Bug fix: GLX backend incompatibility with mesa & others

- Fix a bug that glx_bind_pixmap() doesn't work with mesa drivers.
  Thanks to Janhouse and mkraemer for reporting. (#7)

- Use stencil buffer to attempt to eliminate potential double-paint
  issue in glx_render(). X Fixes doesn't guarantee the rectangles in a
  region do not overlap, and this may cause some regions to be painted
  twice, which would be a problem if we are painting transparent things.
  Now the target window must have a stencil buffer. Compiz uses its own
  region implementation to deal with this, but as a lightweight
  compositor we can't really do the same. It may have a positive or
  negative effort over performance. Callgrind result indicates basically
  no change in performance, but this may or may not be true.

- Correctly distinguish GL extensions and GLX extensions. Sorry. :-D

- Handle screen size. Thanks to tsmithe for reporting. (#7)

- Rename OpenGL backend to GLX backend, because, we might have a EGL
  backend someday.

- Add configuration file option `backend` to specify backend. Add
  `backend` to D-Bus `opts_get`.

- Add OpenGL shader compilation code, but currently unused.

- Minor adjustments.

- Known issue: Window content doesn't get updated in VirtualBox,
  probably because its OpenGL implementation requires constant rebinding
  of texture. But that's really slow...

- Known issue: Blur feature is still unimplemented in GLX backend.

6 files changed, 346 insertions, 52 deletions

diff --git a/common.h b/common.h
index 30c09e3a5..f36edf4ba 100644
--- a/common.h
+++ b/common.h
@@ -89,6 +89,10 @@
 
 // libGL
 #ifdef CONFIG_VSYNC_OPENGL
+#ifdef CONFIG_VSYNC_OPENGL_GLSL
+#define GL_GLEXT_PROTOTYPES
+#endif
+
 #include <GL/glx.h>
 #endif
 
@@ -263,6 +267,7 @@ typedef enum {
 enum backend {
   BKEND_XRENDER,
   BKEND_GLX,
+  NUM_BKEND,
 };
 
 typedef struct _glx_texture glx_texture_t;
@@ -594,6 +599,8 @@ typedef struct {
   // === OpenGL related ===
   /// GLX context.
   GLXContext glx_context;
+  /// Whether we have GL_ARB_texture_non_power_of_two.
+  bool glx_has_texture_non_power_of_two;
   /// Pointer to glXGetVideoSyncSGI function.
   f_GetVideoSync glXGetVideoSyncSGI;
   /// Pointer to glXWaitVideoSyncSGI function.
@@ -874,6 +881,7 @@ typedef enum {
 
 extern const char * const WINTYPES[NUM_WINTYPES];
 extern const char * const VSYNC_STRS[NUM_VSYNC];
+extern const char * const BACKEND_STRS[NUM_BKEND];
 extern session_t *ps_g;
 
 // == Debugging code ==
@@ -1252,6 +1260,20 @@ parse_vsync(session_t *ps, const char *str) {
   return false;
 }
 
+/**
+ * Parse a backend option argument.
+ */
+static inline bool
+parse_backend(session_t *ps, const char *str) {
+  for (enum backend i = 0; i < (sizeof(BACKEND_STRS) / sizeof(BACKEND_STRS[0])); ++i)
+    if (!strcasecmp(str, BACKEND_STRS[i])) {
+      ps->o.backend = i;
+      return true;
+    }
+  printf_errf("(\"%s\"): Invalid backend argument.", str);
+  return false;
+}
+
 timeout_t *
 timeout_insert(session_t *ps, time_ms_t interval,
     bool (*callback)(session_t *ps, timeout_t *ptmout), void *data);
@@ -1543,6 +1565,9 @@ void
 glx_on_root_change(session_t *ps);
 
 bool
+glx_init_blur(session_t *ps);
+
+bool
 glx_bind_pixmap(session_t *ps, glx_texture_t **pptex, Pixmap pixmap,
     int width, int height, int depth);
 
@@ -1554,6 +1579,9 @@ glx_tex_binded(const glx_texture_t *ptex) {
   return ptex && ptex->glpixmap && ptex->texture;
 }
 
