The Mysteries of Registration Points

Question

Hello All!
So, I'm starting to feel like Seinfeld:
Registration Points... What is the deal with those? Am I
right?
Every time I try and educate myself about these things, they
get all mixed in with Transformation Points, the Info Panel,
Convert to Symbol and a million other things. You'd think it would
be a lot more straightforward than that. I mean, the ability to
change the registration point on the fly, or at least in the IDE,
would aid SO many issues as far as placing things on the stage in
an intelligent way.
So, I have researched this a bit and come up with nil, but
I'll ask here again:
- What is the easiest way to change a Bitmap (or Sprite or
Shape or MovieClip) registration point (specif from upper left to
object center)?
- Is it possible to change the registration point using
ActionScript?
Follow up question:
- Wahh! Why not?! It's such a no-brainer!
Many thanks in advance, as always...
Yours,
Graham

Answer 1

alright - here's some thoughts.
all objects have their own coordinate system - graphics,
bitmaps, text, other objects that are 'within' an object can be
placed 'anywhere' within the coordinates of the parent object
including being positioned 'negatively' in relation to the parents
registration point - yet each of those 'also' has it's own
coordinate system.
all registration points are at 0, 0 for all objects - how
that position relates to the 'contents' of the object is what you
want to change . by default, the upper left is the where items get
'laid out' because under the coordinate systems we work with,
positive values run to the right and down - so as a primary
construction method our objects are defined with their widths and
heights incremented to the right and down from 0, 0 in their own
coordinate space.
so in order for us to be able to change the position of the
'graphics' contained within an object, we have to be able to 'point
to' those objects to control them - however, when items are drawn
in the IDE, or are a imported or loaded bitmap, for instance, you
have no reference to the items within the object you wish to move.
now one *can* use the object's transform matrix property to convert
the coordinates to a certain degree - by inverting the x or y scale
to a negative value on any objec will 'flip' the image to the other
side of it's registration point, however the resulting image will
be 'mirrored' rather than 'moved' which may not be desired, but is
handy in many cases.
to your questions MM:
- yes, there is no way to 'move' the registration point from
0,0 - as pointed out above when constructing a object with AS, you
still need to be able to control the position of the 'contents',
and you may need to use a container object to do so. however, if
your using something like the drawing API, then you may be able to
simply 'draw' the shapes into their correct positions within the
parents coordinate space (which
thinkingresearcher, might be better for you situation, eg.
using the drawing API to construct your objects)
- when i said 'full' - I mean that the object must *contain*
something in order to have readable dimension bounds - a newly
constructed Object will have width and height of 0 until something
is placed within it.
- there is no example of an object that doesn't have a
coordinate system, it is merely that one cannot 'change' the
contents without a mechanism to target those contents.
- and yes, that is right, you must 'move' the position of the
contents in relation to it's parent's coordinate system, regardless
of how you achieve that, to change the apparent registration point
of the 'parent' container.
Here is a basic container example that repositions a box with
the registration point in the upper left, to the parent container,
so that the container can then be scaled from the registration
point which now resides at the 'apparent' lower left - (place on
any new AS3 doc, first frame):