Add example for applying ONNX model to in-memory images (dotnet#3851)

* Add example for applying ONNX model to in-memory images

* Add expected outputs

docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/ApplyONNXModelWithInMemoryImages.cs

@@ -0,0 +1,98 @@
+using System;
+using System.Drawing;
+using System.Linq;
+using Microsoft.ML;
+using Microsoft.ML.Data;
+using Microsoft.ML.Transforms.Image;
+
+namespace Samples.Dynamic
+{
+    public static class ApplyOnnxModelWithInMemoryImages
+    {
+        // Example of applying ONNX transform on in-memory images.
+        public static void Example()
+        {
+            // Download the squeeznet image model from ONNX model zoo, version 1.2
+            // https://github.com/onnx/models/tree/master/squeezenet or use
+            // Microsoft.ML.Onnx.TestModels nuget.
+            // It's a multiclass classifier. It consumes an input "data_0" and produces
+            // an output "softmaxout_1".
+            var modelPath = @"squeezenet\00000001\model.onnx";
+
+            // Create ML pipeline to score the data using OnnxScoringEstimator
+            var mlContext = new MLContext();
+
+            // Create in-memory data points. Its Image/Scores field is the input/output of the used ONNX model.
+            var dataPoints = new ImageDataPoint[]
+            {
+                new ImageDataPoint(Color.Red),
+                new ImageDataPoint(Color.Green)
+            };
+
+            // Convert training data to IDataView, the general data type used in ML.NET.
+            var dataView = mlContext.Data.LoadFromEnumerable(dataPoints);
+
+            // Create a ML.NET pipeline which contains two steps. First, ExtractPixle is used to convert the 224x224 image to a 3x224x224 float tensor.
+            // Then the float tensor is fed into a ONNX model with an input called "data_0" and an output called "softmaxout_1". Note that "data_0" and
+            // "softmaxout_1" are model input and output names stored in the used ONNX model file. Users may need to inspect their own models to
+            // get the right input and output column names.
+            var pipeline = mlContext.Transforms.ExtractPixels("data_0", "Image")                   // Map column "Image" to column "data_0"
+                .Append(mlContext.Transforms.ApplyOnnxModel("softmaxout_1", "data_0", modelPath)); // Map column "data_0" to column "softmaxout_1"
+            var model = pipeline.Fit(dataView);
+            var onnx = model.Transform(dataView);
+
+            // Convert IDataView back to IEnumerable<ImageDataPoint> so that user can inspect the output, column "softmaxout_1", of the ONNX transform.
+            // Note that Column "softmaxout_1" would be stored in ImageDataPont.Scores because the added attributed [ColumnName("softmaxout_1")]
+            // tells that ImageDataPont.Scores is equivalent to column "softmaxout_1".
+            var transformedDataPoints = mlContext.Data.CreateEnumerable<ImageDataPoint>(onnx, false).ToList();
+
+            // The scores are probabilities of all possible classes, so they should all be positive. 
+            foreach (var dataPoint in transformedDataPoints)
+            {
+                var firstClassProb = dataPoint.Scores.First();
+                var lastClassProb = dataPoint.Scores.Last();
+                Console.WriteLine($"The probability of being the first class is {firstClassProb * 100}%.");
+                Console.WriteLine($"The probability of being the last class is {lastClassProb * 100}%.");
+            }
+
+            // Expected output:
+            //  The probability of being the first class is 0.002542659%.
+            //  The probability of being the last class is 0.0292684%.
+            //  The probability of being the first class is 0.02258059%.
+            //  The probability of being the last class is 0.394428%.
+        }
+
+        // This class is used in Example() to describe data points which will be consumed by ML.NET pipeline.
+        private class ImageDataPoint
+        {
+            // Height of Image.
+            private const int height = 224;
+
+            // Width of Image.
+            private const int width = 224;
+
+            // Image will be consumed by ONNX image multiclass classification model.
+            [ImageType(height, width)]
+            public Bitmap Image { get; set; }
+
+            // Expected output of ONNX model. It contains probabilities of all classes.
+            // Note that the ColumnName below should match the output name in the used
+            // ONNX model file.
+            [ColumnName("softmaxout_1")]
+            public float[] Scores { get; set; }
+
+            public ImageDataPoint()
+            {
+                Image = null;
+            }
+
+            public ImageDataPoint(Color color)
+            {
+                Image = new Bitmap(width, height);
+                for (int i = 0; i < width; ++i)
+                    for (int j = 0; j < height; ++j)
+                        Image.SetPixel(i, j, color);
+            }
+        }
+    }
+}

src/Microsoft.ML.ImageAnalytics/ExtensionsCatalog.cs

@@ -118,6 +118,7 @@ internal static ImageLoadingEstimator LoadImages(this TransformsCatalog catalog,
         /// <format type="text/markdown">
         /// <![CDATA[
         ///  [!code-csharp[ExtractPixels](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/ImageAnalytics/ExtractPixels.cs)]
+        ///  [!code-csharp[ApplyOnnxModel](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/ApplyONNXModelWithInMemoryImages.cs)]
         /// ]]></format>
         /// </example>
         public static ImagePixelExtractingEstimator ExtractPixels(this TransformsCatalog catalog,

