Enable the standard derivatives extension if needed

We need to find the functions defined by this extension for fragment
shaders, and enable it if the shader uses them.  Note that this
extension is automatically enabled in GL ES 2.0 but not in WebGL.

1 files changed, 40 insertions, 0 deletions

diff --git a/src/library_gl.js b/src/library_gl.js
index d99d59ec..1bb94253 100644
--- a/src/library_gl.js
+++ b/src/library_gl.js
@@ -24,6 +24,31 @@ var LibraryGL = {
       }
       return 0;
     },
+
+    // Find a token in a shader source string
+    findToken: function(source, token) {
+      function isIdentChar(ch) {
+        if (ch >= 48 && ch <= 57) // 0-9
+          return true;
+        if (ch >= 65 && ch <= 90) // A-Z
+          return true;
+        if (ch >= 97 && ch <= 122) // a-z
+          return true;
+        return false;
+      }
+      var i = source.indexOf(token);
+      if (i < 0) {
+        return false;
+      }
+      if (i > 0 && isIdentChar(source[i - 1])) {
+        return false;
+      }
+      i += token.length;
+      if (i < source.length - 1 && isIdentChar(source[i + 1])) {
+        return false;
+      }
+      return true;
+    },
   },
 
   glGetString: function(name_) {
@@ -529,6 +554,21 @@ var LibraryGL = {
       }
       source += frag;
     }
+    // Let's see if we need to enable the standard derivatives extension
+    type = Module.ctx.getShaderParameter(GL.shaders[shader], 0x8B4F /* GL_SHADER_TYPE */);
+    if (type == 0x8B30 /* GL_FRAGMENT_SHADER */) {
+      if (GL.findToken(source, "dFdx") ||
+          GL.findToken(source, "dFdy") ||
+          GL.findToken(source, "fwidth")) {
+        source = "#extension GL_OES_standard_derivatives : enable\n" + source;
+        var extension = Module.ctx.getExtension("OES_standard_derivatives");
+#if GL_DEBUG
+        if (!extension) {
+          Module.printErr("Shader attempts to use the standard derivatives extension which is not available.");
+        }
+#endif
+      }
+    }
     Module.ctx.shaderSource(GL.shaders[shader], source);
   },