Re: Image transformations

[Eli Zaretskii <eliz@gnu.org>]

> From: Alp Aker <alptekin.aker@gmail.com>
> Date: Thu, 13 Jun 2019 05:19:52 -0400
> Cc: Alan Third <alan@idiocy.org>, Emacs devel <emacs-devel@gnu.org>
> 
> > This already goes contrary to my geometric intuition, please bear with
> > me.  The rotation is around the (0,0) origin, i.e. around the top-left
> > corner of the original image, right?  If so, the rotation should have
> > been followed by a translation along the X axis, not Y
> 
> The last sentence is problematic.  When a transformation matrix describes a
> rotation followed by a translation, the translation is specified relative
> to the fixed coordinate axes.  The rotation doesn't affect the
> direction of the translation.

That's right, but this is exactly what I was trying to describe.  When
I wrote "translation along the X axis", I meant the original X axis,
which is unaffected by the rotation.  Are you saying that my
expectations are incorrect in that interpretation of "X axis"?

> >     +------------------+> X        +----------+-------------------> X
> >     |                  |           |          |
> >     |                  |           |          |
> >     |                  |           |          |
> >     |                  |   ===>    |          |
> >     +------------------+           |          |
> >     |                              |          |
> >     |                              |          |
> >     |                              |          |
> >     |                              +----------+
> >     |                                         |
> >     V                                         V
> >     Y                                         Y
> >
> > The above is just after the rotation around (0,0).  Is that correct,
> > or am I missing something?
> 
> That's correct.
> 
> > I also tried to approach this from the matrix notation aspect.  Is the
> > following the correct equations of computing (x',y'), the new
> > coordinates of any pixel of the image, from its original coordinates
> > (x,y)?
> >
> >   x' = m11 * x + m12 * y + tx
> >   y' = m21 * x + m22 * y + ty
> >
> > where the factors are related to the matrix as follows:
> >
> >    m[0][0] = m11 | m[0][1] = m12 | m[0][2] = 0
> >    --------------+---------------+-------------
> >    m[1][0] = m21 | m[1][1] = m22 | m[1][2] = 0
> >    --------------+---------------+-------------
> >    m[2][0] = tx  | m[2][1] = ty  | m[2][2] = 1
> 
> I confess I'm not sure how to interpret that matrix.  I just looked through
> image_set_rotation and found it somewhat confusing, as it seems to use
> column-major representation where I'd expect row-major.  E.g., the above
> matrix
> looks odd to me, because tx and ty would normally be in m[0][2] and m[1][2]
> (and
> I'd expect m[2][0] == m[2][1] == 0).  Similarly, the rotation matrix used in
> image_set_rotation:
> 
>   [0][0] = cos_r, [0][1] = -sin_r
>   [1][0] = sin_r, [1][1] = cos_r
> 
> would normally describe a counter-clockwise rotation by r, not a clockwise
> rotation.

Maybe that's the problem: if the rotation is counter-clockwise, then
the translation should indeed be along the Y axis.

> That said, if I correctly understand the layout of the data, the equations
> should be:
> 
>    x' = m11 * x + m21 * y + tx
>    y' = m12 * x + m22 * y + ty

AFAIU, this indeed describes a counter-clockwise rotation, not a
clockwise rotation.

> > the correct coordinates should be (233,0), not (0,232).
> > What am I missing here?
> 
> The transformation described by the matrix is: rotate 90 degrees
> around the origin, then translate by 232 along the y axis.  The
> first operation leaves (0, 0) unmoved, then the second operation
> moves it to (0, 232).

But if the rotation is clockwise, the result should be (233,0), right?

Thank you for helping me figure out this stuff.