src/Microsoft.ML.OnnxTransformer/OnnxCatalog.cs

@@ -45,6 +45,13 @@ public static OnnxScoringEstimator ApplyOnnxModel(this TransformsCatalog catalog
         /// <param name="modelFile">The path of the file containing the ONNX model.</param>
         /// <param name="gpuDeviceId">Optional GPU device ID to run execution on, <see langword="null" /> to run on CPU.</param>
         /// <param name="fallbackToCpu">If GPU error, raise exception or fallback to CPU.</param>
+        /// <example>
+        /// <format type="text/markdown">
+        /// <![CDATA[
+        /// [!code-csharp[ApplyOnnxModel](~/../docs/samples/docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/ApplyONNXModelWithInMemoryImages.cs)]
+        /// ]]>
+        /// </format>
+        /// </example>
         public static OnnxScoringEstimator ApplyOnnxModel(this TransformsCatalog catalog,
             string outputColumnName,
             string inputColumnName,

test/Microsoft.ML.OnnxTransformerTest/OnnxTransformTests.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Drawing;
 using System.IO;
 using System.Linq;
 using Microsoft.ML;
@@ -14,6 +15,7 @@
 using Microsoft.ML.StaticPipe;
 using Microsoft.ML.TestFramework.Attributes;
 using Microsoft.ML.Tools;
+using Microsoft.ML.Transforms.Image;
 using Microsoft.ML.Transforms.StaticPipe;
 using Xunit;
 using Xunit.Abstractions;
@@ -316,5 +318,85 @@ public void TestUnknownDimensions()
             Assert.Equal(0, predictions[1].argmax[0]);
             Assert.Equal(2, predictions[2].argmax[0]);
         }
+
+        /// <summary>
+        /// This class is used in <see cref="OnnxModelInMemoryImage"/> to describe data points which will be consumed by ML.NET pipeline.
+        /// </summary>
+        private class ImageDataPoint
+        {
+            /// <summary>
+            /// Height of <see cref="Image"/>.
+            /// </summary>
+            private const int height = 224;
+
+            /// <summary>
+            /// Width of <see cref="Image"/>.
+            /// </summary>
+            private const int width = 224;
+
+            /// <summary>
+            /// Image will be consumed by ONNX image multiclass classification model.
+            /// </summary>
+            [ImageType(height, width)]
+            public Bitmap Image { get; set; }
+
+            /// <summary>
+            /// Output of ONNX model. It contains probabilities of all classes.
+            /// </summary>
+            [ColumnName("softmaxout_1")]
+            public float[] Scores { get; set; }
+
+            public ImageDataPoint()
+            {
+                Image = null;
+            }
+
+            public ImageDataPoint(Color color)
+            {
+                Image = new Bitmap(width, height);
+                for (int i = 0; i < width; ++i)
+                    for (int j = 0; j < height; ++j)
+                        Image.SetPixel(i, j, color);
+            }
+        }
+
+        /// <summary>
+        /// Test applying ONNX transform on in-memory image.
+        /// </summary>
+        [OnnxFact]
+        public void OnnxModelInMemoryImage()
+        {
+            // Path of ONNX model. It's a multiclass classifier. It consumes an input "data_0" and produces an output "softmaxout_1".
+            var modelFile = "squeezenet/00000001/model.onnx";
+
+            // Create in-memory data points. Its Image/Scores field is the input/output of the used ONNX model.
+            var dataPoints = new ImageDataPoint[]
+            {
+                new ImageDataPoint(Color.Red),
+                new ImageDataPoint(Color.Green)
+            };
+
+            // Convert training data to IDataView, the general data type used in ML.NET.
+            var dataView = ML.Data.LoadFromEnumerable(dataPoints);
+
+            // Create a ML.NET pipeline which contains two steps. First, ExtractPixle is used to convert the 224x224 image to a 3x224x224 float tensor.
+            // Then the float tensor is fed into a ONNX model with an input called "data_0" and an output called "softmaxout_1". Note that "data_0" and
+            // "softmaxout_1" are model input and output names stored in the used ONNX model file. Users may need to inspect their own models to
+            // get the right input and output column names.
+            var pipeline = ML.Transforms.ExtractPixels("data_0", "Image")                   // Map column "Image" to column "data_0"
+                .Append(ML.Transforms.ApplyOnnxModel("softmaxout_1", "data_0", modelFile)); // Map column "data_0" to column "softmaxout_1"
+            var model = pipeline.Fit(dataView);
+            var onnx = model.Transform(dataView);
+
+            // Convert IDataView back to IEnumerable<ImageDataPoint> so that user can inspect the output, column "softmaxout_1", of the ONNX transform.
+            // Note that Column "softmaxout_1" would be stored in ImageDataPont.Scores because the added attributed [ColumnName("softmaxout_1")]
+            // tells that ImageDataPont.Scores is equivalent to column "softmaxout_1".
+            var transformedDataPoints = ML.Data.CreateEnumerable<ImageDataPoint>(onnx, false).ToList();
+
+            // The scores are probabilities of all possible classes, so they should all be positive.
+            foreach (var dataPoint in transformedDataPoints)
+                foreach (var score in dataPoint.Scores)
+                    Assert.True(score > 0);
+        }
     }
 }