PHP GD2 圖片縮圖類別：支援裁切與等比縮放

前置需求

PHP 必須編譯安裝 GD2 擴充 (--with-gd)，支援 JPEG、PNG、GIF 格式。

ResizeImage 類別完整程式碼

<?php
class ResizeImage {
    private $srcImg, $dstImg, $type;
    private $width, $height;
    private $resizeW, $resizeH, $cut;

    public function __construct($src, $dstW, $dstH, $cut, $dstPath) {
        $this->srcImg   = $src;
        $this->resizeW  = $dstW;
        $this->resizeH  = $dstH;
        $this->cut      = $cut; // 1=裁切填滿, 0=等比縮放含留白
        $this->type     = strtolower(substr(strrchr($src, '.'), 1));
        $this->initImage();
        $this->dstImg   = $dstPath;
        $this->process();
    }

    private function initImage() {
        switch ($this->type) {
            case 'jpg': case 'jpeg': $this->im = imagecreatefromjpeg($this->srcImg); break;
            case 'gif':               $this->im = imagecreatefromgif($this->srcImg); break;
            case 'png':               $this->im = imagecreatefrompng($this->srcImg); break;
            default: throw new Exception("Unsupported format: {$this->type}");
        }
        $this->width  = imagesx($this->im);
        $this->height = imagesy($this->im);
    }

    private function process() {
        $srcRatio = $this->width / $this->height;
        $dstRatio = $this->resizeW / $this->resizeH;

        if ($this->cut) {
            // 裁切模式：以目標比例裁切源圖再縮放
            if ($srcRatio >= $dstRatio) {
                // 來源較寬 → 高度優先，左右裁切
                $cropH = $this->height;
                $cropW = $cropH * $dstRatio;
                $srcX  = ($this->width - $cropW) / 2;
                $srcY  = 0;
            } else {
                // 來源較高 → 寬度優先，上下裁切
                $cropW = $this->width;
                $cropH = $cropW / $dstRatio;
                $srcX  = 0;
                $srcY  = ($this->height - $cropH) / 2;
            }
            $newImg = imagecreatetruecolor($this->resizeW, $this->resizeH);
            imagecopyresampled($newImg, $this->im, 0, 0, $srcX, $srcY,
                               $this->resizeW, $this->resizeH, $cropW, $cropH);
        } else {
            // 等比縮放模式：完整保留畫面，留白處透明/背景色
            if ($srcRatio >= $dstRatio) {
                $newW = $this->resizeW;
                $newH = $newW / $srcRatio;
            } else {
                $newH = $this->resizeH;
                $newW = $newH * $srcRatio;
            }
            $newImg = imagecreatetruecolor($newW, $newH);
            // 設定透明背景 (PNG/GIF)
            if ($this->type == 'png' || $this->type == 'gif') {
                imagealphablending($newImg, false);
                imagesavealpha($newImg, true);
                $transparent = imagecolorallocatealpha($newImg, 0,0,0,127);
                imagefill($newImg, 0, 0, $transparent);
            }
            imagecopyresampled($newImg, $this->im, 0, 0, 0, 0, $newW, $newH, $this->width, $this->height);
        }

        // 輸出
        switch ($this->type) {
            case 'jpg': case 'jpeg': imagejpeg($newImg, $this->dstImg, 90); break;
            case 'gif':              imagegif($newImg, $this->dstImg); break;
            case 'png':              imagepng($newImg, $this->dstImg, 6); break;
        }
        imagedestroy($newImg);
        imagedestroy($this->im);
    }
}
?>

使用範例

require_once 'ResizeImage.php';

// 等比縮放，最大寬 200、高 100
new ResizeImage("photo.jpg", 200, 100, 0, "thumb_photo.jpg");

// 裁切填滿 200x100
new ResizeImage("photo.jpg", 200, 100, 1, "thumb_crop.jpg");

進階功能擴充

• 支援 imagefilter() 銳化、灰階、負片效果
• 加入浮水印文字/圖片疊加
• 輸出 WebP 格式 (imagewebp())
• 批次處理目錄下所有圖片
• 記憶體優化：大圖分塊處理或使用 Imagick 擴充

常見問題