Quickstart: Generate a thumbnail using the Computer Vision SDK and C#

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In this quickstart, you will generate a smart-cropped thumbnail from an image using the Computer Vision SDK for C#. If you wish, you can download the code in this guide as a complete sample app from the Cognitive Services Csharp Vision repo on GitHub.

Prerequisites

GenerateThumbnailAsync method

You can use these methods to 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.

To run the sample, do the following steps:

  1. Create a new Visual C# Console App in Visual Studio.

  2. Install the Computer Vision client library 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 "Microsoft.Azure.CognitiveServices.Vision.ComputerVision".
    3. Select Microsoft.Azure.CognitiveServices.Vision.ComputerVision when it displays, then click the checkbox next to your project name, and Install.

  3. Replace Program.cs with the following code. The GenerateThumbnailAsync and GenerateThumbnailInStreamAsync methods wrap the Get Thumbnail API for remote and local images, respectively.

    using Microsoft.Azure.CognitiveServices.Vision.ComputerVision;

using System;
using System.IO;
using System.Threading.Tasks;

namespace ImageThumbnail
{
    class Program
    {
        private const bool writeThumbnailToDisk = false;

        // subscriptionKey = "0123456789abcdef0123456789ABCDEF"
        private const string subscriptionKey = "<SubscriptionKey>";

        // localImagePath = @"C:\Documents\LocalImage.jpg"
        private const string localImagePath = @"<LocalImage>";

        private const string remoteImageUrl =
            "https://upload.wikimedia.org/wikipedia/commons/9/94/Bloodhound_Puppy.jpg";

        private const int thumbnailWidth = 100;
        private const int thumbnailHeight = 100;

        static void Main(string[] args)
        {
            ComputerVisionClient computerVision = new ComputerVisionClient(
                new ApiKeyServiceClientCredentials(subscriptionKey),
                new System.Net.Http.DelegatingHandler[] { });

            computerVision.Endpoint = "https://api.cognitive.azure.cn";

            Console.WriteLine("Images being analyzed ...\n");
            var t1 = GetRemoteThumbnailAsync(computerVision, remoteImageUrl);
            var t2 = GetLocalThumbnailAsnc(computerVision, localImagePath);

            Task.WhenAll(t1, t2).Wait(5000);
            Console.WriteLine("Press ENTER to exit");
            Console.ReadLine();
        }

        // Create a thumbnail from a remote image
        private static async Task GetRemoteThumbnailAsync(
            ComputerVisionClient computerVision, string imageUrl)
        {
            if (!Uri.IsWellFormedUriString(imageUrl, UriKind.Absolute))
            {
                Console.WriteLine(
                    "\nInvalid remoteImageUrl:\n{0} \n", imageUrl);
                return;
            }

            Stream thumbnail = await computerVision.GenerateThumbnailAsync(
                thumbnailWidth, thumbnailHeight, imageUrl, true);

            string path = Environment.CurrentDirectory;
            string imageName = imageUrl.Substring(imageUrl.LastIndexOf('/') + 1);
            string thumbnailFilePath =
                path + "\\" + imageName.Insert(imageName.Length - 4, "_thumb");

            // Save the thumbnail to the current working directory,
            // using the original name with the suffix "_thumb".
            SaveThumbnail(thumbnail, thumbnailFilePath);
        }

        // Create a thumbnail from a local image
        private static async Task GetLocalThumbnailAsnc(
            ComputerVisionClient computerVision, string imagePath)
        {
            if (!File.Exists(imagePath))
            {
                Console.WriteLine(
                    "\nUnable to open or read localImagePath:\n{0} \n", imagePath);
                return;
            }

            using (Stream imageStream = File.OpenRead(imagePath))
            {
                Stream thumbnail = await computerVision.GenerateThumbnailInStreamAsync(
                    thumbnailWidth, thumbnailHeight, imageStream, true);

                string thumbnailFilePath =
                    localImagePath.Insert(localImagePath.Length - 4, "_thumb");

                // Save the thumbnail to the same folder as the original image,
                // using the original name with the suffix "_thumb".
                SaveThumbnail(thumbnail, thumbnailFilePath);
            }
        }

        // Save the thumbnail locally.
        // NOTE: This will overwrite an existing file of the same name.
        private static void SaveThumbnail(Stream thumbnail, string thumbnailFilePath)
        {
            if (writeThumbnailToDisk)
            {
                using (Stream file = File.Create(thumbnailFilePath))
                {
                    thumbnail.CopyTo(file);
                }
            }
            Console.WriteLine("Thumbnail {0} written to: {1}\n",
                writeThumbnailToDisk ? "" : "NOT", thumbnailFilePath);
        }
    }
}

  4. Replace <Subscription Key> with your valid subscription key.

  5. Change computerVision.Endpoint to the Azure region associated with your subscription keys, if necessary.

  6. Optionally, replace <LocalImage> with the path and file name of a local image (will be ignored if not set).

  7. Optionally, set remoteImageUrl to a different image.

  8. Optionally, set writeThumbnailToDisk to true to save the thumbnail to disk.

  9. Run the program.

Examine the response

A successful response saves the thumbnail for each image locally and displays the thumbnail's location, for example:

Thumbnail written to: C:\Documents\LocalImage_thumb.jpg

Thumbnail written to: ...\bin\Debug\Bloodhound_Puppy_thumb.jpg

Next steps

Explore the Computer Vision APIs used to analyze an image, detect celebrities and landmarks, create a thumbnail, and extract printed and handwritten text.

Explore Computer Vision APIs