GIF上的PNG水印图像不会出现

Question

将水印添加到jpg / jpeg并且png工作完美，但是当相同的水印添加到gif图像时会出现奇怪的颜色

the gif image with watermark is here

这里奇怪的颜色是我的png水印图像，如下所示

我的代码

header('content-type: image/png');

        $image = imagecreatefromgif($pic_url);
        $imageSize = getimagesize($pic_url);

        $watermark = imagecreatefrompng('../template/images/logo-watermark2.png');

        $watermark_o_width = imagesx($watermark);
        $watermark_o_height = imagesy($watermark);


        $newWatermarkWidth = ($imageSize[0]-20)/2;
        $newWatermarkHeight = $watermark_o_height * $newWatermarkWidth / $watermark_o_width;

        imagecopyresized($image, $watermark, $imageSize[0]/2 - $newWatermarkWidth/2, $imageSize[1]/2 - $newWatermarkHeight/2, 0, 0, $newWatermarkWidth, $newWatermarkHeight, imagesx($watermark), imagesy($watermark));

        imagegif($image);

        imagedestroy($image);
        imagedestroy($watermark);

将gif图像转换为真彩色图像后解决此问题

header('content-type: image/png');

        $image = imagecreatefromgif($pic_url);
        $imageSize = getimagesize($pic_url);

        $watermark = imagecreatefrompng('../template/images/logo-watermark2.png');

        $watermark_o_width = imagesx($watermark);
        $watermark_o_height = imagesy($watermark);

        $newWatermarkWidth = ($imageSize[0]-20)/2;
        $newWatermarkHeight = $watermark_o_height * $newWatermarkWidth / $watermark_o_width;

        $tmp = imagecreatetruecolor(imagesx($image), imagesy($image));
        $bg = imagecolorallocate($tmp, 255, 255, 255);
        imagefill($tmp, 0, 0, $bg);
        imagecopy($tmp, $image, 0, 0, 0, 0, imagesx($image), imagesy($image));
        $image = $tmp;


        imagecopyresized($image, $watermark, $imageSize[0]/2 - $newWatermarkWidth/2, $imageSize[1]/2 - $newWatermarkHeight/2, 0, 0, $newWatermarkWidth, $newWatermarkHeight, imagesx($watermark), imagesy($watermark));

        imagegif($image);

        imagedestroy($image);
        imagedestroy($watermark);