title: Font Rendering in OpenGL with Pango and Cairo
date: 2013-08-17 16:00:00
tags: opengl, pango, cairo, font, wsu
summary: A brief tutorial for rendering fonts in OpenGL with libpangocairo
---

I am working towards a 0.1 release of my game development framework
for GNU Guile, [guile-2d](https://github.com/davexunit/guile-2d). One
of the few remaining blockers on my to-do list is font rendering. A
reddit user, [Madsy9](http://www.reddit.com/user/Madsy9), pointed me
in the right direction with this
[comment](http://www.reddit.com/r/scheme/comments/1k739l/guile_2d_game_programming_lib_for_scheme/cbmnyuk). There
are two libraries needed to perform nice font rendering with proper
internationalization support: [Pango](http://www.pango.org/), “a
library for laying out and rendering of text, with an emphasis on
internationalization,” and [Cairo](http://cairographics.org/), “a 2D
graphics library with support for multiple output devices.”

It took me awhile to put together all of the pieces and build a
working sample program. The goal of this post is to help others that
may be trying to accomplish a similar task that have no prior
knowledge of Pango and Cairo. I will assume basic knowledge of C, SDL,
and OpenGL throughout this post.

Let’s get the basic SDL and OpenGL initialization out of the way:

```c
#include <pango/pangocairo.h>
#include <SDL.h>
#include <SDL_opengl.h>

#define WINDOW_WIDTH 800
#define WINDOW_HEIGHT 600
#define FONT "Sans Bold 18"
#define TEXT "The quick brown fox is so かわいい!"

void
init_sdl ()
{
    SDL_Init (SDL_INIT_EVERYTHING);
    SDL_SetVideoMode (WINDOW_WIDTH, WINDOW_HEIGHT, 0, SDL_OPENGL);
}

void
init_gl ()
{
    glClearColor (0.0f, 0.0f, 0.0f, 0.0f);
    glDisable (GL_DEPTH_TEST);
    glEnable (GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable (GL_TEXTURE_2D);
    glViewport (0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    glOrtho (0, WINDOW_WIDTH, WINDOW_HEIGHT, 0, -1, 1);
    glMatrixMode (GL_MODELVIEW);
    glLoadIdentity ();
}
```

`create_texture` simply creates an OpenGL texture given an array of
pixel data and the texture dimensions. Our Cairo surface will use BGRA
color.

```c
unsigned int
create_texture (unsigned int width,
                unsigned int height,
                unsigned char *pixels)
{
    unsigned int texture_id;

    glGenTextures (1, &texture_id);
    glBindTexture (GL_TEXTURE_2D, texture_id);
    glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexImage2D (GL_TEXTURE_2D,
                  0,
                  GL_RGBA,
                  width,
                  height,
                  0,
                  GL_BGRA,
                  GL_UNSIGNED_BYTE,
                  pixels);

    return texture_id;
}
```

`draw_texture` clears the screen, renders a simple textured quad using
OpenGL’s immediate mode, and then swaps buffers.

```c
void
draw_texture (int width,
              int height,
              unsigned int texture_id)
{
    /* Render a texture in immediate mode. */
    glMatrixMode (GL_MODELVIEW);
    glLoadIdentity ();
    glClear (GL_COLOR_BUFFER_BIT);
    glPushMatrix ();
    glBindTexture (GL_TEXTURE_2D, texture_id);
    glColor3f (1.f, 1.0f, 1.0f);

    glBegin (GL_QUADS);
    glTexCoord2f (0.0f, 0.0f);
    glVertex2f (0.0f, 0.0f);
    glTexCoord2f (1.0f, 0.0f);
    glVertex2f (width, 0.0f);
    glTexCoord2f (1.0f, 1.0f);
    glVertex2f (width , height);
    glTexCoord2f (0.0f, 1.0f);
    glVertex2f (0.0f, height);
    glEnd ();

    glPopMatrix ();
    SDL_GL_SwapBuffers();
}
```

`create_cairo_context` is used to make a new Cairo context that draws
to a raw data surface.  The return value, a `cairo_t`, is the main
object in Cairo. All drawing is done via a `cairo_t` object.  A
context needs a surface to draw on.
`cairo_image_surface_create_for_data` creates a raw data surface for
us. We will be translating the surface into a texture later on.

```c
cairo_t*
create_cairo_context (int width,
                      int height,
                      int channels,
                      cairo_surface_t** surf,
                      unsigned char** buffer)
{
    *buffer = calloc (channels * width * height, sizeof (unsigned char));
    *surf = cairo_image_surface_create_for_data (*buffer,
                                                 CAIRO_FORMAT_ARGB32,
                                                 width,
                                                 height,
                                                 channels * width);
    return cairo_create (*surf);
}
```

