Re: Scale transformation of images

[Eli Zaretskii <eliz@gnu.org>]

On June 16, 2019 9:45:56 AM GMT+03:00, mituharu@math.s.chiba-u.ac.jp wrote:
> > If I insert an image into a buffer with insert-image, and then
> position
> > point on the image and press '-' to make it smaller, the scale
> factors I
> > see in the transform matrix are greater than 1.  This is because
> > image_set_size does this:
> >
> >    double xscale = img->width / (double) width;
> >
> > and similarly for yscale.  Shouldn't we divide width by img->width
> > instead?
> 
> No.  Documentation of cairo_pattern_set_matrix and its example
> would help you understand what's happening:
> https://www.cairographics.org/manual/cairo-cairo-pattern-t.html
> 
>   Important: Please note that the direction of this
>   transformation matrix is from user space to pattern space. This
>   means that if you imagine the flow from a pattern to user
>   space (and on to device space), then coordinates in that flow
>   will be transformed by the inverse of the pattern matrix.
> 
>   For example, if you want to make a pattern appear twice as
>   large as it does by default the correct code to use is:
> 
>   1 cairo_matrix_init_scale (&matrix, 0.5, 0.5);
>   2 cairo_pattern_set_matrix (pattern, &matrix);
> 
>   Meanwhile, using values of 2.0 rather than 0.5 in the code
>   above would cause the pattern to appear at half of its default
>   size.


Thanks.  So you are saying that in the equations presented by Richard in https://lists.gnu.org/archive/html/emacs-devel/2019-06/msg00361.html the matrix Fij is the inverse of the matrix computed in image_set_transform, is that right?