mutter/examples/cogl-gles2-gears.c

/*
 * Copyright (C) 1999-2001  Brian Paul   All Rights Reserved.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * BRIAN PAUL BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
 * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

/*
 * Ported to GLES2.
 * Kristian Høgsberg <krh@bitplanet.net>
 * May 3, 2010
 *
 * Improve GLES2 port:
 *   * Refactor gear drawing.
 *   * Use correct normals for surfaces.
 *   * Improve shader.
 *   * Use perspective projection transformation.
 *   * Add FPS count.
 *   * Add comments.
 * Alexandros Frantzis <alexandros.frantzis@linaro.org>
 * Jul 13, 2010
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#define GL_GLEXT_PROTOTYPES

#include <math.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <unistd.h>
#include <GLES2/gl2.h>
#include <cogl/cogl.h>
#include <cogl/cogl-gles2.h>
#include <glib.h>

#define STRIPS_PER_TOOTH 7
#define VERTICES_PER_TOOTH 34
#define GEAR_VERTEX_STRIDE 6

typedef struct _Data
{
    CoglContext *ctx;
    CoglFramebuffer *fb;

    CoglGLES2Context *gles2_ctx;

    GTimer *timer;
    int frames;
    double last_elapsed;
} Data;


/**
 * Struct describing the vertices in triangle strip
 */
struct vertex_strip {
    /** The first vertex in the strip */
    GLint first;
    /** The number of consecutive vertices in the strip after the first */
    GLint count;
};

/* Each vertex consist of GEAR_VERTEX_STRIDE GLfloat attributes */
typedef GLfloat GearVertex[GEAR_VERTEX_STRIDE];

/**
 * Struct representing a gear.
 */
struct gear {
    /** The array of vertices comprising the gear */
    GearVertex *vertices;
    /** The number of vertices comprising the gear */
    int nvertices;
    /** The array of triangle strips comprising the gear */
    struct vertex_strip *strips;
    /** The number of triangle strips comprising the gear */
    int nstrips;
    /** The Vertex Buffer Object holding the vertices in the graphics card */
    GLuint vbo;
};

/** The view rotation [x, y, z] */
static GLfloat view_rot[3] = { 20.0, 30.0, 0.0 };
/** The gears */
static struct gear *gear1, *gear2, *gear3;
/** The current gear rotation angle */
static GLfloat angle = 0.0;
/** The location of the shader uniforms */
static GLuint ModelViewProjectionMatrix_location,
              NormalMatrix_location,
              LightSourcePosition_location,
              MaterialColor_location;
/** The projection matrix */
static GLfloat ProjectionMatrix[16];
/** The direction of the directional light for the scene */
static const GLfloat LightSourcePosition[4] = { 5.0, 5.0, 10.0, 1.0};

#ifndef HAVE_SINCOS
static void
sincos (double x, double *sinx, double *cosx)
{
  *sinx = sin (x);
  *cosx = cos (x);
}
#endif /* HAVE_SINCOS */

/**
 * Fills a gear vertex.
 *
 * @param v the vertex to fill
 * @param x the x coordinate
 * @param y the y coordinate
 * @param z the z coortinate
 * @param n pointer to the normal table
 *
 * @return the operation error code
 */
static GearVertex *
vert (GearVertex *v, GLfloat x, GLfloat y, GLfloat z, GLfloat n[3])
{
    v[0][0] = x;
    v[0][1] = y;
    v[0][2] = z;
    v[0][3] = n[0];
    v[0][4] = n[1];
    v[0][5] = n[2];

    return v + 1;
}

/**
 *  Create a gear wheel.
 *
 *  @param inner_radius radius of hole at center
 *  @param outer_radius radius at center of teeth
 *  @param width width of gear
 *  @param teeth number of teeth
 *  @param tooth_depth depth of tooth
 *
 *  @return pointer to the constructed struct gear
 */
static struct gear *
create_gear (GLfloat inner_radius, GLfloat outer_radius, GLfloat width,
             GLint teeth, GLfloat tooth_depth)
{
    GLfloat r0, r1, r2;
    GLfloat da;
    GearVertex *v;
    struct gear *gear;
    double s[5], c[5];
    GLfloat normal[3];
    int cur_strip = 0;
    int i;

    /* Allocate memory for the gear */
    gear = malloc (sizeof *gear);
    if (gear == NULL)
        return NULL;

