为什么输出图像文件大于使用imagick的输入？

Question

why is the output image bigger than input using imagick? original jpg: http://www.persunmall.com/images/jm3.jpg

/**
 * example
 --------------------------------------
        'jm3.jpg' => 119 k   (453*680)
        'l.jpg' => 275 k   (400*600)        // why the output jpg is much bigger than input jpg?

 */
$jy_image=new jy_image
$jy_image->output_image('jm3.jpg','l.jpg',400,0);

class bellow:

<?php 
class jy_image {
    private $image = null;
    private $type = null;

    // 构造函数
    public function __construct() {
    }


    // 析构函数
    public function __destruct() {
        if ($this->image !== null)
            $this->image->destroy();
    }

    // 载入图像
    public function open($path) {
        $this->image = new Imagick($path);
        if ($this->image) {
            $this->type = strtolower($this->image->getImageFormat());   // jpeg
        }
        return $this->image;
    }


    public function crop($x = 0, $y = 0, $width = null, $height = null) {
        if ($width == null)
            $width = $this->image->getImageWidth() - $x;
        if ($height == null)
            $height = $this->image->getImageHeight() - $y;
        if ($width <= 0 || $height <= 0)
            return;

        if ($this->type == 'gif') {
            $image  = $this->image;
            $canvas = new Imagick();

            $images = $image->coalesceImages();
            foreach ($images as $frame) {
                $img = new Imagick();
                $img->readImageBlob($frame);
                $img->cropImage($width, $height, $x, $y);

                $canvas->addImage($img);
                $canvas->setImageDelay($img->getImageDelay());
                $canvas->setImagePage($width, $height, 0, 0);
            }

            $image->destroy();
            $this->image = $canvas;
        } else {
            $this->image->cropImage($width, $height, $x, $y);
        }
    }

    /*
     * 更改图像大小
     $fit: 适应大小方式
     'force': 把图片强制变形成 $width X $height 大小
     'scale': 按比例在安全框 $width X $height 内缩放图片, 输出缩放后图像大小 不完全等于 $width X $height
     'scale_fill': 按比例在安全框 $width X $height 内缩放图片，安全框内没有像素的地方填充色, 使用此参数时可设置背景填充色 $bg_color = array(255,255,255)(红,绿,蓝, 透明度) 透明度(0不透明-127完全透明))
     其它: 智能模能 缩放图像并载取图像的中间部分 $width X $height 像素大小
     $fit = 'force','scale','scale_fill' 时： 输出完整图像
     $fit = 图像方位值 时, 输出指定位置部分图像 
     字母与图像的对应关系如下:

     north_west   north   north_east

     west         center        east

     south_west   south   south_east

     */
    public function resize_to($width = 100, $height = 100, $fit = 'center', $fill_color = array(255, 255, 255, 0)) {

        // 保证其中一边，另外一边相应缩放
        if($width==0 || $height==0){
            $image = $this->image;
            $src_width = $image->getImageWidth();
            $src_height = $image->getImageHeight();
            if($width>0 && $height==0){
                $width=($width>$src_width)?$src_width:$width;
                $height = $src_height*($width/$src_width);
            }
            else if($width==0 && $height>0){
                $height=($height>$src_height)?$src_height:$height;
                $width = $src_width*($height/$src_height);
            }
            $fit='force';
        }

        switch ($fit) {
            case 'force':   // 强制缩放，会变形
                if ($this->type == 'gif') {
                    $image  = $this->image;
                    $canvas = new Imagick();

                    $images = $image->coalesceImages();
                    foreach ($images as $frame) {
                        $img = new Imagick();
                        $img->readImageBlob($frame);
                        $img->thumbnailImage($width, $height, false);

                        $canvas->addImage($img);
                        $canvas->setImageDelay($img->getImageDelay());
                    }
                    $image->destroy();
                    $this->image = $canvas;
                } else {
                    $this->image->thumbnailImage($width, $height, false);
                }
                break;
            case 'scale':
                if ($this->type == 'gif') {
                    $image  = $this->image;
                    $images = $image->coalesceImages();
                    $canvas = new Imagick();
                    foreach ($images as $frame) {
                        $img = new Imagick();
                        $img->readImageBlob($frame);
                        $img->thumbnailImage($width, $height, true);

                        $canvas->addImage($img);
                        $canvas->setImageDelay($img->getImageDelay());
                    }
                    $image->destroy();
                    $this->image = $canvas;
                } else {
                    $this->image->thumbnailImage($width, $height, true);
                }
                break;
            case 'scale_fill':      // 填充白色
                $size       = $this->image->getImagePage();
                $src_width  = $size['width'];
                $src_height = $size['height'];

                $x = 0;
                $y = 0;

                $dst_width  = $width;
                $dst_height = $height;

                if ($src_width * $height > $src_height * $width) {
                    $dst_height = intval($width * $src_height / $src_width);
                    $y          = intval(($height - $dst_height) / 2);
                } else {
                    $dst_width = intval($height * $src_width / $src_height);
                    $x         = intval(($width - $dst_width) / 2);
                }

                $image  = $this->image;
                $canvas = new Imagick();

                $color = 'rgba(' . $fill_color[0] . ',' . $fill_color[1] . ',' . $fill_color[2] . ',' . $fill_color[3] . ')';
                if ($this->type == 'gif') {
                    $images = $image->coalesceImages();
                    foreach ($images as $frame) {
                        $frame->thumbnailImage($width, $height, true);

                        $draw = new ImagickDraw();
                        $draw->composite($frame->getImageCompose(), $x, $y, $dst_width, $dst_height, $frame);

                        $img = new Imagick();
                        $img->newImage($width, $height, $color, 'gif');
                        $img->drawImage($draw);

                        $canvas->addImage($img);
                        $canvas->setImageDelay($img->getImageDelay());
                        $canvas->setImagePage($width, $height, 0, 0);
                    }
                } else {
                    $image->thumbnailImage($width, $height, true);

                    $draw = new ImagickDraw();
                    $draw->composite($image->getImageCompose(), $x, $y, $dst_width, $dst_height, $image);

                    $canvas->newImage($width, $height, $color, $this->get_type());
                    $canvas->drawImage($draw);
                    $canvas->setImagePage($width, $height, 0, 0);
                }
                $image->destroy();
                $this->image = $canvas;
                break;
            default:
                $size       = $this->image->getImagePage();
                $src_width  = $size['width'];
                $src_height = $size['height'];

                $crop_x = 0;
                $crop_y = 0;

                $crop_w = $src_width;
                $crop_h = $src_height;

                if ($src_width * $height > $src_height * $width) {
                    $crop_w = intval($src_height * $width / $height);   // 100*50 =>50*100 输出：25*50
                } else {
                    $crop_h = intval($src_width * $height / $width);    //50*100 =>100*50 输出：50*25
                }

                switch ($fit) {
                    case 'north_west':
                        $crop_x = 0;
                        $crop_y = 0;
                        break;
                    case 'north':
                        $crop_x = intval(($src_width - $crop_w) / 2);
                        $crop_y = 0;
                        break;
                    case 'north_east':
                        $crop_x = $src_width - $crop_w;
                        $crop_y = 0;
                        break;
                    case 'west':
                        $crop_x = 0;
                        $crop_y = intval(($src_height - $crop_h) / 2);
                        break;
                    case 'center':
                        $crop_x = intval(($src_width - $crop_w) / 2);
                        $crop_y = intval(($src_height - $crop_h) / 2);
                        break;
                    case 'east':
                        $crop_x = $src_width - $crop_w;
                        $crop_y = intval(($src_height - $crop_h) / 2);
                        break;
                    case 'south_west':
                        $crop_x = 0;
                        $crop_y = $src_height - $crop_h;
                        break;
                    case 'south':
                        $crop_x = intval(($src_width - $crop_w) / 2);
                        $crop_y = $src_height - $crop_h;
                        break;
                    case 'south_east':
                        $crop_x = $src_width - $crop_w;
                        $crop_y = $src_height - $crop_h;
                        break;
                    default:
                        $crop_x = intval(($src_width - $crop_w) / 2);
                        $crop_y = intval(($src_height - $crop_h) / 2);
                }

