rx == 0 or ry == 0 ellipse or rectangle can paralyze Cairo port, at least.

[mpsuzuki]

During the investigation of my patch for Issue #67, I found my cairo port is paralyzed by the ellipse or rectangles with rx == 0 or ry == 0. I think Skia and CoreGraphics ports are safe from this issue, because they have direct functions to draw ellipse or rounded-rectangle.

Cairo has no direct functions to draw ellipse or rounded-rectangle, so I inserted cairo_scale() to stretch the circle or rounded-corner of the rounded rectangle. Maybe, cairo_scale(cr, 0, scale_y) or cairo_scale(cr, scale_x, 0) and following cairo_arc() makes cairo confused, and all following objects are not drawn correctly.

So, I drafted a patch to replace "rx == 0 or ry == 0 ellipse" by a squashed bounding-box, and to replace "rx == 0 or ry == 0 rounded rectangle" by normal rectangle.

Another option might be "if an ellipse or a rounded-rectangle with rx==0 or ry==0, do not take it as a drawable element". I wish if the experts give some comments on the direction we should go.

[mpsuzuki]

If rx and ry are non-zero, like,

<svg xmlns="http://www.w3.org/2000/svg" width="300" height="200" viewBox="0 0 300 200">
  <ellipse cx="150" cy="50" rx="25" ry="25"/>
  <rect x="120" y="120" width="60" height="60" rx="15"  ry="15"/>

  <ellipse cx="50"  cy="50" rx="1" ry="25"/>
  <rect x="20"  y="120" width="60" height="60" rx="1"   ry="15"/>

  <ellipse cx="250" cy="50" rx="50" ry="25"/>
  <rect x="220" y="120" width="60" height="60" rx="150" ry="15"/>
</svg>

The result is looking like

[mpsuzuki]

But, if rx == 0, like,

<svg xmlns="http://www.w3.org/2000/svg" width="300" height="200" viewBox="0 0 300 200">
  <ellipse cx="150" cy="50" rx="25" ry="25"/>
  <rect x="120" y="120" width="60" height="60" rx="15"  ry="15"/>

  <ellipse cx="50"  cy="50" rx="0" ry="25"/>
  <rect x="20"  y="120" width="60" height="60" rx="0"   ry="15"/>

  <ellipse cx="250" cy="50" rx="50" ry="25"/>
  <rect x="220" y="120" width="60" height="60" rx="150" ry="15"/>
</svg>

The source at abfb9aa gives such result.
. You can find the rx == 0 elements (at the left side) are not drawn, and, following elements at the right sides are not drawn at all. It is what I call as "paralyzed".

[mpsuzuki]

The source at abfb9aa, plus my 2 patches (741849a and 0df194c), another for rounded-rectangle), the result is like this.

The web browsers would show such results.

[dirkschulze]

According to https://svgwg.org/svg2-draft/shapes.html#EllipseElement a value of 0 for either rx or ry disables the rendering of the element. Same for r on circle or width/height on rect.

So we should skip the element if one of the attributes is 0.