    /* Calculate the radii used in the gear */
    r0 = inner_radius;
    r1 = outer_radius - tooth_depth / 2.0;
    r2 = outer_radius + tooth_depth / 2.0;

    da = 2.0 * M_PI / teeth / 4.0;

    /* Allocate memory for the triangle strip information */
    gear->nstrips = STRIPS_PER_TOOTH * teeth;
    gear->strips = calloc (gear->nstrips, sizeof (*gear->strips));

    /* Allocate memory for the vertices */
    gear->vertices = calloc (VERTICES_PER_TOOTH * teeth, sizeof(*gear->vertices));
    v = gear->vertices;

    for (i = 0; i < teeth; i++) {
        /* Calculate needed sin/cos for varius angles */
        sincos (i * 2.0 * M_PI / teeth, &s[0], &c[0]);
        sincos (i * 2.0 * M_PI / teeth + da, &s[1], &c[1]);
        sincos (i * 2.0 * M_PI / teeth + da * 2, &s[2], &c[2]);
        sincos (i * 2.0 * M_PI / teeth + da * 3, &s[3], &c[3]);
        sincos (i * 2.0 * M_PI / teeth + da * 4, &s[4], &c[4]);

      /* A set of macros for making the creation of the gears easier */
#define  GEAR_POINT(r, da) { (r) * c[(da)], (r) * s[(da)] }
#define  SET_NORMAL(x, y, z) do { \
    normal[0] = (x); normal[1] = (y); normal[2] = (z); \
} while(0)

#define  GEAR_VERT(v, point, sign) vert((v), p[(point)].x, p[(point)].y, (sign) * width * 0.5, normal)

#define START_STRIP do { \
    gear->strips[cur_strip].first = v - gear->vertices; \
} while(0);

#define END_STRIP do { \
    int _tmp = (v - gear->vertices); \
    gear->strips[cur_strip].count = _tmp - gear->strips[cur_strip].first; \
    cur_strip++; \
} while (0)

#define QUAD_WITH_NORMAL(p1, p2) do { \
    SET_NORMAL((p[(p1)].y - p[(p2)].y), -(p[(p1)].x - p[(p2)].x), 0); \
    v = GEAR_VERT(v, (p1), -1); \
    v = GEAR_VERT(v, (p1), 1); \
    v = GEAR_VERT(v, (p2), -1); \
    v = GEAR_VERT(v, (p2), 1); \
} while(0)

        {
            struct point {
                GLfloat x;
                GLfloat y;
            };

            /* Create the 7 points (only x,y coords) used to draw a tooth */
            struct point p[7] = {
                GEAR_POINT (r2, 1), // 0
                GEAR_POINT (r2, 2), // 1
                GEAR_POINT (r1, 0), // 2
                GEAR_POINT (r1, 3), // 3
                GEAR_POINT (r0, 0), // 4
                GEAR_POINT (r1, 4), // 5
                GEAR_POINT (r0, 4), // 6
            };

            /* Front face */
            START_STRIP;
            SET_NORMAL (0, 0, 1.0);
            v = GEAR_VERT (v, 0, +1);
            v = GEAR_VERT (v, 1, +1);
            v = GEAR_VERT (v, 2, +1);
            v = GEAR_VERT (v, 3, +1);
            v = GEAR_VERT (v, 4, +1);
            v = GEAR_VERT (v, 5, +1);
            v = GEAR_VERT (v, 6, +1);
            END_STRIP;

            /* Inner face */
            START_STRIP;
            QUAD_WITH_NORMAL (4, 6);
            END_STRIP;

            /* Back face */
            START_STRIP;
            SET_NORMAL (0, 0, -1.0);
            v = GEAR_VERT (v, 6, -1);
            v = GEAR_VERT (v, 5, -1);
            v = GEAR_VERT (v, 4, -1);
            v = GEAR_VERT (v, 3, -1);
            v = GEAR_VERT (v, 2, -1);
            v = GEAR_VERT (v, 1, -1);
            v = GEAR_VERT (v, 0, -1);
            END_STRIP;

            /* Outer face */
            START_STRIP;
            QUAD_WITH_NORMAL (0, 2);
            END_STRIP;

            START_STRIP;
            QUAD_WITH_NORMAL (1, 0);
            END_STRIP;

