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+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).