PHP GD拇指质量差

Question

            $tmp_im = imagecreatetruecolor($width, $height);
            $x = $this->getX();
            $y = $this->getY();

            $w = floor($resize_height * ($x / $y));
            $h = $resize_height;

            $this->tmp_im = imagecreatetruecolor($w, $h);

                            imagealphablending($this->tmp_im, false);
                            imagesavealpha($this->tmp_im, true);

                            imagecopyresampled($this->tmp_im, $this->im, 0, 0, 0, 0, $w, $h, $this->getX(), $this->getY());

            $this->im = $this->tmp_im;


            $hs = floor(($width - $this->getX())/2);
            $vs = floor(($height - $this->getY())/2);

            imagecopy($tmp_im, $this->im, $hs, $vs, 0, 0, $this->getX(), $this->getY());

            $this->im = $tmp_im;

the results is a poor quality resized image, what im doing wrong? i also tried to use a imagejpeg with quality = 100

Answer 1

Use imagecopytruecolor() instead of

imagecopy($tmp_im, $this->im, $hs, $vs, 0, 0, $this->getX(), $this->getY());

This is a Notice on the official PHP documentation page:

There is a problem due to palette image limitations (255+1 colors). Resampling or filtering an image commonly needs more colors than 255, a kind of approximation is used to calculate the new resampled pixel and its color. With a palette image we try to allocate a new color, if that failed, we choose the closest (in theory) computed color. This is not always the closest visual color. That may produce a weird result, like blank (or visually blank) images. To skip this problem, please use a truecolor image as a destination image, such as one created by imagecreatetruecolor().