                $image  = $this->image;
                $canvas = new Imagick();

                if ($this->type == 'gif') {
                    $images = $image->coalesceImages();
                    foreach ($images as $frame) {
                        $img = new Imagick();
                        $img->readImageBlob($frame);
                        $img->cropImage($crop_w, $crop_h, $crop_x, $crop_y);
                        $img->thumbnailImage($width, $height, true);

                        $canvas->addImage($img);
                        $canvas->setImageDelay($img->getImageDelay());
                        $canvas->setImagePage($width, $height, 0, 0);
                    }
                } else {
                    $image->cropImage($crop_w, $crop_h, $crop_x, $crop_y);
                    $image->thumbnailImage($width, $height, true);
                    $canvas->addImage($image);
                    $canvas->setImagePage($width, $height, 0, 0);
                }
                $image->destroy();
                $this->image = $canvas;
        }

    }




    // 添加水印图片
    public function add_watermark($path, $x = 0, $y = 0) {

        $watermark = new Imagick($path);

        // 默认水印位置 by jimmy 2014-4-11
        if(empty($x) && empty($y)){
            $im_w= $this->image->getImageWidth();
            $im_h = $this->image->getImageHeight();
            $wa_w = $watermark->getImageWidth();
            $wa_h = $watermark->getImageHeight();
            $x=($im_w-$wa_w)/2;
            $y=($im_h-$wa_h-50);
        }


        $draw      = new ImagickDraw();
        $draw->composite($watermark->getImageCompose(), $x, $y, $watermark->getImageWidth(), $watermark->getimageheight(), $watermark);

        if ($this->type == 'gif') {
            $image  = $this->image;
            $canvas = new Imagick();
            $images = $image->coalesceImages();
            foreach ($image as $frame) {
                $img = new Imagick();
                $img->readImageBlob($frame);
                $img->drawImage($draw);

                $canvas->addImage($img);
                $canvas->setImageDelay($img->getImageDelay());
            }
            $image->destroy();
            $this->image = $canvas;
        } else {
            $this->image->drawImage($draw);
        }
    }


    // 添加水印文字
    public function add_text($text, $x = 0, $y = 0, $angle = 0, $style = array()) {
        $draw = new ImagickDraw();
        if (isset($style['font']))
            $draw->setFont($style['font']);
        if (isset($style['font_size']))
            $draw->setFontSize($style['font_size']);
        if (isset($style['fill_color']))
            $draw->setFillColor($style['fill_color']);
        if (isset($style['under_color']))
            $draw->setTextUnderColor($style['under_color']);

        if ($this->type == 'gif') {
            foreach ($this->image as $frame) {
                $frame->annotateImage($draw, $x, $y, $angle, $text);
            }
        } else {
            $this->image->annotateImage($draw, $x, $y, $angle, $text);
        }
    }


    // 保存到指定路径
    public function save_to($path) {
        $this->image->stripImage();
        if ($this->type == 'gif') {
            $this->image->writeImages($path, true);
        } else {
            $this->image->writeImage($path);
        }
    }

    // 输出图像
    public function output($header = true) {
        if ($header)
            header('Content-type: ' . $this->type);
        echo $this->image->getImagesBlob();
    }


    public function get_width() {
        $size = $this->image->getImagePage();
        return $size['width'];
    }

    public function get_height() {
        $size = $this->image->getImagePage();
        return $size['height'];
    }

    // 设置图像类型， 默认与源类型一致
    public function set_type($type = 'png') {
        $this->type = $type;
        $this->image->setImageFormat($type);
    }

    // 获取源图像类型
    public function get_type() {
        return $this->type;
    }


    // 当前对象是否为图片
    public function is_image() {
        if ($this->image)
            return true;
        else
            return false;
    }



    public function thumbnail($width = 100, $height = 100, $fit = true) {
        $this->image->thumbnailImage($width, $height, $fit);
    } // 生成缩略图 $fit为真时将保持比例并在安全框 $width X $height 内生成缩略图片

    /*
    添加一个边框
    $width: 左右边框宽度
    $height: 上下边框宽度
    $color: 颜色: RGB 颜色 'rgb(255,0,0)' 或 16进制颜色 '#FF0000' 或颜色单词 'white'/'red'...
    */
    public function border($width, $height, $color = 'rgb(220, 220, 220)') {
        $color = new ImagickPixel();
        $color->setColor($color);
        $this->image->borderImage($color, $width, $height);
    }

    public function blur($radius, $sigma) {
        $this->image->blurImage($radius, $sigma);
    } // 模糊
    public function gaussian_blur($radius, $sigma) {
        $this->image->gaussianBlurImage($radius, $sigma);
    } // 高斯模糊
    public function motion_blur($radius, $sigma, $angle) {
        $this->image->motionBlurImage($radius, $sigma, $angle);
    } // 运动模糊
    public function radial_blur($radius) {
        $this->image->radialBlurImage($radius);
    } // 径向模糊

    public function add_noise($type = null) {
        $this->image->addNoiseImage($type == null ? imagick::NOISE_IMPULSE : $type);
    } // 添加噪点

    public function level($black_point, $gamma, $white_point) {
        $this->image->levelImage($black_point, $gamma, $white_point);
    } // 调整色阶
    public function modulate($brightness, $saturation, $hue) {
        $this->image->modulateImage($brightness, $saturation, $hue);
    } // 调整亮度、饱和度、色调

    public function charcoal($radius, $sigma) {
        $this->image->charcoalImage($radius, $sigma);
    } // 素描
    public function oil_paint($radius) {
        $this->image->oilPaintImage($radius);
    } // 油画效果

    public function flop() {
        $this->image->flopImage();
    } // 水平翻转
    public function flip() {
        $this->image->flipImage();
    } // 垂直翻转

    /**
     * 压缩图片 by jimmy 2014-4-11
     * ----------------------------------------
     */
    function compress($quality=100){
        $this->image->setImageFormat('JPEG');
        $this->image->setImageCompression(Imagick::COMPRESSION_JPEG);
        $q = $this->image->getImageCompressionQuality();
        $q = $q* $quality/100;
        if($q==0){
            $q = $quality;
        }
        $this->image->setImageCompressionQuality($q);
        $this->image->stripImage();
    }

    /**
     * 综合各个步骤输出 by jimmy 2014-4-11
     * ----------------------------------------
     */
    function output_image($srcFile,$destFile,$w=0,$h=0,$quality=100,$waterFile='',$fit="scale_fill"){
        $this->open($srcFile);
        $this->compress($quality);
        if(!empty($waterFile)){
            $this->add_watermark($waterFile);
        }
        if($w+$h>0){
            $this->resize_to($w,$h,$fit);
        }
        $this->save_to($destFile);
    }

}

Answer 1

First, from your comment "even quality =80 ,the l.jpg is more bigger than jm3.jpg" I think that is wrong. Re-saving the file with lower quality gives the following results.

119,300 = original image          
266,008 = Resized at 100% 
245,145 = Resized at 100% + despeckle
 97,536 = Resized at 90% quality
 88,342 = Resized at 90% quality with despeckle

I also added Imagick::despeckle as resizing can introduce speckles.

However the fundamental question you're asking it "Why does re-saving an JPEG image increase the file size, for the same quality?"

The simple answer is that after because it is a lossy format, after saving an image as a JPEG there can be more information in the new image. To save the image at the same quality, that additional information needs encoded, which takes up additional space.

Consider a simple sine wave; it contains very little information. Then imagine that we 'save it as an image' with a lossy format, which trims the ends off the sine wave i.e. something like:

Whichever way you consider the new shape it has more information that needs to be encoded to retain the new shape, so re-saving it at 100% quality takes a lot more space that the original lossy saved image.

Re-saving it at a lower quality produces a smaller image, but rapidly loses information from the image, as each saving is an approximation of an approximation.

TL:DR use PNG for all intermediate formats, and only use JPG for the final output to customers.