Fix percentage sizes in SVG display (bug#64908)

* src/image.c (svg_css_length_to_pixels): Make percent units always
return zero and handle font size based units better.
(svg_load_image): Don't rely on the width and height values from the
SVG actually having any useful data, even if they're explicitly set.

1 files changed, 52 insertions, 30 deletions

diff --git a/src/image.c b/src/image.c
index 06fc44b5abd..29e88f0d752 100644
--- a/src/image.c
+++ b/src/image.c
@@ -11740,6 +11740,12 @@ svg_css_length_to_pixels (RsvgLength length, double dpi, int font_size)
 {
   double value = length.length;
 
+#if ! LIBRSVG_CHECK_VERSION (2, 48, 0)
+  /* librsvg 2.48 lets us define the font size, but earlier versions
+     default to 12 pixels.  */
+  font_size = 12;
+#endif
+
   switch (length.unit)
     {
     case RSVG_UNIT_PX:
@@ -11764,16 +11770,31 @@ svg_css_length_to_pixels (RsvgLength length, double dpi, int font_size)
     case RSVG_UNIT_IN:
       value *= dpi;
       break;
-#if LIBRSVG_CHECK_VERSION (2, 48, 0)
-      /* We don't know exactly what font size is used on older librsvg
-         versions.  */
     case RSVG_UNIT_EM:
       value *= font_size;
       break;
-#endif
+    case RSVG_UNIT_EX:
+      /* librsvg uses an ex height of half the em height, so we match
+         that here.  */
+      value = value * font_size / 2.0;
+      break;
+    case RSVG_UNIT_PERCENT:
+      /* Percent is a ratio of the containing "viewport".  We don't
+         have a viewport, as such, as we try to draw the image to it's
+         'natural' size rather than dictate the size as if we were
+         drawing icons on a toolbar or similar.  This means that
+         percent values are useless to us and we are best off just
+         drawing the image according to whatever other sizes we can
+         derive.
+
+         If we do set explicit width and height values in the image
+         spec, this will work out correctly as librsvg will still
+         honour the percentage sizes in its final rendering no matter
+         what size we make the image.  */
+      value = 0;
+      break;
     default:
-      /* Probably ex or %.  We can't know what the pixel value is
-         without more information.  */
+      /* We should never reach this.  */
       value = 0;
     }
 
@@ -11904,7 +11925,8 @@ svg_load_image (struct frame *f, struct image *img, char *contents,
     }
   else
     {
-      RsvgRectangle zero_rect, viewbox, out_logical_rect;
+      RsvgRectangle  viewbox;
+      double explicit_width = 0, explicit_height = 0;
 
       /* Try the intrinsic dimensions first.  */
       gboolean has_width, has_height;
@@ -11916,34 +11938,27 @@ svg_load_image (struct frame *f, struct image *img, char *contents,
                                             &has_height, &iheight,
                                             &has_viewbox, &viewbox);
 
-      if (has_width && has_height)
-        {
-          /* Success!  We can use these values directly.  */
-          viewbox_width = svg_css_length_to_pixels (iwidth, dpi,
+      if (has_width)
+        explicit_width = svg_css_length_to_pixels (iwidth, dpi,
+                                                   img->face_font_size);
+      if (has_height)
+        explicit_height = svg_css_length_to_pixels (iheight, dpi,
                                                     img->face_font_size);
-          viewbox_height = svg_css_length_to_pixels (iheight, dpi,
-                                                     img->face_font_size);
 
-          /* Here one dimension could be zero because in percent unit.
-             So calculate this dimension with the other.  */
-          if (! (0 < viewbox_width) && (iwidth.unit == RSVG_UNIT_PERCENT))
-            viewbox_width = (viewbox_height * viewbox.width / viewbox.height)
-              * iwidth.length;
-          else if (! (0 < viewbox_height) && (iheight.unit == RSVG_UNIT_PERCENT))
-            viewbox_height = (viewbox_width * viewbox.height / viewbox.width)
-              * iheight.length;
+      if (explicit_width > 0 && explicit_height > 0)
+        {
+          viewbox_width = explicit_width;
+          viewbox_height = explicit_height;
         }
-      else if (has_width && has_viewbox)
+      else if (explicit_width > 0 && has_viewbox)
         {
-          viewbox_width = svg_css_length_to_pixels (iwidth, dpi,
-                                                    img->face_font_size);
-          viewbox_height = viewbox_width * viewbox.height / viewbox.width;
+          viewbox_width = explicit_width;
+          viewbox_height = explicit_width * viewbox.height / viewbox.width;
         }
-      else if (has_height && has_viewbox)
+      else if (explicit_height > 0 && has_viewbox)
         {
-          viewbox_height = svg_css_length_to_pixels (iheight, dpi,
-                                                     img->face_font_size);
-          viewbox_width = viewbox_height * viewbox.width / viewbox.height;
+          viewbox_height = explicit_height;
+          viewbox_width = explicit_height * viewbox.width / viewbox.height;
         }
       else if (has_viewbox)
         {
@@ -11957,8 +11972,15 @@ svg_load_image (struct frame *f, struct image *img, char *contents,
         {
           /* We haven't found a usable set of sizes, so try working out
              the visible area.  */
+
+          /* FIXME: I'm not sure exactly how librsvg uses this
+             viewport input here, so I'm not sure what values I should
+             set. */
+          RsvgRectangle max_viewport_rect = {0, 0, UINT_MAX, UINT_MAX};
+          RsvgRectangle out_logical_rect;
+
           rsvg_handle_get_geometry_for_layer (rsvg_handle, NULL,
-                                              &zero_rect, &viewbox,
+                                              &max_viewport_rect, &viewbox,
                                               &out_logical_rect, NULL);
           viewbox_width = viewbox.x + viewbox.width;
           viewbox_height = viewbox.y + viewbox.height;