+void
+glx_set_clip(session_t *ps, XserverRegion reg);
+
 bool
 glx_render(session_t *ps, const glx_texture_t *ptex,
     int x, int y, int dx, int dy, int width, int height, int z,
diff --git a/compton.c b/compton.c
index f8c33d265..df39a36f4 100644
--- a/compton.c
+++ b/compton.c
@@ -40,6 +40,12 @@ const char * const VSYNC_STRS[NUM_VSYNC] = {
   "opengl-swc",   // VSYNC_OPENGL_SWC
 };
 
+/// Names of backends.
+const char * const BACKEND_STRS[NUM_BKEND] = {
+  "xrender",      // BKEND_XRENDER
+  "glx",          // BKEND_GLX
+};
+
 /// Function pointers to init VSync modes.
 static bool (* const (VSYNC_FUNCS_INIT[NUM_VSYNC]))(session_t *ps) = {
   [VSYNC_DRM        ] = vsync_drm_init,
@@ -1628,8 +1634,7 @@ paint_all(session_t *ps, XserverRegion region, win *t) {
     reg_paint = region;
   }
 
-  XFixesSetPictureClipRegion(ps->dpy, ps->tgt_buffer, 0, 0, reg_paint);
-
+  set_tgt_clip(ps, reg_paint);
   paint_root(ps, reg_paint);
 
   // Create temporary regions for use during painting
@@ -1666,8 +1671,7 @@ paint_all(session_t *ps, XserverRegion region, win *t) {
 
       // Detect if the region is empty before painting
       if (region == reg_paint || !is_region_empty(ps, reg_paint)) {
-        XFixesSetPictureClipRegion(ps->dpy, ps->tgt_buffer, 0, 0,
-            reg_paint);
+        set_tgt_clip(ps, reg_paint);
 
         win_paint_shadow(ps, w, reg_paint);
       }
@@ -1694,7 +1698,7 @@ paint_all(session_t *ps, XserverRegion region, win *t) {
     }
 
     if (!is_region_empty(ps, reg_paint)) {
-      XFixesSetPictureClipRegion(ps->dpy, ps->tgt_buffer, 0, 0, reg_paint);
+      set_tgt_clip(ps, reg_paint);
       // Blur window background
       if ((ps->o.blur_background && WMODE_SOLID != w->mode)
           || (ps->o.blur_background_frame && w->frame_opacity)) {
@@ -1713,7 +1717,7 @@ paint_all(session_t *ps, XserverRegion region, win *t) {
 
   // Do this as early as possible
   if (!ps->o.dbe)
-    XFixesSetPictureClipRegion(ps->dpy, ps->tgt_buffer, 0, 0, None);
+    set_tgt_clip(ps, None);
 
   if (ps->o.vsync) {
     // Make sure all previous requests are processed to achieve best
@@ -4044,7 +4048,7 @@ usage(void) {
     "    opengl-oml = Try to VSync with OML_sync_control OpenGL extension.\n"
     "      Only work on some drivers. Experimental." WARNING"\n"
     "    opengl-swc = Try to VSync with SGI_swap_control OpenGL extension.\n"
-    "      Only work on some drivers. Works only with OpenGL backend.\n"
+    "      Only work on some drivers. Works only with GLX backend.\n"
     "      Does not actually control paint timing, only buffer swap is\n"
     "      affected, so it doesn't have the effect of --sw-opti unlike\n"
     "      other methods. Experimental." WARNING "\n"
@@ -4098,8 +4102,8 @@ usage(void) {
     "--invert-color-include condition\n"
     "  Specify a list of conditions of windows that should be painted with\n"
     "  inverted color. Resource-hogging, and is not well tested.\n"
-    "--opengl\n"
-    "  Enable the highly experimental OpenGL backend." WARNING "\n"
+    "--backend backend\n"
+    "  Choose backend. Possible choices are xrender and glx" WARNING ".\n"
 #undef WARNING
 #ifndef CONFIG_DBUS
 #define WARNING WARNING_DISABLED
@@ -4450,6 +4454,9 @@ parse_config(session_t *ps, struct options_tmp *pcfgtmp) {
   // --vsync
   if (config_lookup_string(&cfg, "vsync", &sval) && !parse_vsync(ps, sval))
     exit(1);
+  // --backend
+  if (config_lookup_string(&cfg, "backend", &sval) && !parse_backend(ps, sval))
+    exit(1);
   // --alpha-step
   config_lookup_float(&cfg, "alpha-step", &ps->o.alpha_step);
   // --dbe
@@ -4552,6 +4559,7 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
     { "logpath", required_argument, NULL, 287 },
     { "invert-color-include", required_argument, NULL, 288 },
     { "opengl", no_argument, NULL, 289 },
+    { "backend", required_argument, NULL, 290 },
     // Must terminate with a NULL entry
     { NULL, 0, NULL, 0 },
   };
@@ -4809,6 +4817,11 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
         // --opengl
         ps->o.backend = BKEND_GLX;
         break;
+      case 290:
+        // --backend
+        if (!parse_backend(ps, optarg))
+          exit(1);
+        break;
       default:
         usage();
         break;
@@ -5115,7 +5128,7 @@ vsync_opengl_swc_init(session_t *ps) {
 
   if (BKEND_GLX != ps->o.backend) {
     printf_errf("(): I'm afraid glXSwapIntervalSGI wouldn't help if you are "
-        "not using OpenGL backend. You could try, nonetheless.");
+        "not using GLX backend. You could try, nonetheless.");
   }
 
   // Get video sync functions
@@ -5283,11 +5296,13 @@ init_filters(session_t *ps) {
           }
           break;
         }
+#ifdef CONFIG_VSYNC_OPENGL
       case BKEND_GLX:
         {
-          printf_errf("(): OpenGL blur is not implemented yet.");
-          return false;
+          if (!glx_init_blur(ps))
+            return false;
         }
+#endif
     }
   }
 
@@ -5709,6 +5724,7 @@ session_init(session_t *ps_old, int argc, char **argv) {
 
 #ifdef CONFIG_VSYNC_OPENGL
     .glx_context = None,
+    .glx_has_texture_non_power_of_two = false,
     .glXGetVideoSyncSGI = NULL,
     .glXWaitVideoSyncSGI = NULL,
     .glXGetSyncValuesOML = NULL,
@@ -5862,6 +5878,14 @@ session_init(session_t *ps_old, int argc, char **argv) {
       ps->o.dbe = false;
   }
 
+  // Start listening to events on root earlier to catch all possible
+  // root geometry changes
+  XSelectInput(ps->dpy, ps->root,
+    SubstructureNotifyMask
+    | ExposureMask
+    | StructureNotifyMask
+    | PropertyChangeMask);
+
   ps->root_width = DisplayWidth(ps->dpy, ps->scr);
   ps->root_height = DisplayHeight(ps->dpy, ps->scr);
 
@@ -5874,7 +5898,7 @@ session_init(session_t *ps_old, int argc, char **argv) {
 
   // Initialize DBE
   if (ps->o.dbe && BKEND_GLX == ps->o.backend) {
-    printf_errf("(): DBE couldn't be used on OpenGL backend.");
+    printf_errf("(): DBE couldn't be used on GLX backend.");
     ps->o.dbe = false;
   }
 
@@ -5887,7 +5911,7 @@ session_init(session_t *ps_old, int argc, char **argv) {
     if (!glx_init(ps, true))
       exit(1);
 #else
-    printf_errfq(1, "(): OpenGL backend support not compiled in.");
+    printf_errfq(1, "(): GLX backend support not compiled in.");
 #endif
   }
 
@@ -5926,6 +5950,7 @@ session_init(session_t *ps_old, int argc, char **argv) {
     }
   }
 
+  // Initialize filters, must be preceded by OpenGL context creation
   if (!init_filters(ps))
     exit(1);
 
@@ -5947,12 +5972,6 @@ session_init(session_t *ps_old, int argc, char **argv) {
 
   redir_start(ps);
 