            START_STRIP;
            QUAD_WITH_NORMAL (3, 1);
            END_STRIP;

            START_STRIP;
            QUAD_WITH_NORMAL (5, 3);
            END_STRIP;
        }
    }

    gear->nvertices = (v - gear->vertices);

    /* Store the vertices in a vertex buffer object (VBO) */
    glGenBuffers (1, &gear->vbo);
    glBindBuffer (GL_ARRAY_BUFFER, gear->vbo);
    glBufferData (GL_ARRAY_BUFFER, gear->nvertices * sizeof(GearVertex),
                  gear->vertices, GL_STATIC_DRAW);

    return gear;
}

/**
 * Multiplies two 4x4 matrices.
 *
 * The result is stored in matrix m.
 *
 * @param m the first matrix to multiply
 * @param n the second matrix to multiply
 */
static void
multiply (GLfloat *m, const GLfloat *n)
{
    GLfloat tmp[16];
    const GLfloat *row, *column;
    div_t d;
    int i, j;

    for (i = 0; i < 16; i++) {
        tmp[i] = 0;
        d = div(i, 4);
        row = n + d.quot * 4;
        column = m + d.rem;
        for (j = 0; j < 4; j++)
            tmp[i] += row[j] * column[j * 4];
    }
    memcpy (m, &tmp, sizeof tmp);
}

/**
 * Rotates a 4x4 matrix.
 *
 * @param[in,out] m the matrix to rotate
 * @param angle the angle to rotate
 * @param x the x component of the direction to rotate to
 * @param y the y component of the direction to rotate to
 * @param z the z component of the direction to rotate to
 */
static void
rotate (GLfloat *m, GLfloat angle, GLfloat x, GLfloat y, GLfloat z)
{
    double s, c;

    sincos (angle, &s, &c);

    {
        GLfloat r[16] = {
            x * x * (1 - c) + c,     y * x * (1 - c) + z * s, x * z * (1 - c) - y * s, 0,
            x * y * (1 - c) - z * s, y * y * (1 - c) + c,     y * z * (1 - c) + x * s, 0,
            x * z * (1 - c) + y * s, y * z * (1 - c) - x * s, z * z * (1 - c) + c,     0,
            0, 0, 0, 1
        };

        multiply (m, r);
    }
}


/**
 * Translates a 4x4 matrix.
 *
 * @param[in,out] m the matrix to translate
 * @param x the x component of the direction to translate to
 * @param y the y component of the direction to translate to
 * @param z the z component of the direction to translate to
 */
static void
translate(GLfloat *m, GLfloat x, GLfloat y, GLfloat z)
{
    GLfloat t[16] = { 1, 0, 0, 0,  0, 1, 0, 0,  0, 0, 1, 0,  x, y, z, 1 };

    multiply (m, t);
}

/**
 * Creates an identity 4x4 matrix.
 *
 * @param m the matrix make an identity matrix
 */
static void
identity (GLfloat *m)
{
    GLfloat t[16] = {
        1.0, 0.0, 0.0, 0.0,
        0.0, 1.0, 0.0, 0.0,
        0.0, 0.0, 1.0, 0.0,
        0.0, 0.0, 0.0, 1.0,
    };

    memcpy (m, t, sizeof(t));
}

/**
 * Transposes a 4x4 matrix.
 *
 * @param m the matrix to transpose
 */
static void
transpose (GLfloat *m)
{
    GLfloat t[16] = {
        m[0], m[4], m[8],  m[12],
        m[1], m[5], m[9],  m[13],
        m[2], m[6], m[10], m[14],
        m[3], m[7], m[11], m[15]};

    memcpy (m, t, sizeof(t));
}

/**
 * Inverts a 4x4 matrix.
 *
 * This function can currently handle only pure translation-rotation matrices.
 * Read http://www.gamedev.net/community/forums/topic.asp?topic_id=425118
 * for an explanation.
 */
static void
invert (GLfloat *m)
{
    GLfloat t[16];
    identity (t);

    // Extract and invert the translation part 't'. The inverse of a
    // translation matrix can be calculated by negating the translation
    // coordinates.
    t[12] = -m[12]; t[13] = -m[13]; t[14] = -m[14];

    // Invert the rotation part 'r'. The inverse of a rotation matrix is
    // equal to its transpose.
    m[12] = m[13] = m[14] = 0;
    transpose (m);

