Disable coverage adjustment for blended objects

components/sceneutil/mwshadowtechnique.cpp

@@ -904,6 +904,7 @@ void SceneUtil::MWShadowTechnique::setupCastingShader(Shader::ShaderManager & sh
         program->addShader(castingVertexShader);
         program->addShader(shaderManager.getShader("shadowcasting_fragment.glsl", { {"alphaFunc", std::to_string(alphaFunc)},
                                                                                     {"alphaToCoverage", "0"},
+                                                                                    {"adjustCoverage", "1"},
                                                                                     {"useGPUShader4", useGPUShader4}
                                                                                   }, osg::Shader::FRAGMENT));
     }

components/shader/shadervisitor.cpp

@@ -482,6 +482,7 @@ namespace Shader
             removedState = new osg::StateSet();
 
         defineMap["alphaToCoverage"] = "0";
+        defineMap["adjustCoverage"] = "0";
         if (reqs.mAlphaFunc != osg::AlphaFunc::ALWAYS)
         {
             writableStateSet->addUniform(new osg::Uniform("alphaRef", reqs.mAlphaRef));
@@ -505,6 +506,11 @@ namespace Shader
                 defineMap["alphaToCoverage"] = "1";
             }
 
+            // Adjusting coverage isn't safe with blending on as blending requires the alpha to be intact.
+            // Maybe we could also somehow (e.g. userdata) detect when the diffuse map has coverage-preserving mip maps in the future
+            if (!reqs.mAlphaBlend)
+                defineMap["adjustCoverage"] = "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)

files/shaders/alpha.glsl

@@ -22,7 +22,7 @@ float mipmapLevel(vec2 scaleduv)
 
 float coveragePreservingAlphaScale(sampler2D diffuseMap, vec2 uv)
 {
-    #if @alphaFunc != FUNC_ALWAYS && @alphaFunc != FUNC_NEVER
+    #if @adjustCoverage
         vec2 textureSize;
         #if @useGPUShader4
             textureSize = textureSize2D(diffuseMap, 0);