Contao\CoreBundle\Test\Contao\GdImageTest::testConvertToPaletteImage PHP Method

    public function testConvertToPaletteImage()
    {
        $image = imagecreatetruecolor(100, 100);
        // Whole image black
        imagefill($image, 0, 0, imagecolorallocatealpha($image, 0, 0, 0, 0));
        // Bottom right 25% transparent
        imagealphablending($image, false);
        imagefilledrectangle($image, 50, 50, 100, 100, imagecolorallocatealpha($image, 0, 0, 0, 127));
        $image = new GdImage($image);
        $image->convertToPaletteImage();
        $this->assertInternalType('resource', $image->getResource());
        $this->assertFalse(imageistruecolor($image->getResource()));
        $this->assertEquals(['red' => 0, 'green' => 0, 'blue' => 0, 'alpha' => 0], imagecolorsforindex($image->getResource(), imagecolorat($image->getResource(), 0, 0)), 'Left top pixel should be black');
        $this->assertEquals(127, imagecolorsforindex($image->getResource(), imagecolorat($image->getResource(), 75, 75))['alpha'], 'Bottom right quater should be transparent');
    }