Encoding and Conversion](image-conversion.md)
+- [Copies, Native Memory, and Performance](memory-copy-and-performance.md)
+- [Common Errors and Diagnostics](../troubleshooting/common-errors.md)
+- [Native Library Loading](../troubleshooting/native-library-loading.md)
+
+## Official references
+
+- [OpenCV image file reading and writing](https://docs.opencv.org/5.0/main_modules/imgcodecs.html)
+- [Upload files in ASP.NET Core](https://learn.microsoft.com/aspnet/core/mvc/models/file-uploads)
+- [Configure Kestrel server limits](https://learn.microsoft.com/aspnet/core/fundamentals/servers/kestrel/options)
+
+## Related OpenCvSharp API
+
+- [Cv2](xref:OpenCvSharp.Cv2)
+- [Mat](xref:OpenCvSharp.Mat)
+- [ImreadModes](xref:OpenCvSharp.ImreadModes)
+- [ImageEncodingParam](xref:OpenCvSharp.ImageEncodingParam)

docs/docfx/articles/guides/image-conversion.md

@@ -77,6 +77,8 @@ For file uploads, HTTP responses, and database blobs, encode directly with `Cv2.
 
 See [Display Images in .NET Applications](displaying-images-dotnet.md) for WPF, Avalonia, GDI+, UI-thread, and live-frame guidance.
 
+See [ASP.NET Core Image Uploads and Streams](aspnet-image-processing.md) for bounded upload buffering, `IFormFile`, request cancellation, and HTTP responses.
+
 ## Official OpenCV references
 
 - [OpenCV image file reading and writing](https://docs.opencv.org/5.0/main_modules/imgcodecs.html)