private async Task GetPreviewFrameAsSoftwareBitmapAsync()
{
    // Get information about the preview
    var previewProperties = _mediaCapture.VideoDeviceController.GetMediaStreamProperties(MediaStreamType.VideoPreview) as VideoEncodingProperties;

    // Create the video frame to request a SoftwareBitmap preview frame
    var videoFrame = new VideoFrame(BitmapPixelFormat.Bgra8, (int)previewProperties.Width, (int)previewProperties.Height);

    // Capture the preview frame
    using (var currentFrame = await _mediaCapture.GetPreviewFrameAsync(videoFrame))
    {
        // Collect the resulting frame
        SoftwareBitmap previewFrame = currentFrame.SoftwareBitmap;

        // Add a simple green filter effect to the SoftwareBitmap
        EditPixels(previewFrame);
    }
}

private unsafe void EditPixels(SoftwareBitmap bitmap)
{
    // Effect is hard-coded to operate on BGRA8 format only
    if (bitmap.BitmapPixelFormat == BitmapPixelFormat.Bgra8)
    {
        // In BGRA8 format, each pixel is defined by 4 bytes
        const int BYTES_PER_PIXEL = 4;

        using (var buffer = bitmap.LockBuffer(BitmapBufferAccessMode.ReadWrite))
        using (var reference = buffer.CreateReference())
        {
            // Get a pointer to the pixel buffer
            byte* data;
            uint capacity;
            ((IMemoryBufferByteAccess)reference).GetBuffer(out data, out capacity);

            // Get information about the BitmapBuffer
            var desc = buffer.GetPlaneDescription(0);

            // Iterate over all pixels
            for (uint row = 0; row < desc.Height; row++)
            {
                for (uint col = 0; col < desc.Width; col++)
                {
                    // Index of the current pixel in the buffer (defined by the next 4 bytes, BGRA8)
                    var currPixel = desc.StartIndex + desc.Stride * row + BYTES_PER_PIXEL * col;

                    // Read the current pixel information into b,g,r channels (leave out alpha channel)
                    var b = data[currPixel + 0]; // Blue
                    var g = data[currPixel + 1]; // Green
                    var r = data[currPixel + 2]; // Red

                    // Boost the green channel, leave the other two untouched
                    data[currPixel + 0] = b;
                    data[currPixel + 1] = (byte)Math.Min(g + 80, 255);
                    data[currPixel + 2] = r;
                }
            }
        }
    }
}

[ComImport]
[Guid("5b0d3235-4dba-4d44-865e-8f1d0e4fd04d")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
unsafe interface IMemoryBufferByteAccess
{
    void GetBuffer(out byte* buffer, out uint capacity);
}