Mirror writing is a strange thing. I discovered I could mirror-write quite easily when I was 9 or 10 years old. It was fun to write backward messages on paper that could only be read in a mirror or flipped over and held up in front of a bright window. But why can some people do it, and others can't? Why do small children struggle to remember which way a lowercase b or d goes; or a g or p?
I did a search on mirror-writing and came across an interesting article written in 2002 called, Acquired mirror writing and reading: evidence for reflective graphemic representations by Gottfried, Sancar and Chatterjee.
"In summary, we suggest that mirror reading and writing, which at first glance appear to be a neuropsychological oddity, point to general principles underlying the processing and representation of visual forms. The adaptive advantage of mirror equivalency means that the nervous system harbors visual representations in their normal and reflected forms. The need for orientation specificity means that these different forms must also be distinguishable, and thus can be damaged selectively. These organizational principles that apply to objects int he world generalize to other visual objects, such as letters and words, even thought the adaptive advantage for these visual forms no longer applies. Finally, the functional modularity of visual representations themselves means that better access to one kind of mirrored visual form (lexical graphemes in this case) does not mean that access to all mirrored visual forms is privileged."
In real life, we see an object and it can flip over and we know it is still the same object. What is unnatural is that something would remain forever rigid and fixed in one direction orientation. For example, a leaf on a tree doesn't remain fixed, forever pointing in the same direction for the duration of its lifetime. It can move around and flip over and we still see it as a leaf.
What is unnatural is than an object outline or shape would remain fixed in such a way as to always be pointing in the same direction. For example, in the case of lower case letters b and d, children have difficulty learning which way they go because they are the same reflected shape. The direction of the letter is learned, based on knowlege about the letter and its context in words that are also learned. This learning has to override our natural ability to see and know a shape and its mirror reflection as one and the same object. The child has to learn through repetition that the b and the d may look the same, but point in different directions, so they are not the same letter. This learning, applied to two-dimensional representations, runs counter to our natural categorization of visual images in a three-dimensional world. So the child learns that certain letters are directional, really that all writing is directional. But for those people with strong visual mirroring abilities, the learned process of writing letters with a particular direction can easily be inverted.
So, shape direction or orientation (reflection) is an attribute of shape form.