-  XSelectInput(ps->dpy, ps->root,
-    SubstructureNotifyMask
-    | ExposureMask
-    | StructureNotifyMask
-    | PropertyChangeMask);
-
   {
     Window root_return, parent_return;
     Window *children;
diff --git a/compton.h b/compton.h
index 0479b3d0a..49ffcb88f 100644
--- a/compton.h
+++ b/compton.h
@@ -176,7 +176,7 @@ paint_isvalid(session_t *ps, const paint_t *ppaint) {
   return true;
 }
 /**
- * Bind texture in paint_t if we are using OpenGL backend.
+ * Bind texture in paint_t if we are using GLX backend.
  */
 static inline bool
 paint_bind_tex(session_t *ps, paint_t *ppaint, int wid, int hei, int depth) {
@@ -542,6 +542,20 @@ win_render(session_t *ps, win *w, int x, int y, int wid, int hei, double opacity
       pict, (w ? w->paint.ptex: ps->root_tile_paint.ptex), reg_paint);
 }
 
+static inline void
+set_tgt_clip(session_t *ps, XserverRegion reg) {
+  switch (ps->o.backend) {
+    case BKEND_XRENDER:
+      XFixesSetPictureClipRegion(ps->dpy, ps->tgt_buffer, 0, 0, reg);
+      break;
+#ifdef CONFIG_VSYNC_OPENGL
+    case BKEND_GLX:
+      glx_set_clip(ps, reg);
+      break;
+#endif
+  }
+}
+
 static void
 paint_all(session_t *ps, XserverRegion region, win *t);
 
diff --git a/dbus.c b/dbus.c
index 4dd6143f8..25b37fdf3 100644
--- a/dbus.c
+++ b/dbus.c
@@ -876,6 +876,11 @@ cdbus_process_opts_get(session_t *ps, DBusMessage *msg) {
     cdbus_reply_string(ps, msg, VSYNC_STRS[ps->o.vsync]);
     return true;
   }
+  if (!strcmp("backend", target)) {
+    assert(ps->o.backend < sizeof(BACKEND_STRS) / sizeof(BACKEND_STRS[0]));
+    cdbus_reply_string(ps, msg, BACKEND_STRS[ps->o.backend]);
+    return true;
+  }
   cdbus_m_opts_get_do(dbe, cdbus_reply_bool);
   cdbus_m_opts_get_do(vsync_aggressive, cdbus_reply_bool);
 
diff --git a/opengl.c b/opengl.c
index 16e99de02..2ec0b3313 100644
--- a/opengl.c
+++ b/opengl.c
@@ -51,7 +51,7 @@ glx_init(session_t *ps, bool need_render) {
   }
 
   // Ensure GLX_EXT_texture_from_pixmap exists
-  if (need_render && !glx_hasext(ps, "GLX_EXT_texture_from_pixmap"))
+  if (need_render && !glx_hasglxext(ps, "GLX_EXT_texture_from_pixmap"))
     goto glx_init_end;
 
   // Get GLX context
@@ -68,6 +68,24 @@ glx_init(session_t *ps, bool need_render) {
     goto glx_init_end;
   }
 
+  // Ensure we have a stencil buffer. X Fixes does not guarantee rectangles
+  // in regions don't overlap, so we must use stencil buffer to make sure
+  // we don't paint a region for more than one time, I think?
+  if (need_render) {
+    GLint val = 0;
+    glGetIntegerv(GL_STENCIL_BITS, &val);
+    if (!val) {
+      printf_errf("(): Target window doesn't have stencil buffer.");
+      goto glx_init_end;
+    }
+  }
+
+  // Check GL_ARB_texture_non_power_of_two, requires a GLX context and
+  // must precede FBConfig fetching
+  if (need_render)
+    ps->glx_has_texture_non_power_of_two = glx_hasglext(ps,
+        "GL_ARB_texture_non_power_of_two");
+
   // Acquire function addresses
   if (need_render) {
     ps->glXBindTexImageProc = (f_BindTexImageEXT)
@@ -85,24 +103,23 @@ glx_init(session_t *ps, bool need_render) {
     goto glx_init_end;
 
   if (need_render) {
-    // Adjust viewport
-    glViewport(0, 0, ps->root_width, ps->root_height);
-
-    // Initialize settings, copied from dcompmgr
-    glMatrixMode(GL_PROJECTION);
-    glLoadIdentity();
-    glOrtho(0, ps->root_width, ps->root_height, 0, -100.0, 100.0);
-    glMatrixMode(GL_MODELVIEW);
-    glLoadIdentity();
+    glx_on_root_change(ps);
+
     // glEnable(GL_DEPTH_TEST);
     glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
     glDisable(GL_BLEND);
 
