[cairo] What are the unit of the height field in font_metrics_t?

[Richard Cole]

Here is a program. Am I doing something wrong?

cairo_font_extents_t font_extents;

cairo_select_font (
  cr, "Sans", CAIRO_FONT_SLANT_NORMAL, CAIRO_FONT_WEIGHT_BOLD
);

cairo_scale_font (cr, .3);
cairo_current_font_extents(cr, &font_extents);

cairo_save(cr);
  cairo_move_to (cr, 0.04, 0.83);
  cairo_show_text (cr, "Hello");
cairo_restore(cr);

cairo_save(cr);
  cairo_set_line_width(cr, 0.01);
  cairo_rectangle (cr,
    0.04, 0.83 - font_extents.height, 0.5, font_extents.height);
  fprintf(stderr, "TRACE: Font height=%f\n", font_extents.height);
  fprintf(stderr, "TRACE: Font height * font_scale=%f\n", 
font_extents.height * 0.3);
  cairo_stroke(cr);
cairo_restore(cr);

I added this program to cairo-demo/cairo_snippets, ran make and then 
"./cairo_snippets_png". The height being returned is: 89.400000, but the 
font in the rendered image looks to be roughly 0.3 in user co-ordinates. 
So what are the units of font_extents.height and how do I convert these 
to user space? Is there any documentation, even a specification, of the 
behaviour of the cairo_current_font_extents function?

regards,

Richard.