Maybe proceed with a ViewportEffect?

src/core/StelViewportEffect.cpp

@@ -148,7 +148,7 @@ StelViewportDistorterFisheyeToSphericMirror::StelViewportDistorterFisheyeToSpher
 
 				// sharp image up to the border of the fisheye image, at the cost of
 				// accepting clamping artefacts. You can get rid of the clamping
-				// artefacts by specifying a viewport size a little less then
+				// artefacts by specifying a viewport size a little less than
 				// (1<<n)*(1<<n), for instance 1022*1022. With a viewport size
 				// of 512*512 and viewportFovDiameter=512 you will get clamping
 				// artefacts in the 3 otherwise black hills on the bottom of the image.
@@ -177,7 +177,7 @@ StelViewportDistorterFisheyeToSphericMirror::StelViewportDistorterFisheyeToSpher
 		QTextStream in;
 		QString fName = StelFileMgr::findFile(custom_distortion_file);
 		if (fName.isEmpty()) {
-			qWarning() << "WARNING: could not open custom_distortion_file:" << custom_distortion_file;
+			qWarning() << "WARNING: could not find custom_distortion_file:" << custom_distortion_file;
 		} else {
 			file.setFileName(fName);
 			if(file.open(QIODevice::ReadOnly))
@@ -344,3 +344,12 @@ void StelViewportDistorterFisheyeToSphericMirror::paintViewportBuffer(const QOpe
 	GL(gl->glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST));
 }
 
+void StelViewportFaderEffect::alterBuffer(QOpenGLFramebufferObject* buf) const
+{
+	Q_UNUSED(buf)
+	// TODO: I am still unsure about how to use this. When I want to have a scene fading to black (effect of a light echo or showing star trails),
+	// will the main buffer be changed, or can I only apply an effect to the finally displayed image?
+	// https://stackoverflow.com/questions/6810591/how-to-make-fading-to-black-effect-with-opengl
+}
+
+

src/core/StelViewportEffect.hpp

@@ -37,7 +37,7 @@ class StelViewportEffect
 	//! The default implementation does nothing.
 	virtual void alterBuffer(QOpenGLFramebufferObject*) const {;}
 	//! Draw the viewport on the screen.
-	//! @param buf the GL frame buffer containing the Stellarium viewport alreay drawn.
+	//! @param buf the GL frame buffer containing the Stellarium viewport already drawn.
 	//! The default implementation paints the buffer on the fullscreen.
 	virtual void paintViewportBuffer(const QOpenGLFramebufferObject* buf) const;
 	//! Distort an x,y position according to the distortion.
@@ -50,10 +50,10 @@ class StelViewportDistorterFisheyeToSphericMirror : public StelViewportEffect
 {
 public:
 	StelViewportDistorterFisheyeToSphericMirror(int screen_w,int screen_h);
-	~StelViewportDistorterFisheyeToSphericMirror();
-	virtual QString getName() const {return "sphericMirrorDistorter";}
-	virtual void paintViewportBuffer(const QOpenGLFramebufferObject* buf) const;
-	virtual void distortXY(qreal& x, qreal& y) const;
+	~StelViewportDistorterFisheyeToSphericMirror() Q_DECL_OVERRIDE;
+	virtual QString getName() const Q_DECL_OVERRIDE {return "sphericMirrorDistorter";}
+	virtual void paintViewportBuffer(const QOpenGLFramebufferObject* buf) const Q_DECL_OVERRIDE;
+	virtual void distortXY(qreal& x, qreal& y) const Q_DECL_OVERRIDE;
 private:
 	const int screen_w;
 	const int screen_h;
@@ -70,5 +70,18 @@ class StelViewportDistorterFisheyeToSphericMirror : public StelViewportEffect
 	QVector<Vec2f> displayTexCoordList;
 };
 
+class StelViewportFaderEffect : public StelViewportEffect
+{
+public:
+	StelViewportFaderEffect() {}
+	//~StelViewportFaderEffect() Q_DECL_OVERRIDE;
+	virtual QString getName() const Q_DECL_OVERRIDE {return "viewportFaderEffect";}
+	//! Alter the GL frame buffer, this method must not display anything.
+	//! The implementation in this class reduces the brightness of the existing buffer.
+	virtual void alterBuffer(QOpenGLFramebufferObject* buf) const Q_DECL_OVERRIDE;
+	//virtual void paintViewportBuffer(const QOpenGLFramebufferObject* buf) const Q_DECL_OVERRIDE;
+//private:
+};
+
 #endif // STELVIEWPORTEFFECT_HPP