+    // Initialize stencil buffer
+    glClear(GL_STENCIL_BUFFER_BIT);
+    glEnable(GL_STENCIL_TEST);
+    glStencilMask(0x1);
+    glStencilFunc(GL_EQUAL, 0x1, 0x1);
+
     // Clear screen
     glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
-    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
-    glXSwapBuffers(ps->dpy, get_tgt_window(ps));
+    // glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+    // glXSwapBuffers(ps->dpy, get_tgt_window(ps));
   }
 
   success = true;
@@ -141,6 +158,34 @@ glx_destroy(session_t *ps) {
 void
 glx_on_root_change(session_t *ps) {
   glViewport(0, 0, ps->root_width, ps->root_height);
+
+  // Initialize matrix, copied from dcompmgr
+  glMatrixMode(GL_PROJECTION);
+  glLoadIdentity();
+  glOrtho(0, ps->root_width, ps->root_height, 0, -100.0, 100.0);
+  glMatrixMode(GL_MODELVIEW);
+  glLoadIdentity();
+}
+
+/**
+ * Initialize GLX blur filter.
+ */
+bool
+glx_init_blur(session_t *ps) {
+  printf_errf("(): Blur on GLX backend isn't implemented yet, sorry.");
+  return false;
+
+// #ifdef CONFIG_VSYNC_OPENGL_GLSL
+//           static const char *FRAG_SHADER_BLUR = "";
+//           GLuint frag_shader = glx_create_shader(GL_FRAGMENT_SHADER, FRAG_SHADER_BLUR);
+//           if (!frag_shader) {
+//             printf_errf("(): Failed to create fragment shader for blurring.");
+//             return false;
+//           }
+// #else
+//           printf_errf("(): GLSL support not compiled in. Cannot do blur with GLX backend.");
+//           return false;
+// #endif
 }
 
 /**
@@ -346,15 +391,28 @@ glx_bind_pixmap(session_t *ps, glx_texture_t **pptex, Pixmap pixmap,
   if (!ptex->glpixmap) {
     need_release = false;
 
+    // Determine texture target, copied from compiz
+    GLint tex_tgt = 0;
+    if (GLX_TEXTURE_2D_BIT_EXT & pcfg->texture_tgts
+        && ps->glx_has_texture_non_power_of_two)
+      tex_tgt = GLX_TEXTURE_2D_EXT;
+    else if (GLX_TEXTURE_RECTANGLE_BIT_EXT & pcfg->texture_tgts)
+      tex_tgt = GLX_TEXTURE_RECTANGLE_EXT;
+    else if (!(GLX_TEXTURE_2D_BIT_EXT & pcfg->texture_tgts))
+      tex_tgt = GLX_TEXTURE_RECTANGLE_EXT;
+    else
+      tex_tgt = GLX_TEXTURE_2D_EXT;
+
 #ifdef DEBUG_GLX
-    printf_dbgf("(): depth %d rgba %d\n", depth, (GLX_TEXTURE_FORMAT_RGBA_EXT == pcfg->texture_fmt));
+    printf_dbgf("(): depth %d, tgt %#x, rgba %d\n", depth, tex_tgt,
+        (GLX_TEXTURE_FORMAT_RGBA_EXT == pcfg->texture_fmt));
 #endif
 
-    int attrs[] = {
+    GLint attrs[] = {
         GLX_TEXTURE_FORMAT_EXT,
         pcfg->texture_fmt,
-        // GLX_TEXTURE_TARGET_EXT,
-        // ,
+        GLX_TEXTURE_TARGET_EXT,
+        tex_tgt,
         0,
     };
 