    // inv (m) = inv (r) * inv (t)
    multiply (m, t);
}

/**
 * Calculate a perspective projection transformation.
 *
 * @param m the matrix to save the transformation in
 * @param fovy the field of view in the y direction
 * @param aspect the view aspect ratio
 * @param zNear the near clipping plane
 * @param zFar the far clipping plane
 */
static void
perspective (GLfloat *m, GLfloat fovy, GLfloat aspect, GLfloat zNear, GLfloat zFar)
{
    GLfloat tmp[16];
    double sine, cosine, cotangent, deltaZ;
    GLfloat radians = fovy / 2 * M_PI / 180;

    identity (tmp);

    deltaZ = zFar - zNear;
    sincos (radians, &sine, &cosine);

    if ((deltaZ == 0) || (sine == 0) || (aspect == 0))
      return;

    cotangent = cosine / sine;

    tmp[0] = cotangent / aspect;
    tmp[5] = cotangent;
    tmp[10] = -(zFar + zNear) / deltaZ;
    tmp[11] = -1;
    tmp[14] = -2 * zNear * zFar / deltaZ;
    tmp[15] = 0;

    memcpy (m, tmp, sizeof(tmp));
}

/**
 * Draws a gear.
 *
 * @param gear the gear to draw
 * @param transform the current transformation matrix
 * @param x the x position to draw the gear at
 * @param y the y position to draw the gear at
 * @param angle the rotation angle of the gear
 * @param color the color of the gear
 */
static void
draw_gear (struct gear *gear, GLfloat *transform,
           GLfloat x, GLfloat y, GLfloat angle, const GLfloat color[4])
{
    GLfloat model_view[16];
    GLfloat normal_matrix[16];
    GLfloat model_view_projection[16];
    int n;

    /* Translate and rotate the gear */
    memcpy(model_view, transform, sizeof (model_view));
    translate(model_view, x, y, 0);
    rotate(model_view, 2 * M_PI * angle / 360.0, 0, 0, 1);

    /* Create and set the ModelViewProjectionMatrix */
    memcpy (model_view_projection,
            ProjectionMatrix,
            sizeof(model_view_projection));
    multiply (model_view_projection, model_view);

    glUniformMatrix4fv (ModelViewProjectionMatrix_location, 1, GL_FALSE,
                        model_view_projection);

    /*
     * Create and set the NormalMatrix. It's the inverse transpose of the
     * ModelView matrix.
     */
    memcpy (normal_matrix, model_view, sizeof (normal_matrix));
    invert (normal_matrix);
    transpose (normal_matrix);
    glUniformMatrix4fv (NormalMatrix_location, 1, GL_FALSE, normal_matrix);

    /* Set the gear color */
    glUniform4fv (MaterialColor_location, 1, color);

    /* Set the vertex buffer object to use */
    glBindBuffer (GL_ARRAY_BUFFER, gear->vbo);

    /* Set up the position of the attributes in the vertex buffer object */
    glVertexAttribPointer (0, 3, GL_FLOAT, GL_FALSE,
                           6 * sizeof(GLfloat), NULL);
    glVertexAttribPointer (1, 3, GL_FLOAT, GL_FALSE,
                           6 * sizeof(GLfloat), (GLfloat *) 0 + 3);

    /* Enable the attributes */
    glEnableVertexAttribArray (0);
    glEnableVertexAttribArray (1);

    /* Draw the triangle strips that comprise the gear */
    for (n = 0; n < gear->nstrips; n++)
        glDrawArrays (GL_TRIANGLE_STRIP,
                      gear->strips[n].first,
                      gear->strips[n].count);

    /* Disable the attributes */
    glDisableVertexAttribArray (1);
    glDisableVertexAttribArray (0);
}

/**
 * Draws the gears.
 */
static void
gears_draw(void)
{
    const static GLfloat red[4] = { 0.8, 0.1, 0.0, 1.0 };
    const static GLfloat green[4] = { 0.0, 0.8, 0.2, 1.0 };
    const static GLfloat blue[4] = { 0.2, 0.2, 1.0, 1.0 };
    GLfloat transform[16];
    identity(transform);

    glClearColor (0.0, 0.0, 0.0, 0.0);
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    /* Translate and rotate the view */
    translate (transform, 0, 0, -20);
    rotate (transform, 2 * M_PI * view_rot[0] / 360.0, 1, 0, 0);
    rotate (transform, 2 * M_PI * view_rot[1] / 360.0, 0, 1, 0);
    rotate (transform, 2 * M_PI * view_rot[2] / 360.0, 0, 0, 1);

