In response to a post on libcinder.org (http://discourse.libcinder.org/t/super-simple-fade-effect-not-working/317)

FadeoutApp.cpp

#include "cinder/app/App.h"
#include "cinder/app/RendererGl.h"
#include "cinder/gl/gl.h"

using namespace ci;
using namespace ci::app;
using namespace std;

class fadeoutApp : public App {
  public:
	void draw() override;
	void resize() override;

	void mouseMove( MouseEvent event ) override;

	void render();

  private:
	gl::FboRef mFbo;
	ci::ivec2  mMousePos;
};

void fadeoutApp::draw()
{
	// Render scene to Fbo. This allows us to keep the contents of the previous
	// frame.
	render();
	// Now render the scene to the main buffer.
	gl::draw( mFbo->getColorTexture() );
}

void fadeoutApp::mouseMove( MouseEvent event )
{
	// Keep track of the mouse position in window coordinates.
	mMousePos = event.getPos();
}

void fadeoutApp::resize()
{
	// Create an Fbo the size of the window.
	mFbo = gl::Fbo::create( getWindowWidth(), getWindowHeight() );

	// Clear once at setup.
	gl::ScopedFramebuffer scpFbo( mFbo );
	gl::ScopedViewport    scpViewport( mFbo->getSize() );
	gl::clear();
}

void fadeoutApp::render()
{
	// Bind the Fbo and update the viewport.
	gl::ScopedFramebuffer scpFbo( mFbo );
	gl::ScopedViewport    scpViewport( mFbo->getSize() );

	// Enable pre-multiplied alpha blending.
	gl::ScopedBlendPremult scpBlend;

	// Draw transparent black rectangle.
	gl::ScopedColor scpColor( 0, 0, 0, 4.0f / 255.0f );
	gl::drawSolidRect( { 0, 0, (float)getWindowWidth(), (float)getWindowHeight() } );

	// Draw a white circle.
	gl::color( 1, 1, 1 );
	gl::drawSolidCircle( mMousePos, 20 );
}

CINDER_APP( fadeoutApp, RendererGl )