Remove -s USE_WEBGL2=1 and replace it with -s GL_MIN_FEATURE_LEVEL=10…

…/20 and -s GL_MAX_FEATURE_LEVEL=10/20

emcc.py

@@ -1269,6 +1269,9 @@ def is_supported_link_flag(f):
       # stb_image 2.x need to have STB_IMAGE_IMPLEMENTATION defined to include the implementation when compiling
       newargs.append('-DSTB_IMAGE_IMPLEMENTATION')
 
+    if shared.Settings.USE_WEBGL2:
+      shared.Settings.GL_MAX_FEATURE_LEVEL = 20
+
     forced_stdlibs = []
 
     if shared.Settings.ASMFS and final_suffix in JS_CONTAINING_ENDINGS:

site/source/docs/optimizing/Optimizing-WebGL.rst

@@ -25,7 +25,7 @@ By default, if no special GL related linker flags are chosen, Emscripten targets
 
 - When targeting OpenGL ES 3, if one needs to render from client side memory, or the use of ``glMapBuffer*()`` API is needed, pass the linker flag ``-s FULL_ES3=1`` to emulate these features, which core WebGL 2 does not have. This emulation is expected to hurt performance, so using VBOs is recommended instead.
 
-- Even if your application does not need any WebGL 2/OpenGL ES 3 features, consider porting the application to run on WebGL 2, because JavaScript side performance in WebGL 2 has been optimized to generate no temporary garbage, which has been observed to give a solid 3-7% speed improvement, as well as reducing potential stuttering at render time. To enable these optimizations, build with the linker flag ``-s USE_WEBGL2=1`` and make sure to create a WebGL 2 context at GL startup time (OpenGL ES 3 context if using EGL).
+- Even if your application does not need any WebGL 2/OpenGL ES 3 features, consider porting the application to run on WebGL 2, because JavaScript side performance in WebGL 2 has been optimized to generate no temporary garbage, which has been observed to give a solid 3-7% speed improvement, as well as reducing potential stuttering at render time. To enable these optimizations, build with the linker flag ``-s GL_MAX_FEATURE_LEVEL=20`` and make sure to create a WebGL 2 context at GL startup time (OpenGL ES 3 context if using EGL).
 
 How To Profile WebGL
 ====================

site/source/docs/porting/multimedia_and_graphics/OpenGL-support.rst

@@ -25,7 +25,9 @@ To program against the WebGL subset of OpenGL ES, one uses the GL ES 2.0 header
 
 This mode is used by default because it best matches the WebGL features brovided by browsers.
 
-To target WebGL 2, pass the linker flag ``-s USE_WEBGL2=1``. Specifying this flag enables (and defaults to, unless otherwise specified at context creation time) the creation of WebGL 2 contexts at runtime, but it is still possible to create WebGL 1 contexts, so applications can choose whether to require WebGL 2 or whether to support a fallback to WebGL 1.
+To target WebGL 2, pass the linker flag ``-s GL_MAX_FEATURE_LEVEL=20``. Specifying this flag enables (and defaults to, unless otherwise specified at context creation time) the creation of WebGL 2 contexts at runtime, but it is still possible to create WebGL 1 contexts, so applications can choose whether to require WebGL 2 or whether to support a fallback to WebGL 1.
+
+To only target WebGL 2 and drop support for WebGL 1 altogether to save code size, pass the linker flags ``-s GL_MIN_FEATURE_LEVEL=20`` and ``-s GL_MAX_FEATURE_LEVEL=20``.
 
 .. _opengl-support-opengl-es2-0-emulation:
 

src/library_browser.js

@@ -312,10 +312,14 @@ var LibraryBrowser = {
         var contextAttributes = {
           antialias: false,
           alpha: false,
-#if USE_WEBGL2 // library_browser.js defaults: use the WebGL version chosen at compile time (unless overridden below)
+#if GL_MIN_FEATURE_LEVEL >= 20
+          majorVersion: 2,
+#else
+#if GL_MAX_FEATURE_LEVEL >= 20 // library_browser.js defaults: use the WebGL version chosen at compile time (unless overridden below)
           majorVersion: (typeof WebGL2RenderingContext !== 'undefined') ? 2 : 1,
 #else
           majorVersion: 1,
+#endif
 #endif
         };
 

src/library_egl.js

@@ -345,14 +345,14 @@ var LibraryEGL = {
       }
       contextAttribs += 8;
     }
-#if USE_WEBGL2
+#if GL_MAX_FEATURE_LEVEL >= 20
     if (glesContextVersion < 2 || glesContextVersion > 3) {
 #else
     if (glesContextVersion != 2) {
 #endif
 #if GL_ASSERTIONS
       if (glesContextVersion == 3) {
-        err('When initializing GLES3/WebGL2 via EGL, one must build with -s USE_WEBGL2=1 !');
+        err('When initializing GLES3/WebGL2 via EGL, one must build with -s GL_MAX_FEATURE_LEVEL=20 !');
       } else {
         err('When initializing GLES2/WebGL1 via EGL, one must pass EGL_CONTEXT_CLIENT_VERSION = 2 to GL context attributes! GLES version ' + glesContextVersion + ' is not supported!');
       }

src/library_glemu.js

@@ -3339,7 +3339,7 @@ var LibraryGLEmulation = {
   glRotatef: 'glRotated',
 
   glDrawBuffer: function() { throw 'glDrawBuffer: TODO' },
-#if !USE_WEBGL2
+#if GL_MAX_FEATURE_LEVEL < 20
   glReadBuffer: function() { throw 'glReadBuffer: TODO' },
 #endif