meta: build system for Dear ImGui + GL3W + FreeType2

* Add build instructions for Dear ImGui that build the SDL +
  OpenGL 3 / OpenGL ES 2 backend.
* Use Dear ImGui's bundled GL3W as a loader (including in azur itself,
  which has not been using a loader until now).
* Properly select headers for OpenGL ES 2.0 (with the VAO extension) and
  attributes for WebGL; clear up OpenGL 4 error codes.
* If FreeType2 is available through pkg-config, or if empscripten is
  used (since it has a FreeType port), use FreeType to render fonts in
  Dear ImGui for much-appreciated hinting quality.

Minor changes:
* Add window title to azur_init().
* Use emscripten's infinite loop simulation to make sure everything
  stays in the same thread. This is needed for Dear ImGui to work.

CMakeLists.txt

@@ -45,6 +45,7 @@ if(FACETS_PLATFORM STREQUAL linux)
   pkg_check_modules(sdl2 REQUIRED sdl2 IMPORTED_TARGET)
   pkg_check_modules(sdl2_image REQUIRED SDL2_image IMPORTED_TARGET)
   pkg_check_modules(opengl REQUIRED opengl IMPORTED_TARGET)
+  pkg_check_modules(glx REQUIRED glx IMPORTED_TARGET)
 
   link_libraries(PkgConfig::sdl2_image PkgConfig::sdl2 PkgConfig::opengl -lm)
 endif()
@@ -64,4 +65,9 @@ endif()
 
 #---
 
+# This provides Dear ImGui but also GL3W as an OpenGL loader
+if(NOT FACETS_PLATFORM STREQUAL gint)
+  add_subdirectory(imgui-builds)
+endif()
+
 add_subdirectory(azur)

azur/CMakeLists.txt

@@ -47,6 +47,11 @@ if(AZUR_PLATFORM_EMSCRIPTEN)
   set_target_properties(azur PROPERTIES LINK_DEPENDS "${ASSETS_ABSOLUTE}")
 endif()
 
+# Use ImGui's integrated GL3W loader (which we build separately)
+if(AZUR_GRAPHICS_OPENGL_3_3)
+  target_link_libraries(azur PUBLIC gl3w)
+endif()
+
 target_include_directories(azur
   PUBLIC "${PROJECT_SOURCE_DIR}/include"
   PUBLIC "${PROJECT_BINARY_DIR}/include"

azur/include/azur/azur.h

@@ -14,7 +14,7 @@ AZUR_BEGIN_DECLS
 
    On GINT_CG, the window size is fixed to 396x224 and ignored.
    TODO: On GINT_CG, accept super-resolution in azur_init(). */
-int azur_init(int window_width, int window_height);
+int azur_init(char const *title, int window_width, int window_height);
 
 /* azur_main_loop(): Run the update/render loop.
 

azur/include/azur/sdl_opengl/gl.h

@@ -6,9 +6,19 @@
 #include <azur/defs.h>
 AZUR_BEGIN_DECLS
 
-#define GL_GLEXT_PROTOTYPES
-#include <SDL2/SDL_opengl.h>
-#include <SDL2/SDL_opengl_glext.h>
+#ifdef AZUR_GRAPHICS_OPENGL_3_3
+# include "GL/gl3w.h"
+# include <SDL2/SDL_opengl.h>
+#endif /* OpenGL 3.3 */
+
+#ifdef AZUR_GRAPHICS_OPENGL_ES_2_0
+# define GL_GLEXT_PROTOTYPES
+# include <SDL2/SDL_opengles2.h>
+
+# define glGenVertexArrays glGenVertexArraysOES
+# define glBindVertexArray glBindVertexArrayOES
+# define glDeleteVertexArrays glDeleteVertexArraysOES
+#endif /* OpenGL ES 2.0 */
 
 /* azgl_error(): String description of a GLenum error code.
    The returned pointer might be a static buffer modified by further calls. */

azur/src/sdl_opengl/init.c

@@ -10,7 +10,7 @@ static SDL_GLContext glcontext = NULL;
 
 static void main_loop_quit(void);
 
-int azur_init(int window_width, int window_height)
+int azur_init(char const *title, int window_width, int window_height)
 {
     int rc = SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER);
     if(rc < 0) {
@@ -24,16 +24,24 @@ int azur_init(int window_width, int window_height)
         return 1;
     }
 
-    /* Select OpenGL 3.3 core, but not on the OpenGL ES/emscripten backend,
-       which does not support context attributes. */
+    /* Select OpenGL 3.3 core */
     #ifdef AZUR_GRAPHICS_OPENGL_3_3
-    SDL_GL_SetAttribute(
-        SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, 0);
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK,
+        SDL_GL_CONTEXT_PROFILE_CORE);
     SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
     SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
     #endif
 
-    window = SDL_CreateWindow("facets-floating",
+    /* ... or select Open GL ES 2.0 */
+    #ifdef AZUR_GRAPHICS_OPENGL_ES_2_0
+    SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, 0);
+    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);
+    #endif
+
+    window = SDL_CreateWindow(title,
         SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
         window_width, window_height,
         SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
@@ -47,11 +55,21 @@ int azur_init(int window_width, int window_height)
         azlog(FATAL, "SDL_GL_CreateContext: %s\n", SDL_GetError());
         return 1;
     }
+    SDL_GL_MakeCurrent(window, glcontext);
 
     rc = SDL_GL_SetSwapInterval(0);
     if(rc < 0)
         azlog(ERROR, "SDL_GL_SetSwapInterval: %s\n", SDL_GetError());
 
