First part of #1988 fix

data/shaders/degraded_ibl.frag

@@ -0,0 +1,18 @@
+uniform sampler2D ntex;
+
+out vec4 Diff;
+out vec4 Spec;
+
+vec3 DecodeNormal(vec2 n);
+vec4 getPosFromUVDepth(vec3 uvDepth, mat4 InverseProjectionMatrix);
+vec3 DiffuseIBL(vec3 normal);
+vec3 SpecularIBL(vec3 normal, vec3 V, float roughness);
+
+void main(void)
+{
+    vec2 uv = gl_FragCoord.xy / screen;
+    vec3 normal = normalize(DecodeNormal(2. * texture(ntex, uv).xy - 1.));
+
+    Diff = vec4(0.25 * DiffuseIBL(normal), 1.);
+    Spec = vec4(0., 0., 0., 1.);
+}

src/config/user_config.hpp

@@ -654,6 +654,10 @@ namespace UserConfigParams
             PARAM_DEFAULT( IntUserConfigParam(0,
                            "shadows_resoltion", &m_graphics_quality,
                            "Shadow resolution (0 = disabled") );
+    PARAM_PREFIX BoolUserConfigParam          m_degraded_IBL
+        PARAM_DEFAULT(BoolUserConfigParam(false,
+        "Degraded_IBL", &m_graphics_quality,
+        "Disable specular IBL"));
 
     // ---- Misc
     PARAM_PREFIX BoolUserConfigParam        m_cache_overworld

src/graphics/post_processing.cpp

@@ -220,11 +220,22 @@ void PostProcessing::renderEnvMap(const float *bSHCoeff, const float *gSHCoeff,
     glBlendEquation(GL_FUNC_ADD);
     glBlendFunc(GL_ONE, GL_ONE);
 
-    glUseProgram(FullScreenShader::IBLShader::getInstance()->Program);
+    if (UserConfigParams::m_degraded_IBL)
-    glBindVertexArray(SharedObject::FullScreenQuadVAO);
+    {
+        glUseProgram(FullScreenShader::DegradedIBLShader::getInstance()->Program);
+        glBindVertexArray(SharedObject::FullScreenQuadVAO);
 
-    FullScreenShader::IBLShader::getInstance()->SetTextureUnits(irr_driver->getRenderTargetTexture(RTT_NORMAL_AND_DEPTH), irr_driver->getDepthStencilTexture(), skybox);
+        FullScreenShader::DegradedIBLShader::getInstance()->SetTextureUnits(irr_driver->getRenderTargetTexture(RTT_NORMAL_AND_DEPTH));
-    FullScreenShader::IBLShader::getInstance()->setUniforms();
+        FullScreenShader::DegradedIBLShader::getInstance()->setUniforms();
+    }
+    else
+    {
+        glUseProgram(FullScreenShader::IBLShader::getInstance()->Program);
+        glBindVertexArray(SharedObject::FullScreenQuadVAO);
+
+        FullScreenShader::IBLShader::getInstance()->SetTextureUnits(irr_driver->getRenderTargetTexture(RTT_NORMAL_AND_DEPTH), irr_driver->getDepthStencilTexture(), skybox);
+        FullScreenShader::IBLShader::getInstance()->setUniforms();
+    }
 
     glDrawArrays(GL_TRIANGLES, 0, 3);
     glBindVertexArray(0);

src/graphics/shaders.cpp

@@ -1641,6 +1641,19 @@ namespace FullScreenShader
         AssignSamplerNames(Program, 0, "ntex", 1, "dtex", 2, "probe");
     }
 
+    DegradedIBLShader::DegradedIBLShader()
+    {
+        Program = LoadProgram(OBJECT,
+            GL_VERTEX_SHADER, file_manager->getAsset("shaders/screenquad.vert").c_str(),
+            GL_FRAGMENT_SHADER, file_manager->getAsset("shaders/utils/decodeNormal.frag").c_str(),
+            GL_FRAGMENT_SHADER, file_manager->getAsset("shaders/utils/getPosFromUVDepth.frag").c_str(),
+            GL_FRAGMENT_SHADER, file_manager->getAsset("shaders/utils/DiffuseIBL.frag").c_str(),
+            GL_FRAGMENT_SHADER, file_manager->getAsset("shaders/utils/SpecularIBL.frag").c_str(),
+            GL_FRAGMENT_SHADER, file_manager->getAsset("shaders/degraded_ibl.frag").c_str());
+        AssignUniforms();
+        AssignSamplerNames(Program, 0, "ntex");
+    }
+
     ShadowedSunLightShaderPCF::ShadowedSunLightShaderPCF()
     {
         Program = LoadProgram(OBJECT,

src/graphics/shaders.hpp

@@ -426,6 +426,12 @@ public:
     IBLShader();
 };
 
+class DegradedIBLShader : public ShaderHelperSingleton<DegradedIBLShader>, public TextureRead<Nearest_Filtered>
+{
+public:
+    DegradedIBLShader();
+};
+
 class ShadowedSunLightShaderPCF : public ShaderHelperSingleton<ShadowedSunLightShaderPCF, float, float, float, float, float>, public TextureRead<Nearest_Filtered, Nearest_Filtered, Shadow_Sampler>
 {
 public: