#304 Avoid accessing OpenGL if we are using "-platform offscreen"

Add description of how to start ResInsight in headless mode

ApplicationCode/Application/RiaGuiApplication.cpp

@@ -394,10 +394,13 @@ bool RiaGuiApplication::useShaders() const
 {
     if ( !m_preferences->useShaders() ) return false;
 
-    bool isShadersSupported = caf::Viewer::isShadersSupported();
-    if ( !isShadersSupported ) return false;
+    bool isShadersSupported = true;
+    if ( platformName() != "offscreen" ) // Avoid opengl access if we are in qt offscreen mode
+    {
+        isShadersSupported = caf::Viewer::isShadersSupported();
+    }
 
-    return true;
+    return isShadersSupported;
 }
 
 //--------------------------------------------------------------------------------------------------

doc/ResInsightHeadlessSnapshots.txt

@@ -0,0 +1,18 @@
+
+Investigations on using ResInsight to render plots in a compleatly Headless state
+
+- Qt 5.8 or higher needed, it seems. Tested with Qt 5.14.1. Tested with Qt 5.6.1 on CentOS 6 and the plots was rendered without fonts.
+- Start ResInsight with the additional options -platform offscreen
+- The project can not contain visible 3D views. If it does, ResInsight will crash
+- Might add -stylesheet <path to stylesheet definition file> to explicitly control fonts
+  example contents of stylesheets file:
+  
+	* { font: bold italic large "Times New Roman" }
+	
+  The "*" might be substituted with specific classes or widgets.
+  It seems as the Qwt plot axis-titles are not affected by the stylesheet set above
+  
+- Example command line:
+
+  ./ResInsight --project FPPLot.rsp --savesnapshots all --snapshotsize 500 500 -stylesheet stylesheet.txt -platform offscreen
+