[cairo] Handling degenerate matrices

[Behdad Esfahbod]

Hi,

It started with http://bugzilla.gnome.org/show_bug.cgi?id=431990
In short, thanks to Chris Wilson's lots of error-path fixes, we now only
go into an error state if a font is rendered at size zero.  Of course I
want to make it work with no errors.  So far I've got this:

--- a/src/cairo-scaled-font.c
+++ b/src/cairo-scaled-font.c
@@ -367,7 +367,11 @@ _cairo_scaled_font_init (cairo_scaled_font_t               *scaled_font,
     inverse = scaled_font->scale;
     status = cairo_matrix_invert (&inverse);
     if (status)
-       return status;
+      {
+        cairo_matrix_init (&inverse, 0, 0, 0, 0, 0, 0);
+       inverse.x0 = -scaled_font->scale.x0;
+       inverse.y0 = -scaled_font->scale.y0;
+      }
 
     scaled_font->glyphs = _cairo_cache_create (_cairo_scaled_glyph_keys_equal,
                                               _cairo_scaled_glyph_destroy,


But I'm going to start hacking on that long overdue cairo_transform_t
thing.  The main problems are:

  - What to do when a matrix maps a 2d space to a 1d space?

  - How to keep track of the shear matrix when a matrix is transformed
(eg. scaled) to 0?  This is needed when using a zero matrix as a pattern
transform.  You would need the axes still for CAIRO_EXTEND_PAD.


Just a heads up and to steer discussion.

-- 
behdad
http://behdad.org/

"Those who would give up Essential Liberty to purchase a little
 Temporary Safety, deserve neither Liberty nor Safety."
        -- Benjamin Franklin, 1759