title: Libgdx blending contrast issue in render();
body: <p>In a Libgdx 1.x Stage, my goal is to apply a dark semi transparent mask on my screen except an area that should display the original screen.</p>
  
  <p>My global render method:</p>
  
  <pre><code>public void render() {
      Gdx.gl.glClearColor(0, 0, 0, 1);        
      Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT); 
      batch.setShader(null); 
      update();
      stage.act(Gdx.graphics.getDeltaTime());     
      stage.draw();       
  
      batch.begin();      
      darkener.draw(batch, 0.7f);     
      spotlight.draw(batch, 1f);      
      batch.end();                
  }
  </code></pre>
  
  <p>Both instances of objects Darkener and Spotlight are Sprites made from a pure white 4x4 texture:</p>
  
  <pre><code>spotlight = new Sprite(new Texture(Gdx.files.internal("data/ui/whitepixel.png")));
  
  darkener = new Sprite(new Texture(Gdx.files.internal("data/ui/whitepixel.png")));
  </code></pre>
  
  <p>I give my Darkener object the size of the screen, and my Spotlight object the size of the area I want not to be darkened.</p>
  
  <p>Here is the draw method of my object darkener:</p>
  
  <pre><code>public void draw(Batch batch, float alpha) {        
      darkener.draw(batch, alpha);        
  }
  </code></pre>
  
  <p>And the draw method of my Spotlight object:</p>
  
  <pre><code>public void draw(Batch batch, float alpha) {
      batch.setBlendFunction(GL20.GL_DST_COLOR, GL20.GL_SRC_ALPHA);
      spotlight.draw(batch, alpha);
      batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
  }
  </code></pre>
  
  <p>My problem is about blending:
  With the parameters: </p>
  
  <pre><code>darkener.draw(batch, 0.7f);     
  spotlight.draw(batch, 1f);  
  </code></pre>
  
  <p>The preserved (spotlight) area is darker than the original.</p>
  
  <p>With the parameters:</p>
  
  <pre><code>darkener.draw(batch, 0.3f);     
  spotlight.draw(batch, 1f);  
  </code></pre>
  
  <p>The preserved (spotlight) area is brighter (saturated) than the original.</p>
  
  <p>With the paremeters:</p>
  
  <pre><code>darkener.draw(batch, 0.5f);     
  spotlight.draw(batch, 1f);  
  </code></pre>
  
  <p>The preserved (spotlight) area is exactly like the original (so, as I want it). The problem is that I want to be able to set different values for my Darkener either &lt; 0.5f or > 0.5f.</p>
  
  <p>What I am doing wrong?</p>