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Is custom preprocessing available? #2492

sob3kx opened this Issue Feb 10, 2019 · 2 comments


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sob3kx commented Feb 10, 2019


I am working on productivization of Keras's VGG16 model in C#. The VGG16's weights are adapted from Caffe and demands turning image into BGR and substracting mean of channels over dataset from the channels of pixel. If my explaination is not understandable, you can see the code here in the first else branch of _preprocess_numpy_input.

Is there an option to do such thing in ML.NET? If not, is there an option to skip the graph definition of preprocessing steps and defining own preprocessing? If not, are there any plans for such feature?



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Ivanidzo4ka commented Feb 10, 2019
It's not optimal but doable.
Regaridng BGR order, our pixel extractor right now doing ARGB order, and it's possible what till 1.0 release it will remain the same, but as workaround you can always create three transforms which would extract Blue, Green and Red channel and then combine them together.

Also if your preprocessing step is just to offset by certain value, you can use offset parameter in transform (we also have scale if you need to scale your values)

      var pipeline = mlContext.Transforms.LoadImages(imagesFolder, ("ImageObject", "ImagePath"))
                          .Append(mlContext.Transforms.Resize("ImageObject", imageWidth: 100, imageHeight: 100))
                          .Append(mlContext.Transforms.ExtractPixels(new ImageAnalytics.ImagePixelExtractingEstimator.ColumnInfo("Green", "ImageObject",
                          colors: ImageAnalytics.ImagePixelExtractingEstimator.ColorBits.Blue, offset: 103.939f))
                          .Append(mlContext.Transforms.ExtractPixels(new ImageAnalytics.ImagePixelExtractingEstimator.ColumnInfo("Red", "ImageObject",
                          colors: ImageAnalytics.ImagePixelExtractingEstimator.ColorBits.Blue, offset: 116.779f))
                          .Append(mlContext.Transforms.ExtractPixels(new ImageAnalytics.ImagePixelExtractingEstimator.ColumnInfo("Blue", "ImageObject",
                          colors: ImageAnalytics.ImagePixelExtractingEstimator.ColorBits.Blue, offset: 123.68f)))
                          .Append(mlContext.Transforms.Concatenate("GRB", "Green", "Red", "Blue"));

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sob3kx commented Feb 11, 2019

the channels processing part seems to be doing good work, although I would probably need more "combine" kind of function, than concatenate. Concatenate will take 3 columns and concatenate them in [a, b, c] order, while for image classification you need a[0], b[0], c[0], a[1], b[1], c[1] ...

I am going to try to find out how to combine columns in such way tommorow and report back.

@Ivanidzo4ka Ivanidzo4ka referenced a pull request that will close this issue Feb 18, 2019


[WIP] Introduce order for pixel extraction #2602

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