@@ -404,6 +462,51 @@ glx_release_pixmap(session_t *ps, glx_texture_t *ptex) {
 }
 
 /**
+ * Set clipping region on the target window.
+ */
+void
+glx_set_clip(session_t *ps, XserverRegion reg) {
+  glClear(GL_STENCIL_BUFFER_BIT);
+
+  if (reg) {
+    glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
+    glDepthMask(GL_FALSE);
+    glStencilOp(GL_REPLACE, GL_KEEP, GL_KEEP);
+
+    int nrects = 0;
+    XRectangle *rects = XFixesFetchRegion(ps->dpy, reg, &nrects);
+
+    glBegin(GL_QUADS);
+
+    for (int i = 0; i < nrects; ++i) {
+      GLint rx = rects[i].x;
+      GLint ry = rects[i].y;
+      GLint rxe = rx + rects[i].width;
+      GLint rye = ry + rects[i].height;
+      GLint z = 0;
+
+#ifdef DEBUG_GLX
+      printf_dbgf("(): Rect %d: %f, %f, %f, %f\n", i, rx, ry, rxe, rye);
+#endif
+
+      glVertex3i(rx, ry, z);
+      glVertex3i(rxe, ry, z);
+      glVertex3i(rxe, rye, z);
+      glVertex3i(rx, rye, z);
+    }
+
+    glEnd();
+
+    if (rects)
+      XFree(rects);
+
+    glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
+    glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
+    glDepthMask(GL_TRUE);
+  }
+}
+
+/**
  * @brief Render a region with texture data.
  */
 bool
@@ -440,11 +543,6 @@ glx_render(session_t *ps, const glx_texture_t *ptex,
     }
   }
 
-  glEnable(GL_TEXTURE_2D);
-  glBindTexture(GL_TEXTURE_2D, ptex->texture);
-
-  glBegin(GL_QUADS);
-
   {
     XserverRegion reg_new = None;
 
@@ -459,9 +557,10 @@ glx_render(session_t *ps, const glx_texture_t *ptex,
     int nrects = 1;
 
 #ifdef DEBUG_GLX
-    printf_dbgf("(): Draw: %d, %d, %d, %d -> %d, %d (%d, %d) z %d \n", x, y, width, height, dx, dy, ptex->width, ptex->height, z);
+    printf_dbgf("(): Draw: %d, %d, %d, %d -> %d, %d (%d, %d) z %d\n", x, y, width, height, dx, dy, ptex->width, ptex->height, z);
 #endif
 
+    /*
     if (reg_tgt) {
       reg_new = XFixesCreateRegion(ps->dpy, &rec_all, 1);
       XFixesIntersectRegion(ps->dpy, reg_new, reg_new, reg_tgt);
@@ -469,6 +568,12 @@ glx_render(session_t *ps, const glx_texture_t *ptex,
       nrects = 0;
       rects = XFixesFetchRegion(ps->dpy, reg_new, &nrects);
     }
+    */
+
+    glEnable(GL_TEXTURE_2D);
+    glBindTexture(GL_TEXTURE_2D, ptex->texture);
+
+    glBegin(GL_QUADS);
 
     for (int i = 0; i < nrects; ++i) {
       GLfloat rx = (double) (rects[i].x - dx + x) / ptex->width;
@@ -504,11 +609,12 @@ glx_render(session_t *ps, const glx_texture_t *ptex,
       glVertex3i(rdx, rdye, z);
     }
 
+    glEnd();
+
     if (rects && rects != &rec_all)
       XFree(rects);
     free_region(ps, &reg_new);
   }
-  glEnd();
 
   // Cleanup
   glBindTexture(GL_TEXTURE_2D, 0);
@@ -519,3 +625,85 @@ glx_render(session_t *ps, const glx_texture_t *ptex,
 
