bug#64908: 29.1; svg parse failure

[Alan Third]

On Sat, Aug 05, 2023 at 10:30:38AM +0200, David Ponce wrote:
> Hello,
> 
> I attached a better patch, because it is possible to have svg image
> definition that combines viewBox, and percentage of width/height, like
> in this example:
> 
> (insert-image
>   '(image :type svg :data
>      "<svg xmlns=\"http://www.w3.org/2000/svg\"; viewBox=\"0 0 22 22\" 
> width=\"110%\" height=\"80%\">
>   <defs id=\"defs3051\">
>     <style type=\"text/css\" id=\"current-color-scheme\">
>       .ColorScheme-Text {
>         color:#232629;
>       }
>       </style>
>   </defs>
>  <path
>     style=\"fill:currentColor;fill-opacity:1;stroke:none\"
>     d=\"m7.707031 3l-.707031.707031 6.125 6.125 1.167969 1.167969-1.167969 
> 1.167969-6.125 6.125.707031.707031 6.125-6.125 
> 1.875-1.875-1.875-1.875-6.125-6.125\"
>     class=\"ColorScheme-Text\"
>     />
> </svg>"
>     :scale 1))
> 
> I hope Alan will find a simpler implementation for all these cases!

Hi David, I did a bit of thinking about what a percentage in the width
or height attributes actually means and how librsvg handles it.

I think we should always ignore it.

A percentage is intended as a percentage of the enclosing box. this
means if we set an image width of 100 pixels in Emacs, and the width
in the SVG is 50%, the image should be 50 pixels wide.

However we requested a width of 100 pixels, and Emacs will give us
that no matter what, even scaling it back up.

Additionally, librsvg will scale the drawn image to a width of 50%
anyway.

What this means is if we set the image width to 100 pixels, librsvg
will scale its drawing down by 50% and it should come out the correct
size within the 100 pixel wide image.

I hope that made sense, it took me a bit of thinking about it before I
came to this conclusion. It was really the fact that librsvg scales
its drawing anyway that made me decide there's no point in us also
scaling it.

I've attached a patch, can you give it a go and let me know how it
works.

Or if you disagree with my analysis above, let me know, because I'm
not 100% sure of it.
-- 
Alan Third

0001-Fix-percentage-sizes-in-SVG-display-bug-64908.patch
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