GLGears for SDL2 + OpenGL ES 2

sdl2gles2gears.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 <[email protected]>
 * 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 <[email protected]>
 * Jul 13, 2010
 */

/*
 * Ported to SDL2
 * David Ludwig <[email protected]>
 * Feb 14, 2015
 */

#define GL_GLEXT_PROTOTYPES
#define EGL_EGLEXT_PROTOTYPES

#define _GNU_SOURCE

#include <math.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <SDL.h>
#include <SDL_opengles2.h>

#ifndef _MSC_VER
#include <unistd.h>
#else
#pragma warning(disable:4244 4305)  /* disable float <--> double conversion warnings */
#endif

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

/**
 * 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};

/** The SDL window */
static SDL_Window * window = NULL;
/** The SDL OpenGL context */
static SDL_GLContext gl_context = NULL;
/** Should the main loop be running? */
static SDL_bool app_running = SDL_TRUE;

#ifndef __GLIBC__
/**
 * sincos() for platforms that do not support it.
 */
void sincos(double x, double *_sin, double *_cos)
{
   *_sin = sin(x);
   *_cos = cos(x);
}
#endif

/** 
 * 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 = (struct 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 = (struct vertex_strip *)calloc(gear->nstrips, sizeof (*gear->strips));

   /* Allocate memory for the vertices */
   gear->vertices = (GearVertex *)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
 */
void perspective(GLfloat *m, GLfloat fovy, GLfloat aspect, GLfloat zNear, GLfloat zFar)
{
   GLfloat tmp[16];
   identity(tmp);

   double sine, cosine, cotangent, deltaZ;
   GLfloat radians = fovy / 2 * M_PI / 180;

   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];

   /* 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 */
   int n;
   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);
   
   SDL_GL_SwapWindow(window);
}

/** 
 * 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);
}

/** 
 * Handles SDL events.
 * 
 * @param event the event to handle.
 */
static void
gears_event(SDL_Event * event)
{
   switch (event->type) {
      case SDL_KEYDOWN:
      {
         switch (event->key.keysym.sym) {
            case SDLK_LEFT:
               view_rot[1] += 5.0;
               break;
            case SDLK_RIGHT:
               view_rot[1] -= 5.0;
               break;
            case SDLK_UP:
               view_rot[0] += 5.0;
               break;
            case SDLK_DOWN:
               view_rot[0] -= 5.0;
               break;
         }
         break;
      }
      
      case SDL_QUIT:
         app_running = SDL_FALSE;
         break;
      
      case SDL_WINDOWEVENT:
      {
         switch (event->window.event) {
            case SDL_WINDOWEVENT_SIZE_CHANGED:
               gears_reshape(event->window.data1, event->window.data2);
               break;
         }
         break;
      }
   }
}

static void
gears_idle(void)
{
   static int frames = 0;
   static double tRot0 = -1.0, tRate0 = -1.0;
   double dt, t = SDL_GetTicks() / 1000.0;

   if (tRot0 < 0.0)
      tRot0 = t;
   dt = t - tRot0;
   tRot0 = t;

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

   //eglutPostRedisplay();
   frames++;

   if (tRate0 < 0.0)
      tRate0 = t;
   if (t - tRate0 >= 5.0) {
      GLfloat seconds = t - tRate0;
      GLfloat fps = frames / seconds;
      SDL_Log("%d frames in %3.1f seconds = %6.3f FPS\n", frames, seconds,
              fps);
      tRate0 = t;
      frames = 0;
   }
}

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);
   SDL_Log("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);
   SDL_Log("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);
   SDL_Log("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 throught 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);
}

void gears_frame(void *userdata)
{
   /* Process events */
   SDL_Event event;
   while (SDL_PollEvent(&event)) {
      gears_event(&event);
   }
   
   /* Update gears */
   gears_idle();
   
   /* Draw */
   gears_draw();
}

int
main(int argc, char *argv[])
{
   int w, h;
   void *native_window = NULL;

   if (argc >= 3 && (SDL_strcmp("--native_window", argv[1]) == 0)) {
      native_window = (void *)SDL_strtoull(argv[2], NULL, 10);
   }

   if (native_window) {
      SDL_SetMainReady();
   }

   if (SDL_Init(SDL_INIT_VIDEO) != 0) {
      SDL_Log("Unable to initialize SDL video subsystem: %s\n", SDL_GetError());
      return 1;
   }

   /* Initialize the window */
   if (native_window) {
      window = SDL_CreateWindowFrom(native_window);
   } else {
      Uint32 window_flags = SDL_WINDOW_OPENGL;
#if __IPHONEOS__ || __ANDROID__ || __WINRT__
      window_flags |= SDL_WINDOW_FULLSCREEN_DESKTOP;
#endif
      window = SDL_CreateWindow("es2gears", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 300, 300, window_flags);
   }

   if (window == NULL) {
      SDL_Log("Unable to create window: %s", SDL_GetError());
      return 1;
   }
   
   SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
   SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
   SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);
   
   gl_context = SDL_GL_CreateContext(window);
   if (gl_context == NULL) {
      SDL_Log("Unable to create GL context: %s", SDL_GetError());
      return 1;
   }
   
   /* Initialize the gears */
   gears_init();
   SDL_GetWindowSize(window, &w, &h);
   gears_reshape(w, h);

#if __WINRT__ && SDL_VIDEO_XAML
   if (native_window) {
      SDL_WinRTXAMLSetAnimationCallback(&gears_frame, NULL);
   }
#endif  /* __WINRT__ && SDL_VIDEO_XAML */

   /* Run the main loop */
   if (!native_window) {
      while (app_running) {
         gears_frame(NULL);
      }
   }

   return 0;
}

Updated: added support for in-progress WinRT/XAML support

Update: made optional OS-native window get passed into main(int,char**), rather than through gears_app_init

Update: enabled optional compilation as C++ code (at least, in MSVC 2013)

Update: white-space cleanups

Update: disabled 'float to/from double' conversion warnings in MSVC

Thanks! I used this to get the gears demo running on my TV under WebOS.

gcc sdl2gles2gears.c $(sdl2-config --cflags --libs) -lm -lGLESv2 -ogles && ./gles # basilar compile and run (works for me)