Image hashing in UWP/C#, the image repeats horizontally after scaling and graying

Question

I am following this tutorial on image hashing.

So far I have achieved the following:

Code:

    private async Task ProcessI         


        

Answer 1

Your ConvertToGrayAsync method should simply return the converted WriteableBitmap. The pixel conversion code inside the loop could also be simplified, and the method does not need to operate on a source and destination buffer. It could as well manipulate the pixel values in place.

private async Task<WriteableBitmap> ConvertToGrayAsync(WriteableBitmap srcBitmap)
{
    var pixels = srcBitmap.PixelBuffer.ToArray();

    for (int i = 0; i < pixels.Length; i += 4)
    {
        var b = pixels[i];
        var g = pixels[i + 1];
        var r = pixels[i + 2];
        var f = (byte)(0.21 * r + 0.71 * g + 0.07 * b);
        pixels[i] = f;
        pixels[i + 1] = f;
        pixels[i + 2] = f;
    }

    var dstBitmap = new WriteableBitmap(srcBitmap.PixelWidth, srcBitmap.PixelHeight);

    using (var pixelStream = dstBitmap.PixelBuffer.AsStream())
    {
        await pixelStream.WriteAsync(pixels, 0, pixels.Length);
    }

    return dstBitmap;
}

The following method loads a WriteableBitmap with a predefined size:

private async Task<WriteableBitmap> LoadWriteableBitmapAsync(
    StorageFile file, int width, int height)
{
    using (var fileStream = await file.OpenReadAsync())
    using (var memoryStream = new InMemoryRandomAccessStream())
    {
        var decoder = await BitmapDecoder.CreateAsync(fileStream);
        var transform = new BitmapTransform
        {
            ScaledWidth = (uint)width,
            ScaledHeight = (uint)height,
            InterpolationMode = BitmapInterpolationMode.Cubic
        };
        var pixelData = await decoder.GetPixelDataAsync(
            BitmapPixelFormat.Bgra8, BitmapAlphaMode.Straight, transform,
            ExifOrientationMode.RespectExifOrientation,
            ColorManagementMode.ColorManageToSRgb);
        var pixels = pixelData.DetachPixelData();

        var encoder = await BitmapEncoder.CreateAsync(
            BitmapEncoder.PngEncoderId, memoryStream);

        encoder.SetPixelData(
            BitmapPixelFormat.Bgra8, BitmapAlphaMode.Straight,
            (uint)width, (uint)height, 96, 96, pixels);

        await encoder.FlushAsync();
        memoryStream.Seek(0);

        var bitmap = new WriteableBitmap(width, height);
        await bitmap.SetSourceAsync(memoryStream);
        return bitmap;
    }
}

Your ProcessImageAsync method would now look like this:

private async Task<WriteableBitmap> ProcessImageAsync(
    StorageFile file, int width, int height)
{
    return await ConvertToGrayAsync(
        await LoadWriteableBitmapAsync(file, width, height));
}