    /* Draw the gears */
    draw_gear (gear1, transform, -3.0, -2.0, angle, red);
    draw_gear (gear2, transform, 3.1, -2.0, -2 * angle - 9.0, green);
    draw_gear (gear3, transform, -3.1, 4.2, -2 * angle - 25.0, blue);
}

static gboolean
paint_cb (void *user_data)
{
    Data *data = user_data;
    double elapsed = g_timer_elapsed (data->timer, NULL);
    double dt = elapsed - data->last_elapsed;
    CoglError *error = NULL;

    /* Draw scene with GLES2 */
    if (!cogl_push_gles2_context (data->ctx,
                                  data->gles2_ctx,
                                  data->fb,
                                  data->fb,
                                  &error))
    {
        g_error ("Failed to push gles2 context: %s\n", error->message);
    }

    gears_draw ();

    cogl_pop_gles2_context (data->ctx);

    cogl_onscreen_swap_buffers (COGL_ONSCREEN (data->fb));

    /* advance rotation for next frame */
    angle += 70.0 * dt;  /* 70 degrees per second */
    if (angle > 3600.0)
        angle -= 3600.0;

    data->frames++;

    if (elapsed > 5.0) {
        GLfloat fps = data->frames / elapsed;
        printf ("%d frames in %3.1f seconds = %6.3f FPS\n",
                data->frames, elapsed, fps);
        g_timer_reset (data->timer);
        data->last_elapsed = 0;
        data->frames = 0;
    }else
      data->last_elapsed = elapsed;

    return FALSE; /* remove the callback */
}

static void
frame_event_cb (CoglOnscreen *onscreen,
                CoglFrameEvent event,
                CoglFrameInfo *info,
                void *user_data)
{
    if (event == COGL_FRAME_EVENT_SYNC)
        paint_cb (user_data);
}

/**
 * Handles a new window size or exposure.
 *
 * @param width the window width
 * @param height the window height
 */
static void
gears_reshape (int width, int height)
{
    /* Update the projection matrix */
    perspective (ProjectionMatrix, 60.0, width / (float)height, 1.0, 1024.0);

    /* Set the viewport */
    glViewport (0, 0, (GLint) width, (GLint) height);
}

#if 0
/**
 * Handles special eglut events.
 *
 * @param special the event to handle.
 */
static void
gears_special(int special)
{
    switch (special) {
      case EGLUT_KEY_LEFT:
        view_rot[1] += 5.0;
        break;
      case EGLUT_KEY_RIGHT:
        view_rot[1] -= 5.0;
        break;
      case EGLUT_KEY_UP:
        view_rot[0] += 5.0;
        break;
      case EGLUT_KEY_DOWN:
        view_rot[0] -= 5.0;
        break;
    }
}
#endif

static const char vertex_shader[] =
    "attribute vec3 position;\n"
    "attribute vec3 normal;\n"
    "\n"
    "uniform mat4 ModelViewProjectionMatrix;\n"
    "uniform mat4 NormalMatrix;\n"
    "uniform vec4 LightSourcePosition;\n"
    "uniform vec4 MaterialColor;\n"
    "\n"
    "varying vec4 Color;\n"
    "\n"
    "void main(void)\n"
    "{\n"
    "    // Transform the normal to eye coordinates\n"
    "    vec3 N = normalize(vec3(NormalMatrix * vec4(normal, 1.0)));\n"
    "\n"
    "    // The LightSourcePosition is actually its direction for directional light\n"
    "    vec3 L = normalize(LightSourcePosition.xyz);\n"
    "\n"
    "    // Multiply the diffuse value by the vertex color (which is fixed in this case)\n"
    "    // to get the actual color that we will use to draw this vertex with\n"
    "    float diffuse = max(dot(N, L), 0.0);\n"
    "    Color = diffuse * MaterialColor;\n"
    "\n"
    "    // Transform the position to clip coordinates\n"
    "    gl_Position = ModelViewProjectionMatrix * vec4(position, 1.0);\n"
    "}";

static const char fragment_shader[] =
    "precision mediump float;\n"
    "varying vec4 Color;\n"
    "\n"
    "void main(void)\n"
    "{\n"
    "    gl_FragColor = Color;\n"
    "}";

static void
gears_init(void)
{
    GLuint v, f, program;
    const char *p;
    char msg[512];

    glEnable (GL_CULL_FACE);
    glEnable (GL_DEPTH_TEST);

