Continue a bit the trail fader viewport effect

Started by Georg in the previous commit.  This can be enabled by setting

    [video]
    viewport_effect = viewportFaderEffect

in the config file.

This effect simply applies a mix of the current frame with the one
before.  Need to see if there is a way to improve the effect somehow.

src/core/StelApp.cpp

@@ -1375,6 +1375,10 @@ void StelApp::setViewportEffect(const QString& name)
 	{
 		viewportEffect = new StelViewportDistorterFisheyeToSphericMirror(w, h);
 	}
+	else if (name == "viewportFaderEffect")
+	{
+		viewportEffect = new StelViewportFaderEffect();
+	}
 	else
 	{
 		qDebug() << "unknown viewport effect name:" << name;

src/core/StelViewportEffect.cpp

@@ -345,12 +345,13 @@ void StelViewportDistorterFisheyeToSphericMirror::paintViewportBuffer(const QOpe
 	GL(gl->glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST));
 }
 
-void StelViewportFaderEffect::alterBuffer(QOpenGLFramebufferObject* buf) const
+void StelViewportFaderEffect::paintViewportBuffer(const 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
+	StelPainter sPainter(StelApp::getInstance().getCore()->getProjection2d());
+	QOpenGLFunctions* gl = sPainter.glFuncs();
+	sPainter.setColor(1,1,1);
+	GL(gl->glBindTexture(GL_TEXTURE_2D, buf->texture()));
+	GL(gl->glBlendColor(1, 1, 1, 0.08));
+	sPainter.setBlending(true, GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA);
+	sPainter.drawRect2d(0, 0, buf->width(), buf->height());
 }
-
-

src/core/StelViewportEffect.hpp

@@ -74,13 +74,11 @@ 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:
+	// virtual void alterBuffer(QOpenGLFramebufferObject* buf) const Q_DECL_OVERRIDE;
+	virtual void paintViewportBuffer(const QOpenGLFramebufferObject* buf) const Q_DECL_OVERRIDE;
 };
 
 #endif // STELVIEWPORTEFFECT_HPP