visioneventing-csharp.md

Create Vision Handler - C#

Create Vision Handler

Let's start with creating an empty ASP.NET Core app:

dotnet new web -o vision

Inside the vision/csharp folder, update Startup.cs to match with what we have.

Change the log level in appsettings.json to Information:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information"
    }
  },
  "AllowedHosts": "*"
}

Add Vision API

Add Vision API NuGet package to our project:

dotnet add package Google.Cloud.Vision.V1

Startup.cs checks for storage#object events. These events are emitted by Cloud Storage when a file is uploaded.

dynamic json = JValue.Parse(content);
if (json == null) return;

var kind = json.kind;
if (kind == null || kind != "storage#object") return;

Next, we extract the Cloud Storage URL of the file from the event:

var storageUrl = (string)ConstructStorageUrl(json);

private string ConstructStorageUrl(dynamic json)
{
    return json == null? null 
        : string.Format("gs://{0}/{1}", json.bucket, json.name);
}

Finally, make a call to Vision API to extract labels from the image:

var labels = await ExtractLabelsAsync(storageUrl);

var message = "This picture is labelled: " + labels;
_logger.LogInformation(message);
await context.Response.WriteAsync(message);

private async Task<string> ExtractLabelsAsync(string storageUrl)
{
    var visionClient = ImageAnnotatorClient.Create();
    var labels = await visionClient.DetectLabelsAsync(Image.FromUri(storageUrl), maxResults: 10);

    var orderedLabels = labels
        .OrderByDescending(x => x.Score)
        .TakeWhile((x, i) => i <= 2 || x.Score > 0.50)
        .Select(x => x.Description)
        .ToList();

    return string.Join(",", orderedLabels.ToArray());
}

You can see the full code in Startup.cs.

Before building the Docker image, make sure the app has no compilation errors:

dotnet build

Create Dockerfile

Create a Dockerfile for the image.

What's Next?

Back to Integrate with Vision API