Examples: Add OpenGL ES 2.0 support to SDL2 example

examples/example_sdl_opengl3/Makefile

@@ -21,13 +21,14 @@ SOURCES += $(IMGUI_DIR)/imgui.cpp $(IMGUI_DIR)/imgui_demo.cpp $(IMGUI_DIR)/imgui
 SOURCES += $(IMGUI_DIR)/backends/imgui_impl_sdl.cpp $(IMGUI_DIR)/backends/imgui_impl_opengl3.cpp
 OBJS = $(addsuffix .o, $(basename $(notdir $(SOURCES))))
 UNAME_S := $(shell uname -s)
+LINUX_GL_LIBS = -lGL
 
 CXXFLAGS = -I$(IMGUI_DIR) -I$(IMGUI_DIR)/backends
 CXXFLAGS += -g -Wall -Wformat
 LIBS =
 
 ##---------------------------------------------------------------------
-## OPENGL LOADER
+## OPENGL LOADER / OPENGL ES
 ##---------------------------------------------------------------------
 
 ## Using OpenGL loader: gl3w [default]
@@ -56,13 +57,20 @@ CXXFLAGS += -I../libs/gl3w -DIMGUI_IMPL_OPENGL_LOADER_GL3W
 # CXXFLAGS += -DIMGUI_IMPL_OPENGL_LOADER_GLBINDING2
 # LIBS += -lglbinding
 
+## Using OpenGL ES, no loader required
+## This assumes a GL ES library available in the system, e.g. libGLESv2.so
+# CXXFLAGS += -DIMGUI_IMPL_OPENGL_ES2
+# LINUX_GL_LIBS = -lGLESv2
+## If you're on a Raspberry Pi, use the following instead:
+# LINUX_GL_LIBS = -L/opt/vc/lib -lbrcmGLESv2
+
 ##---------------------------------------------------------------------
 ## BUILD FLAGS PER PLATFORM
 ##---------------------------------------------------------------------
 
 ifeq ($(UNAME_S), Linux) #LINUX
 	ECHO_MESSAGE = "Linux"
-	LIBS += -lGL -ldl `sdl2-config --libs`
+	LIBS += $(LINUX_GL_LIBS) -ldl `sdl2-config --libs`
 
 	CXXFLAGS += `sdl2-config --cflags`
 	CFLAGS = $(CXXFLAGS)

examples/example_sdl_opengl3/main.cpp

@@ -10,6 +10,9 @@
 #include <stdio.h>
 #include <SDL.h>
 
+#ifdef IMGUI_IMPL_OPENGL_ES2
+#include <GLES2/gl2.h>
+#else
 // About Desktop OpenGL function loaders:
 //  Modern desktop OpenGL doesn't have a standard portable header file to load OpenGL function pointers.
 //  Helper libraries are often used for this purpose! Here we are supporting a few common ones (gl3w, glew, glad).
@@ -35,6 +38,7 @@ using namespace gl;
 #else
 #include IMGUI_IMPL_OPENGL_LOADER_CUSTOM
 #endif
+#endif
 
 // Main code
 int main(int, char**)
@@ -56,13 +60,22 @@ int main(int, char**)
     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, 2);
+#else
+#ifdef IMGUI_IMPL_OPENGL_ES2
+    // GL ES 2.0 + GLSL 100
+    const char* glsl_version = "#version 100";
+    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);
 #else
     // GL 3.0 + GLSL 130
     const char* glsl_version = "#version 130";
     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, 0);
+#endif
 #endif
 
     // Create window with graphics context