   return true;
 }
+
+#ifdef CONFIG_VSYNC_OPENGL_GLSL
+GLuint
+glx_create_shader(GLenum shader_type, const char *shader_str) {
+  bool success = false;
+  GLuint shader = glCreateShader(shader_type);
+  if (!shader) {
+    printf_errf("(): Failed to create shader with type %d.", shader_type);
+    goto glx_create_shader_end;
+  }
+  glShaderSource(shader, 1, &shader_str, NULL);
+  glCompileShader(shader);
+
+  // Get shader status
+  {
+    GLint status = GL_FALSE;
+    glGetShaderiv(shader, GL_COMPILE_STATUS, &status);
+    if (GL_FALSE == status) {
+      GLint log_len = 0;
+      glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &log_len);
+      if (log_len) {
+        char log[log_len + 1];
+        glGetShaderInfoLog(shader, log_len, NULL, log);
+        printf_errf("(): Failed to compile shader with type %d: %s",
+            shader_type, log);
+      }
+      goto glx_create_shader_end;
+    }
+  }
+
+  success = true;
+
+glx_create_shader_end:
+  if (shader && !success) {
+    glDeleteShader(shader);
+    shader = 0;
+  }
+
+  return shader;
+}
+
+GLuint
+glx_create_program(GLenum shader_type, const GLuint * const shaders,
+    int nshaders) {
+  bool success = false;
+  GLuint program = glCreateProgram();
+  if (!program) {
+    printf_errf("(): Failed to create program.");
+    goto glx_create_program_end;
+  }
+
+  for (int i = 0; i < nshaders; ++i)
+    glAttachShader(program, shaders[i]);
+  glLinkProgram(program);
+
+  // Get program status
+  {
+    GLint status = GL_FALSE;
+    glGetProgramiv(program, GL_LINK_STATUS, &status);
+    if (GL_FALSE == status) {
+      GLint log_len = 0;
+      glGetProgramiv(program, GL_INFO_LOG_LENGTH, &log_len);
+      if (log_len) {
+        char log[log_len + 1];
+        glGetProgramInfoLog(program, log_len, NULL, log);
+        printf_errf("(): Failed to link program: %s", log);
+      }
+      goto glx_create_program_end;
+    }
+  }
+  success = true;
+
+glx_create_program_end:
+  if (program && !success) {
+    glDeleteProgram(program);
+    program = 0;
+  }
+
+  return program;
+}
+#endif
+
diff --git a/opengl.h b/opengl.h
index b48b441d0..7f6dae696 100644
--- a/opengl.h
+++ b/opengl.h
@@ -13,21 +13,61 @@
 #include <ctype.h>
 
 /**
+ * Check if a word is in string.
+ */
+static inline bool
+wd_is_in_str(const char *haystick, const char *needle) {
+  if (!haystick)
+    return false;
+
+  assert(*needle);
+
+  const char *pos = haystick - 1;
+  while ((pos = strstr(pos + 1, needle))) {
+    // Continue if it isn't a word boundary
+    if (((pos - haystick) && !isspace(*(pos - 1)))
+        || (strlen(pos) > strlen(needle) && !isspace(pos[strlen(needle)])))
+      continue;
+    return true;
+  }
+
+  return false;
+}
+
+/**
  * Check if a GLX extension exists.
  */
 static inline bool
-glx_hasext(session_t *ps, const char *ext) {
+glx_hasglxext(session_t *ps, const char *ext) {
   const char *glx_exts = glXQueryExtensionsString(ps->dpy, ps->scr);
-  const char *pos = strstr(glx_exts, ext);
-  // Make sure the extension string is matched as a whole word
-  if (!pos
-      || ((pos - glx_exts) && !isspace(*(pos - 1)))
-      || (strlen(pos) > strlen(ext) && !isspace(pos[strlen(ext)]))) {
-    printf_errf("(): Missing OpenGL extension %s.", ext);
+  if (!glx_exts) {
+    printf_errf("(): Failed get GLX extension list.");
     return false;
   }
 
-  return true;
+  bool found = wd_is_in_str(glx_exts, ext);
+  if (!found)
+    printf_errf("(): Missing GLX extension %s.", ext);
+
+  return found;
+}
+
+/**
+ * Check if a GLX extension exists.
+ */
+static inline bool
+glx_hasglext(session_t *ps, const char *ext) {
+  const char *gl_exts = (const char *) glGetString(GL_EXTENSIONS);
+  if (!gl_exts) {
+    printf_errf("(): Failed get GL extension list.");
+    return false;
+  }
+
+  bool found = wd_is_in_str(gl_exts, ext);
+  if (!found)
+    printf_errf("(): Missing GL extension %s.", ext);
+
+  return found;
 }
 
 static inline XVisualInfo *