Some images are being rotated when resized

Question

In a nutshell the purpose of the following code is to resize an image based on the target size and the multiplier (1x, 2x, 3x). This works fine except for some reason I have

Answer 1

Thanks to the excellent help by Chris Farmer¹ and Mark Ransom² I was able to answer my own question.

The core problem was the EXIF data on some of the images was altering the orientation of the images. When I resized the images all the EXIF data was stripped off. Since the EXIF data was stripped the viewers didn't know the image should be orientated differently. That caused me to think the resizer code was rotating some images. It is worth noting that this orientation information doesn't show up in the details view when you right click on the image in Windows Explorer. You need to search for it in the image property objects or use an online view like this one.

Using the data from here³ I was able to create the following named constants:

private const int OrientationKey = 0x0112;
private const int NotSpecified = 0;
private const int NormalOrientation = 1;
private const int MirrorHorizontal = 2;
private const int UpsideDown = 3;
private const int MirrorVertical = 4;
private const int MirrorHorizontalAndRotateRight = 5;
private const int RotateLeft = 6;
private const int MirorHorizontalAndRotateLeft = 7;
private const int RotateRight = 8;

Using those named constants I extended my ResizeImage method to read:

public void ResizeImage(TargetSize targetSize, ResizeMultiplier multiplier, Stream input, Stream output)
{
    using (var image = Image.FromStream(input))
    {
        // Calculate the resize factor
        var scaleFactor = targetSize.CalculateScaleFactor(image.Width, image.Height);
        scaleFactor /= (int)multiplier; 

        var newWidth = (int)Math.Floor(image.Width / scaleFactor);
        var newHeight = (int)Math.Floor(image.Height / scaleFactor);
        using (var newBitmap = new Bitmap(newWidth, newHeight))
        {
            using (var imageScaler = Graphics.FromImage(newBitmap))
            {
                imageScaler.CompositingQuality = CompositingQuality.HighQuality;
                imageScaler.SmoothingMode = SmoothingMode.HighQuality;
                imageScaler.InterpolationMode = InterpolationMode.HighQualityBicubic;

                var imageRectangle = new Rectangle(0, 0, newWidth, newHeight);
                imageScaler.DrawImage(image, imageRectangle);

                // Fix orientation if needed.
                if (image.PropertyIdList.Contains(OrientationKey))
                {
                    var orientation = (int)image.GetPropertyItem(OrientationKey).Value[0];
                    switch (orientation)
                    {
                        case NotSpecified: // Assume it is good.
                        case NormalOrientation:
                            // No rotation required.
                            break;
                        case MirrorHorizontal:
                            newBitmap.RotateFlip(RotateFlipType.RotateNoneFlipX);
                            break;
                        case UpsideDown:
                            newBitmap.RotateFlip(RotateFlipType.Rotate180FlipNone);
                            break;
                        case MirrorVertical:
                            newBitmap.RotateFlip(RotateFlipType.Rotate180FlipX);
                            break;
                        case MirrorHorizontalAndRotateRight:
                            newBitmap.RotateFlip(RotateFlipType.Rotate90FlipX);
                            break;
                        case RotateLeft:
                            newBitmap.RotateFlip(RotateFlipType.Rotate90FlipNone);
                            break;
                        case MirorHorizontalAndRotateLeft:
                            newBitmap.RotateFlip(RotateFlipType.Rotate270FlipX);
                            break;
                        case RotateRight:
                            newBitmap.RotateFlip(RotateFlipType.Rotate270FlipNone);
                            break;
                        default:
                            throw new NotImplementedException("An orientation of " + orientation + " isn't implemented.");
                    }
                }
                newBitmap.Save(output, image.RawFormat);
            }
        }
    }
}

An observant person would notice that most of the additional code was pulled from this answer to a related question.

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¹: ^{Who was right on the money but I didn't understand what he was driving at.}

²:^{Who showed me what Chris Farmer was trying to say.}

³:^{The Orientation key value was confirmed here.}