public function setAlpha($alpha)
    {
        $this->assertChannelValue($alpha, self::CHANNEL_ALPHA);
        $this->alpha = $alpha;

        return $this;
    }
public function setAlpha($alpha)
    {
        $this->assertChannelValue($alpha, self::CHANNEL_ALPHA);
        $this->alpha = $alpha;

        return $this;
    }
public function setAlpha ( $ alpha ) { $ this -> assertChannelValue ( $ alpha , self :: CHANNEL_ALPHA ) ; $ this -> alpha = $ alpha ; return $ this ; }
set alpha value

@param integer $alpha in range (0,255)

@return \Jaguar\Color\RGBColor
@throws \InvalidArgumentException
set alpha value