[cairo] A (LONG) Pango/Cairo Question

Behdad Esfahbod behdad at behdad.org
Wed Aug 27 11:20:29 PDT 2008

Jeremy Moles wrote:
> Hello everyone! I've been somewhat active in the IRC channel lately with
> various questions, so I figured I'd make a thorough post here instead of
> asking each question individually.
> A few months back I made a post inquiring as to how I could utilize
> Pango (and Cairo) to render glyphs to an OpenGL texture. I was directed
> to the Clutter project, which solved this problem by creating a custom
> PangoRenderer object and providing various callbacks (particularly a
> "draw_glyphs" callback) which they used to render an individual glyph,
> cache it inside and OpenGL texture, and later query that when needed
> again as they build and position OpenGL quads based on the information
> provided to them by the Pango layout.
> I've been able to dissect their code a great deal over the last two
> weeks and at this point I'm quite familiar with many of the high-level
> concepts needed to accomplish what I'm trying to do. I have my own "from
> scratch" implementation (though highly inspired by the Clutter project)
> and I'm running into a few problems.
> Firstly, I want to change the origin of my cairo image surface (which is
> always properly sized to be compatible with OpenGL texture requirements)
> to the bottom-left corner, which will also make it's coordinate space
> consistent with OpenGL. To accomplish this, I use the following code:
> 	cairo_matrix_t ms, mt, m;
> 	cairo_matrix_init_scale(&ms, 1.0f, -1.0f);
> 	cairo_matrix_init_translate(&mt, 0.0f, -height);
> 	cairo_matrix_multiply(&m, &mt, &ms);

You can do this much easier using cairo_matrix_scale() and

> 	cairo_set_matrix(cr, &m);

Or if you want to set it on cr, just use cairo_identity_matrix(),
cairo_scale(), and cairo_transform().

> I have confirmed that this works as intended as best I can, by drawing a
> line from 0, 0 to 512, 512--which creates a line from the bottom-left to
> the top-right of the image.
> Secondly, I've hooked into the Pango API and have created my own custom
> renderer class. During it's "draw_glyphs" callback, it iterates over the
> list of PangoGlyph structures, creates a cairo_glyph based on this data,
> and requests that Cairo draw this glyph using "cairo_show_glyphs".
> However, what is currently happening is that my glyphs are getting
> rendered upside-down, and I've attached a screenshot to demonstrate
> this. My first question then is what else should I be doing to inform
> Cairo that the glyph should be rendered properly for my case? I've tried
> setting the font matrix in countless ways, but everything I do simply
> causes the text to disappear.

Just do a scale(1,-1) on the font matrix.


> For a bit of additional information, here is how I actually render the
> glyph:
> 	pango_font_get_glyph_extents(font, glyph, &ink_rect, 0);
> 	pango_extents_to_pixels(&ink_rect, 0);
> 	cairo_set_scaled_font(cr, pango_cairo_font_get_scaled_font(...))
> 	// cairo_set_font_matrix(...)
> 	cairo_glyph.x = -ink_rect.x;
> 	cairo_glyph.y = -ink_rect.y;
> 	cairo_glyph.index = glyph;
> In the above code, I'm somewhat unclear as to why I set the
> cairo_glyph{x, y} values with the negative versions of the ones from the
> ink_rect object queried using the Pango API; I got this part directly
> from the Clutter project. This, then, leads me to my last question,
> which is what are these values and how should I use them? I understand
> that Cairo defines them as offsets from a 0, 0 origin, but does this
> mean that Pango works with a different origin than Cairo does?
> I know this is a huge e-mail, but any advice would be greatly
> appreciated. :) In summation, I will explain again what I'm trying to
> accomplish:
> I want to use Cairo and Pango to create an accelerated OpenGL
> font/texturing library (for use in OpenSceneGraph). To do this, I would
> like to change my Cairo origin to the bottom left (to be consistent with
> OpenGL) and begin rendering/caching glyphs from left-to-right,
> bottom-to-top. Furthermore, rather than create a new surface for each
> individual glyph and then copy that to the "master" texture, I would
> like to use cairo_translate() and the information provided by Pango's
> glyph structure to move around the texture and render the glyphs
> directly. Is this possible, or am I missing some crucial concept
> somewhere? :)
> Thanks!
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