We reflect over the X axis because in the normalized coordinate frame the y-axis has increasing coordinates upward, whereas in the image coordinate frame the y-axis has increasing coordinates downward.
As to why the image space has the y-axis pointing downward -- no reason. Just convention!
Why do we need to reflect about x?
We reflect over the X axis because in the normalized coordinate frame the y-axis has increasing coordinates upward, whereas in the image coordinate frame the y-axis has increasing coordinates downward.
As to why the image space has the y-axis pointing downward -- no reason. Just convention!