[cairo] Possible bug/misuse in set_scaled_font/set_font_face & FreeType

Behdad Esfahbod behdad at behdad.org
Mon Dec 16 13:58:10 PST 2013

On 13-12-16 02:06 PM, Jeremy Moles wrote:
> (This is a resubmission; my intial post was rejected because I attached a huge
> font file, sorry.)
> Hello all! I am developing a (pretty cool) 2D text scenegraph library that
> uses Cairo, Harfbuzz, and glyphy. While working on some of the core
> components, I think I have discovered a bug (or perhaps more likely, a misuse)
> in/of Cairo. Attached is a very simple demo application.
> You'll notice that I only use FreeType to load the initial font file, and then
> quickly create a cairo_font_fact_t* from that directly. I never call
> FT_Set_Char_Size, nor do I want to manage the font size with FreeType directly.
> The issue I've found is that unless I call:
> cairo_get_scaled_font()
> ...after a call to cairo_set_font_size(), then the font glyphs seem to have
> wrong/null origins. I know that that the implementation is probably calling
> FT_Set_Char_Size underneath the hood, but it doesn't seem to actually "apply"
> until after the get_scaled_font() call. I don't know how else to best describe
> it, so I just attached a really simple example demonstrating it.

Correct.  You are misusing these.  You should use
cairo_ft_scaled_font_lock_face() to retrieve the face with correct size set.

> Hopefully I can get some advice/comments on my usage.
> Thanks!


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