    /* Compile the vertex shader */
    p = vertex_shader;
    v = glCreateShader (GL_VERTEX_SHADER);
    glShaderSource (v, 1, &p, NULL);
    glCompileShader (v);
    glGetShaderInfoLog (v, sizeof msg, NULL, msg);
    printf ("vertex shader info: %s\n", msg);

    /* Compile the fragment shader */
    p = fragment_shader;
    f = glCreateShader (GL_FRAGMENT_SHADER);
    glShaderSource (f, 1, &p, NULL);
    glCompileShader (f);
    glGetShaderInfoLog (f, sizeof msg, NULL, msg);
    printf ("fragment shader info: %s\n", msg);

    /* Create and link the shader program */
    program = glCreateProgram ();
    glAttachShader (program, v);
    glAttachShader (program, f);
    glBindAttribLocation (program, 0, "position");
    glBindAttribLocation (program, 1, "normal");

    glLinkProgram (program);
    glGetProgramInfoLog (program, sizeof msg, NULL, msg);
    printf ("info: %s\n", msg);

    /* Enable the shaders */
    glUseProgram (program);

    /* Get the locations of the uniforms so we can access them */
    ModelViewProjectionMatrix_location =
      glGetUniformLocation (program, "ModelViewProjectionMatrix");
    NormalMatrix_location =
      glGetUniformLocation (program, "NormalMatrix");
    LightSourcePosition_location =
      glGetUniformLocation (program, "LightSourcePosition");
    MaterialColor_location =
      glGetUniformLocation (program, "MaterialColor");

    /* Set the LightSourcePosition uniform which is constant throughout
     * the program */
    glUniform4fv (LightSourcePosition_location, 1, LightSourcePosition);

    /* make the gears */
    gear1 = create_gear (1.0, 4.0, 1.0, 20, 0.7);
    gear2 = create_gear (0.5, 2.0, 2.0, 10, 0.7);
    gear3 = create_gear (1.3, 2.0, 0.5, 10, 0.7);
}

int
main (int argc, char **argv)
{
    Data data;
    CoglOnscreen *onscreen;
    CoglError *error = NULL;
    GSource *cogl_source;
    GMainLoop *loop;
    CoglRenderer *renderer;
    CoglDisplay *display;

    renderer = cogl_renderer_new ();
    cogl_renderer_add_constraint (renderer,
                                  COGL_RENDERER_CONSTRAINT_SUPPORTS_COGL_GLES2);
    display = cogl_display_new (renderer, NULL);
    data.ctx = cogl_context_new (display, NULL);

    onscreen = cogl_onscreen_new (data.ctx, 300, 300);
    cogl_onscreen_show (onscreen);
    data.fb = onscreen;

    data.gles2_ctx = cogl_gles2_context_new (data.ctx, &error);
    if (!data.gles2_ctx)
        g_error ("Failed to create GLES2 context: %s\n", error->message);

    /* Draw scene with GLES2 */
    if (!cogl_push_gles2_context (data.ctx,
                                  data.gles2_ctx,
                                  data.fb,
                                  data.fb,
                                  &error))
    {
        g_error ("Failed to push gles2 context: %s\n", error->message);
    }

    gears_reshape (cogl_framebuffer_get_width (data.fb),
                   cogl_framebuffer_get_height (data.fb));

    /* Initialize the gears */
    gears_init();

    cogl_pop_gles2_context (data.ctx);

    cogl_source = cogl_glib_source_new (data.ctx, G_PRIORITY_DEFAULT);

    g_source_attach (cogl_source, NULL);

    cogl_onscreen_add_frame_callback (COGL_ONSCREEN (data.fb),
                                      frame_event_cb,
                                      &data,
                                      NULL); /* destroy notify */

    g_idle_add (paint_cb, &data);

    data.timer = g_timer_new ();
    data.frames = 0;
    data.last_elapsed = 0;

    loop = g_main_loop_new (NULL, TRUE);
    g_main_loop_run (loop);

    return 0;
}