title: Clear screen with transparency in libgdx to draw a particle trail
body: <p>I'm working on an android app using libgdx, and I'm trying to draw some type of wind particles that leave a trail behind, something similar to  this: <a href="http://hint.fm/wind/" rel="nofollow noreferrer">http://hint.fm/wind/</a>. </p>
  
  <p>I've managed to draw the particles, which are lines that connect the previous position to the newest. However, if I clean the screen every time, it will only draw the current positions, which will be similar to little dots moving, as expected. </p>
  
  <p>What I want to do is to draw a trail behind those particles, that fades away. To do this, I tried creating a list of the previous particle positions and drawing them with a decreasing alpha, but there are to many particles to draw and it proved to be very slow. So I searched, and there is another way to achieve this, which is clearing a screen with some some transparency while I draw the new particles. My solution right now is using this: <a href="https://stackoverflow.com/questions/10859567/unexpected-results-implementing-simple-motion-blur-in-libgdx">Unexpected results implementing simple motion blur in Libgdx</a>, and while my desktop rendering works as expected, in the android there is this purple background formed by the white particles that I don't understand. How can I solve this?</p>
  
  <p>This are the screenshots of the android and desktop respectively: <a href="https://imgur.com/a/JM2sz" rel="nofollow noreferrer">http://imgur.com/a/JM2sz</a></p>
  
  <p>Code:</p>
  
  <pre><code>Pixmap screenClearPixmap = new Pixmap(480, 548, Format.RGBA8888);
  screenClearPixmap.setColor(Color.rgba8888(0f, 0f, 0f, 0.1f));
  screenClearPixmap.fill();
  screenClearTexture = new Texture(screenClearPixmap);
  screenClearSprite = new Sprite(screenClearTexture);
  screenClearSprite.setSize(480, 548);
  screenClearPixmap.dispose();
  
  public void render() {
  
  batch.begin();
  screenClearSprite.draw(batch);
  batch.end();
  animate();
  }
  
  public void animate() {
  moveParticles();
  drawParticles();
  }
  
  public void drawParticles() {
  
  shapeRenderer.begin(ShapeType.Line);
  shapeRenderer.setColor(Color.WHITE);
  ...
  shapeRenderer.line((float) particle.getOldX(), (float) particle.getOldY(), (float) particle.getX(), (float) particle.getY());
  Gdx.gl20.glLineWidth(Global.LINEWIDTH);
  shapeRenderer.end();
  }
  </code></pre>