Check for EXT_gpu_shader4 CPU-side

Mesa lies and always defines GL_EXT_gpu_shader4 even when the extension
isn't present.

apps/openmw/mwrender/renderingmanager.cpp

@@ -252,6 +252,7 @@ namespace MWRender
         globalDefines["clamp"] = Settings::Manager::getBool("clamp lighting", "Shaders") ? "1" : "0";
         globalDefines["preLightEnv"] = Settings::Manager::getBool("apply lighting to environment maps", "Shaders") ? "1" : "0";
         globalDefines["radialFog"] = Settings::Manager::getBool("radial fog", "Shaders") ? "1" : "0";
+        globalDefines["useGPUShader4"] = "0";
 
         // It is unnecessary to stop/start the viewer as no frames are being rendered yet.
         mResourceSystem->getSceneManager()->getShaderManager().setGlobalDefines(globalDefines);

components/sceneutil/mwshadowtechnique.cpp

@@ -892,12 +892,17 @@ void SceneUtil::MWShadowTechnique::setupCastingShader(Shader::ShaderManager & sh
     // This can't be part of the constructor as OSG mandates that there be a trivial constructor available
 
     osg::ref_ptr<osg::Shader> castingVertexShader = shaderManager.getShader("shadowcasting_vertex.glsl", {}, osg::Shader::VERTEX);
+    osg::ref_ptr<osg::GLExtensions> exts = osg::GLExtensions::Get(0, false);
+    std::string useGPUShader4 = exts && exts->isGpuShader4Supported ? "1" : "0";
     for (int alphaFunc = GL_NEVER; alphaFunc <= GL_ALWAYS; ++alphaFunc)
     {
         auto& program = _castingPrograms[alphaFunc - GL_NEVER];
         program = new osg::Program();
         program->addShader(castingVertexShader);
-        program->addShader(shaderManager.getShader("shadowcasting_fragment.glsl", { {"alphaFunc", std::to_string(alphaFunc)}, {"alphaToCoverage", "0"} }, osg::Shader::FRAGMENT));
+        program->addShader(shaderManager.getShader("shadowcasting_fragment.glsl", { {"alphaFunc", std::to_string(alphaFunc)},
+                                                                                    {"alphaToCoverage", "0"},
+                                                                                    {"useGPUShader4", useGPUShader4}
+                                                                                  }, osg::Shader::FRAGMENT));
     }
 }
 

components/shader/shadervisitor.cpp

@@ -2,6 +2,7 @@
 
 #include <osg/AlphaFunc>
 #include <osg/Geometry>
+#include <osg/GLExtensions>
 #include <osg/Material>
 #include <osg/Multisample>
 #include <osg/Texture>
@@ -411,6 +412,12 @@ namespace Shader
                 writableStateSet->setMode(GL_SAMPLE_ALPHA_TO_COVERAGE_ARB, osg::StateAttribute::ON);
                 defineMap["alphaToCoverage"] = "1";
             }
+
+            // Preventing alpha tested stuff shrinking as lower mip levels are used requires knowing the texture size
+            osg::ref_ptr<osg::GLExtensions> exts = osg::GLExtensions::Get(0, false);
+            if (exts && exts->isGpuShader4Supported)
+                defineMap["useGPUShader4"] = "1";
+            // We could fall back to a texture size uniform if EXT_gpu_shader4 is missing
         }
 
         if (writableStateSet->getMode(GL_ALPHA_TEST) != osg::StateAttribute::INHERIT)

files/shaders/alpha.glsl

@@ -24,10 +24,10 @@ float coveragePreservingAlphaScale(sampler2D diffuseMap, vec2 uv)
 {
     #if @alphaFunc != FUNC_ALWAYS && @alphaFunc != FUNC_NEVER
         vec2 textureSize;
-        #ifdef GL_EXT_gpu_shader4
+        #if @useGPUShader4
             textureSize = textureSize2D(diffuseMap, 0);
         #else
-            textureSize = 256.0;
+            textureSize = vec2(256.0);
         #endif
             return 1.0 + mipmapLevel(uv * textureSize) * 0.25;
     #else

files/shaders/objects_fragment.glsl

@@ -1,6 +1,8 @@
 #version 120
 
-#extension EXT_gpu_shader4: enable
+#if @useGPUShader4
+    #extension EXT_gpu_shader4: require
+#endif
 
 #if @diffuseMap
 uniform sampler2D diffuseMap;