PHP 圖片縮放與水印實戰：ImageCopyResampled vs ImageCopyResized、SimpleImage 庫、imagecopymerge 透明水印

兩大核心縮放函數對比

參數一致：dst, src, dx, dy, sx, sy, dw, dh, sw, sh (目標/源 資源、目標/源 坐標、目標/源 寬高)。

基礎縮放範例 (等比縮小 50%)

<?php
$srcPath = 'test.jpg';
$dstPath = 'thumb.jpg';
$scale = 0.5;

list($w, $h) = getimagesize($srcPath);
$nw = $w * $scale;
$nh = $h * $scale;

$src = imagecreatefromjpeg($srcPath);
$dst = imagecreatetruecolor($nw, $nh);

// 關鍵：保持透明通道 (PNG/GIF)
imagealphablending($dst, false);
imagesavealpha($dst, true);

imagecopyresampled($dst, $src, 0, 0, 0, 0, $nw, $nh, $w, $h);
imagejpeg($dst, $dstPath, 85); // 質量 85

imagedestroy($src);
imagedestroy($dst);
?>

通用封裝：支援 JPEG/PNG/GIF/WebP、自動等比、裁剪模式

<?php
function resizeImage($srcPath, $dstPath, $maxW, $maxH, $quality = 85, $crop = false) {
    if (!file_exists($srcPath)) return false;
    
    list($w, $h, $type) = getimagesize($srcPath);
    $createFunc = [
        IMAGETYPE_JPEG => 'imagecreatefromjpeg',
        IMAGETYPE_PNG  => 'imagecreatefrompng',
        IMAGETYPE_GIF  => 'imagecreatefromgif',
        IMAGETYPE_WEBP => 'imagecreatefromwebp',
    ][$type] ?? null;
    if (!$createFunc) return false;
    
    $src = $createFunc($srcPath);
    if ($crop) {
        // 裁剪模式：填滿目標區域，居中裁剪
        $ratio = max($maxW/$w, $maxH/$h);
        $nw = $w * $ratio; $nh = $h * $ratio;
        $sx = ($nw - $maxW) / 2 / $ratio;
        $sy = ($nh - $maxH) / 2 / $ratio;
        $sw = $maxW / $ratio; $sh = $maxH / $ratio;
        $dw = $maxW; $dh = $maxH; $dx = $dy = 0;
    } else {
        // 等比縮放：完整顯示
        $ratio = min($maxW/$w, $maxH/$h);
        $nw = $w * $ratio; $nh = $h * $ratio;
        $sx = $sy = 0; $sw = $w; $sh = $h;
        $dw = $nw; $dh = $nh; $dx = $dy = 0;
    }
    
    $dst = imagecreatetruecolor($dw, $dh);
    imagealphablending($dst, false);
    imagesavealpha($dst, true);
    imagecopyresampled($dst, $src, $dx, $dy, $sx, $sy, $dw, $dh, $sw, $sh);
    
    $outputFunc = [
        IMAGETYPE_JPEG => fn() => imagejpeg($dst, $dstPath, $quality),
        IMAGETYPE_PNG  => fn() => imagepng($dst, $dstPath, 9),
        IMAGETYPE_GIF  => fn() => imagegif($dst, $dstPath),
        IMAGETYPE_WEBP => fn() => imagewebp($dst, $dstPath, $quality),
    ][$type];
    $outputFunc();
    
    imagedestroy($src); imagedestroy($dst);
    return true;
}
?>

推薦第三方庫：SimpleImage (單文件、鏈式調用)

require 'SimpleImage.php';
$img = new \claviska\SimpleImage('photo.jpg');
$img->resize(300, 200)           // 等比縮放至長邊 300
    ->crop(200, 200, 'center')   // 中心裁剪正方形
    ->overlay('logo.png', 'bottom right', 10, 10, 0.5) // 水印
    ->save('output.jpg', 90);

GitHub: claviska/SimpleImage

水印實戰：文字水印 + 圖片水印 (支援透明 PNG)

<?php
function addWatermark($srcPath, $dstPath, $wmPath, $position = 'bottom-right', $opacity = 50, $margin = 10) {
    $src = imagecreatefromstring(file_get_contents($srcPath));
    $wm  = imagecreatefrompng($wmPath); // 必須 PNG 透明
    
    $srcW = imagesx($src); $srcH = imagesy($src);
    $wmW  = imagesx($wm);  $wmH  = imagesy($wm);
    
    // 計算位置
    $positions = [
        'top-left'      => [$margin, $margin],
        'top-right'     => [$srcW - $wmW - $margin, $margin],
        'bottom-left'   => [$margin, $srcH - $wmH - $margin],
        'bottom-right'  => [$srcW - $wmW - $margin, $srcH - $wmH - $margin],
        'center'        => [($srcW-$wmW)/2, ($srcH-$wmH)/2],
    ];
    [$dx, $dy] = $positions[$position] ?? $positions['bottom-right'];
    
    // 透明度合成 (PHP 5.4+ imagecopymerge_alpha 或手動混合)
    imagealphablending($src, true);
    imagecopymerge($src, $wm, $dx, $dy, 0, 0, $wmW, $wmH, $opacity);
    
    imagepng($src, $dstPath);
    imagedestroy($src); imagedestroy($wm);
}

// 使用
addWatermark('photo.jpg', 'watermarked.png', 'logo.png', 'bottom-right', 60);
?>

進階：文字水印用 imagettftext() 需 TTF 字體文件，支援旋轉、陰影、漸變。

性能優化建議

1. 大圖處理調大 memory_limit (如 256M+)。
2. 頻繁操作考慮 Imagick (ImageMagick PHP 擴展)，多線程、格式支援更全、速度更快。
3. 異步隊列處理 (上傳 → 入隊 → Worker 消費縮放/水印 → 回調)。
4. CDN 邊緣處理 (Cloudflare Workers、阿里云 CDN 圖片處理參數) 卸載源站。