Quickstart: Generate a thumbnail using the REST API and C# in Computer Vision

In this quickstart, you generate a thumbnail from an image by using Computer Vision's REST API. With the Get Thumbnail method, you can generate a thumbnail of an image. You specify the height and width, which can differ from the aspect ratio of the input image. Computer Vision uses smart cropping to intelligently identify the area of interest and generate cropping coordinates based on that region.

If you don't have an Azure subscription, create a free account before you begin.


Create and run the sample application

To create the sample in Visual Studio, do the following steps:

  1. Create a new Visual Studio solution in Visual Studio, using the Visual C# Console App template.
  2. Install the Newtonsoft.Json NuGet package.
    1. On the menu, click Tools, select NuGet Package Manager, then Manage NuGet Packages for Solution.
    2. Click the Browse tab, and in the Search box type "Newtonsoft.Json".
    3. Select Newtonsoft.Json when it displays, then click the checkbox next to your project name, and Install.
  3. Replace the code in Program.cs with the following code, and then make the following changes in code where needed:
    1. Replace the value of subscriptionKey with your subscription key.
    2. Replace the value of uriBase with the endpoint URL for the Get Thumbnail method from the Azure region where you obtained your subscription keys, if necessary.
  4. Run the program.
  5. At the prompt, enter the path to a local image.
using Newtonsoft.Json.Linq;
using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

namespace CSHttpClientSample
    static class Program
        // Replace <Subscription Key> with your valid subscription key.
        const string subscriptionKey = "<Subscription Key>";

        // You must use the same Azure region in your REST API method as you used to
        // get your subscription keys. For example, if you got your subscription keys
        // from the West US region, replace "westcentralus" in the URL
        // below with "westus".
        // Free trial subscription keys are generated in the "westus" region.
        // If you use a free trial subscription key, you shouldn't need to change
        // this region.
        const string uriBase =

        static void Main()
            // Get the path and filename to process from the user.
                "Enter the path to the image you wish to use to create a thumbnail image: ");
            string imageFilePath = Console.ReadLine();

            if (File.Exists(imageFilePath))
                // Call the REST API method.
                Console.WriteLine("\nWait a moment for the results to appear.\n");
                Console.WriteLine("\nInvalid file path");
            Console.WriteLine("\nPress Enter to exit...");

        /// <summary>
        /// Gets a thumbnail image from the specified image file by using
        /// the Computer Vision REST API.
        /// </summary>
        /// <param name="imageFilePath">The image file to use to create the thumbnail image.</param>
        static async Task MakeThumbNailRequest(string imageFilePath)
                HttpClient client = new HttpClient();

                // Request headers.
                    "Ocp-Apim-Subscription-Key", subscriptionKey);

                // Request parameters.
                // The width and height parameters specify a thumbnail that's 
                // 200 pixels wide and 150 pixels high.
                // The smartCropping parameter is set to true, to enable smart cropping.
                string requestParameters = "width=200&height=150&smartCropping=true";

                // Assemble the URI for the REST API method.
                string uri = uriBase + "?" + requestParameters;

                HttpResponseMessage response;

                // Read the contents of the specified local image
                // into a byte array.
                byte[] byteData = GetImageAsByteArray(imageFilePath);

                // Add the byte array as an octet stream to the request body.
                using (ByteArrayContent content = new ByteArrayContent(byteData))
                    // This example uses the "application/octet-stream" content type.
                    // The other content types you can use are "application/json"
                    // and "multipart/form-data".
                    content.Headers.ContentType =
                        new MediaTypeHeaderValue("application/octet-stream");

                    // Asynchronously call the REST API method.
                    response = await client.PostAsync(uri, content);

                // Check the HTTP status code of the response. If successful, display
                // display the response and save the thumbnail.
                if (response.IsSuccessStatusCode)
                    // Display the response data.
                    Console.WriteLine("\nResponse:\n{0}", response);

                    // Get the image data for the thumbnail from the response.
                    byte[] thumbnailImageData =
                        await response.Content.ReadAsByteArrayAsync();

                    // Save the thumbnail to the same folder as the original image,
                    // using the original name with the suffix "_thumb".
                    // Note: This will overwrite an existing file of the same name.
                    string thumbnailFilePath =
                        imageFilePath.Insert(imageFilePath.Length - 4, "_thumb");
                    File.WriteAllBytes(thumbnailFilePath, thumbnailImageData);
                    Console.WriteLine("\nThumbnail written to: {0}", thumbnailFilePath);
                    // Display the JSON error data.
                    string errorString = await response.Content.ReadAsStringAsync();
            catch (Exception e)
                Console.WriteLine("\n" + e.Message);

        /// <summary>
        /// Returns the contents of the specified file as a byte array.
        /// </summary>
        /// <param name="imageFilePath">The image file to read.</param>
        /// <returns>The byte array of the image data.</returns>
        static byte[] GetImageAsByteArray(string imageFilePath)
            // Open a read-only file stream for the specified file.
            using (FileStream fileStream =
                new FileStream(imageFilePath, FileMode.Open, FileAccess.Read))
                // Read the file's contents into a byte array.
                BinaryReader binaryReader = new BinaryReader(fileStream);
                return binaryReader.ReadBytes((int)fileStream.Length);

Examine the response

A successful response is returned as binary data, which represents the image data for the thumbnail. If the request succeeds, the thumbnail is saved to the same folder as the local image, using the original name with the suffix "_thumb". If the request fails, the response contains an error code and a message to help determine what went wrong.

The sample application displays a successful response in the console window, similar to the following example:


StatusCode: 200, ReasonPhrase: 'OK', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
  Pragma: no-cache
  apim-request-id: 131eb5b4-5807-466d-9656-4c1ef0a64c9b
  Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
  x-content-type-options: nosniff
  Cache-Control: no-cache
  Date: Tue, 06 Jun 2017 20:54:07 GMT
  X-AspNet-Version: 4.0.30319
  X-Powered-By: ASP.NET
  Content-Length: 5800
  Content-Type: image/jpeg
  Expires: -1

Clean up resources

When no longer needed, delete the Visual Studio solution. To do so, open File Explorer, navigate to the folder in which you created the Visual Studio solution, and delete the folder.

Next steps

Explore a basic Windows application that uses Computer Vision to perform optical character recognition (OCR); create smart-cropped thumbnails; plus detect, categorize, tag, and describe visual features, including faces, in an image. To rapidly experiment with the Computer Vision APIs, try the Open API testing console.