保存和加载图像后使用LockBits / UnlockBits时数据损坏

Question

I'm writing a steganography program which supports multiple file types. There is a working implementation for WAV files and then one for image files, which is currently not working.

The image carrier is basically a wrapper around the .NET classes Bitmap and Image. It uses LockBits/UnlockBits to access the array of bytes contained in images. When the program is finished making changes to the byte array, it will copy it back into the Bitmap.

When the data has been copied and the Bitmap has been unlocked, I save it using:

image.Save(destination, ImageFormat.Png);

The next time I load the Image, I would expect that the byte array I saved earlier is identical to the one that I am going to extract from the saved Image (since PNG is lossless). However, this is not always the case.

The byte arrays are ALMOST identical, but SOME of the bytes with a value of 1 have been changed to 0. Other than this there is not really any pattern to it, seeing as it appears to happen to all channels.

Is there something about Image.Save or the PNG format that I have missed?

edit: When embedding the exact same data, it works for some images but not for all, even if they share the same attributes (both 24 bpp and PNG).

Answer 1

When saving an image you can specify different compression algorithms. The default one might produce this undesired effect. Try another one.

See:
- System.Drawing.Imaging.Encoder Compression
- System.Drawing.Imaging.EncoderValue Enumeration
- Image.Save Method (String, ImageCodecInfo, EncoderParameters)

Don't confuse these compression algorithms with the image quality. Image quality should not affect PNGs, as they use lossless compression.