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Re: Image transformations

From: Eli Zaretskii
Subject: Re: Image transformations
Date: Thu, 13 Jun 2019 19:20:40 +0300

> From: Alp Aker <address@hidden>
> Date: Thu, 13 Jun 2019 11:57:34 -0400
> Cc: Alan Third <address@hidden>, Emacs devel <address@hidden>
> > Maybe that's the problem: if the rotation is counter-clockwise, then
> > the translation should indeed be along the Y axis.
> The translation doesn't depend on the rotation in any way.

I think the final value of translation does depend on the rotation,
because the center of the image's position after the rotation depends
on the rotation angle.

> The general form of an affine transformation is F(x) + b, where F is
> a linear transformation (rotation, shear, scaling) and addition by b
> is translation.

Yes, but our transformation includes translation, rotation, and
another translation.  Not just one translation.  IOW, it isn't the
transformation matrix that is given; it's the operation on the image.
The transformation matrix surely depends on whether the rotation is
clockwise or counter-clockwise.

Anyway, until we agree on the equations that convert (x,y) into
(x',y'), it is IMO pointless to argue about details.  So can anyone
tell where the meaning of the matrix passed to XRender is described?

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