`create_layout_context` also makes a new Cairo context, but this
context is for PangoLayout objects.  In Pango, a layout describes the
style of a paragraph of text.  The layout needs a context in order to
function. We use `cairo_image_surface_create` with dimensions of 0x0
because we won’t actually be rendering to this surface. Instead, we
will layout our text and use `create_cairo_context` to build a context
with a surface that is the size of the rendered text.  Cairo uses
reference counting for dynamically allocated objects, so we need to
call `cairo_surface_destroy` when we’re done with the temporary
surface.  The context still maintains a reference to the surface, so
the memory for the surface will not be freed until the context is.

```c
cairo_t*
create_layout_context ()
{
    cairo_surface_t *temp_surface;
    cairo_t *context;

    temp_surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 0, 0);
    context = cairo_create (temp_surface);
    cairo_surface_destroy (temp_surface);

    return context;
}
```

`get_text_size` tells us the size of the text that’s in the layout, in
pixels.  Pango’s units are not in pixels, so we must divide by
`PANGO_SCALE` in order to get pixel units.

```c

void
get_text_size (PangoLayout *layout,
                unsigned int *width,
                unsigned int *height)
{
    pango_layout_get_size (layout, width, height);
    /* Divide by pango scale to get dimensions in pixels. */
    *width /= PANGO_SCALE;
    *height /= PANGO_SCALE;
}
```

`render_text` is where all of the magic happens.  First, we create a
layout with a layout context and set the text that we will render with
this layout.  `TEXT` is defined earlier in the program as "The quick
brown fox is so かわいい!"

Then we create a `PangoFontDescription` object.  This object
represents the font that we want to render.  Earlier in the program,
`FONT` is defined as "Sans Bold 18".  Pango is able to figure out how
to load a font from a string in this format.  Your system must be able
to recognize the font family and font face, though.  I haven’t yet
figured out how to have Pango render an arbitrary font from a `*.ttf`
file.

Next, we create a rendering context by getting the layout’s size and
creating a context with a surface big enough to show all of the
rendered text.

Finally, we set the font color to white, render the text to the
surface with `pango_cairo_show_layout`, and create an OpenGL texture
from the surface. We also clean up all the objects that we no longer
need before returning.

```c
unsigned int
render_text (const char *text,
             unsigned int *text_width,
             unsigned int *text_height,
             unsigned int *texture_id)
{
    cairo_t *layout_context;
    cairo_t *render_context;
    cairo_surface_t *temp_surface;
    cairo_surface_t *surface;
    unsigned char* surface_data = NULL;
    PangoFontDescription *desc;
    PangoLayout *layout;

    layout_context = create_layout_context ();

    /* Create a PangoLayout, set the font and text */
    layout = pango_cairo_create_layout (layout_context);
    pango_layout_set_text (layout, text, -1);

    /* Load the font */
    desc = pango_font_description_from_string (FONT);
    pango_layout_set_font_description (layout, desc);
    pango_font_description_free (desc);

    /* Get text dimensions and create a context to render to */
    get_text_size (layout, text_width, text_height);
    render_context = create_cairo_context (*text_width,
                                           *text_height,
                                           4,
                                           &surface,
                                           &surface_data);

    /* Render */
    cairo_set_source_rgba (render_context, 1, 1, 1, 1);
    pango_cairo_show_layout (render_context, layout);
    *texture_id = create_texture(*text_width, *text_height, surface_data);

    /* Clean up */
    free (surface_data);
    g_object_unref (layout);
    cairo_destroy (layout_context);
    cairo_destroy (render_context);
    cairo_surface_destroy (surface);
}
```

`main` is pretty simple.  We initialize SDL and OpenGL, render text
to a texture, and enter the rendering loop.  The program will run
until you click the close button, press "enter", or press "q".

```c
int main (int argc, char **argv)
{
    SDL_Event event;
    int keep_running = 1;
    unsigned int texture_id;
    unsigned int text_width = 0;
    unsigned int text_height = 0;

    init_sdl ();
    init_gl ();
    render_text(TEXT,
                &texture_id,
                &text_width,
                &text_height);

    /* Update/render loop */
    while (keep_running) {
        SDL_PollEvent (&event);

        switch (event.type) {
        case SDL_QUIT :
            keep_running = 0;
            break;

        case SDL_KEYDOWN :
            if (event.key.keysym.sym == SDLK_ESCAPE)
                keep_running = 0;
            if (event.key.keysym.sym == SDLK_q)
                keep_running = 0;
            break;
        }

        draw_texture (texture_id, text_width, text_height);
        SDL_Delay (16);
    }

    /* Clean up */
    glDeleteTextures (1, &texture_id);

    SDL_Quit();

    return 0;
}
```

And we’re done! You should now be able to render some text in an
OpenGL context. I hope this brief tutorial was helpful. Font rendering
isn’t easy, and it’s not really my area of interest. I’m glad that
Pango exists to do all of the real work for me so that I can more
quickly move on to the parts of graphics programming that I actually
enjoy.

You can download the full source code [here](/src/pangocairo.tar.gz).