ArcGISPortalViewer.Model.TileService.DarkenImageBottom C# (CSharp) Method

        private async static Task DarkenImageBottom(string filename, string outfilename)
        {
            var file = await Windows.Storage.ApplicationData.Current.LocalFolder.GetFileAsync(filename);
            BitmapDecoder decoder = null;
            byte[] sourcePixels = null;
            using (IRandomAccessStream fileStream = await file.OpenReadAsync())
            {
                decoder = await BitmapDecoder.CreateAsync(fileStream);
                // Scale image to appropriate size 
                BitmapTransform transform = new BitmapTransform();
                PixelDataProvider pixelData = await decoder.GetPixelDataAsync(
                    BitmapPixelFormat.Bgra8, // WriteableBitmap uses BGRA format 
                    BitmapAlphaMode.Straight,
                    transform,
                    ExifOrientationMode.IgnoreExifOrientation, // This sample ignores Exif orientation 
                    ColorManagementMode.DoNotColorManage
                );
                // An array containing the decoded image data, which could be modified before being displayed 
                sourcePixels = pixelData.DetachPixelData();
                fileStream.Dispose();
            }

            for (uint col = 0; col < decoder.PixelWidth; col++)
            {
                for (uint row = 0; row < decoder.PixelHeight; row++)
                {
                    if (row < decoder.PixelHeight * .6) continue;
                    uint idx = (row * decoder.PixelWidth + col) * 4;
                    if (decoder.BitmapPixelFormat == BitmapPixelFormat.Bgra8 ||
                        decoder.BitmapPixelFormat == BitmapPixelFormat.Rgba8)
                    {
                        var frac = 1 - Math.Sin(((row / (double)decoder.PixelHeight) - .6) * (1 / .4));
                        byte b = sourcePixels[idx];
                        byte g = sourcePixels[idx + 1];
                        byte r = sourcePixels[idx + 2];
                        sourcePixels[idx] = (byte)(b * frac);
                        sourcePixels[idx + 1] = (byte)(g * frac);
                        sourcePixels[idx + 2] = (byte)(r * frac);
                    }
                }
            }

            var file2 = await Windows.Storage.ApplicationData.Current.LocalFolder.CreateFileAsync(outfilename, CreationCollisionOption.ReplaceExisting);

            var str = await file2.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite);
            BitmapEncoder enc = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, str);
            enc.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore, decoder.PixelWidth, decoder.PixelHeight,
                decoder.DpiX, decoder.DpiY, sourcePixels);
            await enc.FlushAsync();
            str.Dispose();
        }
    }