+    #ifdef AZUR_GRAPHICS_OPENGL_3_3
+    rc = gl3wInit();
+    if(rc != 0) {
+        azlog(FATAL, "gl3wInit: %d\n", rc);
+        return 1;
+    }
+    #endif
+
+    glEnable(GL_SCISSOR_TEST);
     glEnable(GL_BLEND);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
 
@@ -100,14 +118,18 @@ struct params {
     bool tied, started;
 };
 
+static void nop(void)
+{
+}
+
 static EM_BOOL frame(double time, void *data)
 {
     struct params *params = data;
 
     if(params->tied && params->started)
-        params->update();
+        if(params->update) params->update();
 
-    params->render();
+    if(params->render) params->render();
     params->started = true;
 
     return EM_TRUE;
@@ -129,7 +151,13 @@ int azur_main_loop(
 
     if(!(flags & AZUR_MAIN_LOOP_TIED)) {
         void (*update_v)(void) = (void *)update;
-        emscripten_set_main_loop(update_v, update_ups, false);
+        emscripten_set_main_loop(update_v, update_ups, true);
+    }
+    else {
+        /* Even if no update is requested, we want emscripten to simulate an
+           infinite loop, as it is needed for Dear ImGui to work due to
+           threading considerations */
+        emscripten_set_main_loop(nop, 1, true);
     }
 
     return 0;
@@ -193,7 +221,7 @@ int azur_main_loop(
     while(1) {
         if(update_tick && !(flags & AZUR_MAIN_LOOP_TIED)) {
             update_tick = 0;
-            if(update()) break;
+            if(update && update()) break;
         }
 
         /* When vsync is enabled, render() is indirectly blocking so we just
@@ -203,9 +231,9 @@ int azur_main_loop(
 
             /* Tied renders and updates */
             if(started && (flags & AZUR_MAIN_LOOP_TIED)) {
-                if(update()) break;
+                if(update && update()) break;
             }
-            render();
+            if(render) render();
             started = true;
         }
 

azur/src/sdl_opengl/util.c

@@ -45,10 +45,6 @@ char const *azgl_error(GLenum ec)
         return "Invalid value";
     case GL_INVALID_OPERATION:
         return "Invalid operation";
-    case GL_STACK_OVERFLOW:
-        return "Stack overflow";
-    case GL_STACK_UNDERFLOW:
-        return "Stack underflow";
     case GL_OUT_OF_MEMORY:
         return "Out of memory";
     }

imgui-builds/CMakeLists.txt

@@ -0,0 +1,62 @@
+# The GL3W library which is generated along in Dear ImGui
+if(AZUR_GRAPHICS_OPENGL_3_3)
+  add_library(gl3w
+    ../imgui/examples/libs/gl3w/GL/gl3w.c)
+  target_include_directories(gl3w PUBLIC
+    ../imgui/examples/libs/gl3w)
+  # Link with GLX for GL3X to find the extensions, and -ldl
+  target_link_libraries(gl3w INTERFACE PkgConfig::glx -ldl)
+endif()
+
+# The Dear ImGui library for SDL + OpenGL3/OpenGLES2
+set(SOURCES
+  ../imgui/imgui.cpp
+  ../imgui/imgui_demo.cpp
+  ../imgui/imgui_draw.cpp
+  ../imgui/imgui_tables.cpp
+  ../imgui/imgui_widgets.cpp
+  ../imgui/backends/imgui_impl_opengl3.cpp
+  ../imgui/backends/imgui_impl_sdl.cpp)
+
+# Use Freetype if the library is installed, or the emscripten port
+if(AZUR_PLATFORM_EMSCRIPTEN)
+  list(APPEND SOURCES ../imgui/misc/freetype/imgui_freetype.cpp)
+else()
+  pkg_check_modules(freetype2 freetype2 IMPORTED_TARGET)
+  if(freetype2_FOUND)
+    list(APPEND SOURCES ../imgui/misc/freetype/imgui_freetype.cpp)
+  endif()
+endif()
+
+add_library(imgui ${SOURCES})
+
+# Try to reduce the size of the binary code by including individual functions
+# from the archive, instead of whole files
+target_compile_options(imgui PRIVATE -ffunction-sections)
+target_compile_options(imgui INTERFACE -Wl,--gc-sections)
+
+# Use the pre-built GL3W loader on desktop
+if(AZUR_GRAPHICS_OPENGL_3_3)
+  target_compile_definitions(imgui PUBLIC
+    -DIMGUI_IMPL_OPENGL_LOADER_GL3W="GL/gl3w.h")
+  target_link_libraries(imgui PUBLIC gl3w)
+# Use the autonomous GLES 2.0 loader otherwise
+elseif(AZUR_GRAPHICS_OPENGL_ES_2_0)
+  target_compile_definitions(imgui PUBLIC
+    -DIMGUI_IMPL_OPENGL_ES2)
+endif()
+
+# FreeType2 settings
+if(freetype2_FOUND)
+  target_compile_definitions(imgui PUBLIC -DIMGUI_ENABLE_FREETYPE)
+  target_include_directories(imgui PRIVATE ../imgui/misc/freetype)
+  target_link_libraries(imgui PUBLIC PkgConfig::freetype2)
+elseif(AZUR_PLATFORM_EMSCRIPTEN)
+  target_compile_definitions(imgui PUBLIC -DIMGUI_ENABLE_FREETYPE)
+  target_include_directories(imgui PRIVATE ../imgui/misc/freetype)
+  target_compile_options(imgui PUBLIC -sUSE_FREETYPE=2)
+  target_link_options(imgui INTERFACE -sUSE_FREETYPE=2)
+endif()
+
+# Provide include directories
+target_include_directories(imgui PUBLIC ../imgui ../imgui/backends)