中文(繁體)
中文(繁體)
<span><span><span class="hljs-meta"><?php</span></span><span>
</span><span><span class="hljs-comment">// 本文為示例文章的代碼框架</span></span><span>
</span><span><span class="hljs-comment">// 文章與代碼無關，以下僅為分隔前的內容</span></span><span>
</span><span><span class="hljs-meta">?></span></span><span>
<hr>
</span><span><span class="hljs-comment"># 怎樣通過 imagecreatetruecolor 和 imagealphablending 實現圖像的半透明效果？</span></span><span>
在 PHP 的圖像處理庫 GD 中，`</span><span><span class="hljs-title function_ invoke__">imagecreatetruecolor</span></span><span>()` 與 `</span><span><span class="hljs-title function_ invoke__">imagealphablending</span></span><span>()` 是兩個經常配合使用的函數。它們能夠幫助開發者在生成圖像時實現半透明效果，從而讓圖像疊加更加自然。本文將介紹它們的基本原理和實際應用。
</span><span><span class="hljs-comment">## 1. imagecreatetruecolor 的作用</span></span><span>
`</span><span><span class="hljs-title function_ invoke__">imagecreatetruecolor</span></span><span>()` 用於創建一個真彩色圖像資源。與 `</span><span><span class="hljs-title function_ invoke__">imagecreate</span></span><span>()` 不同，它支持更多的顏色深度和更高的質量。調用方式如下：
```php
</span><span><span class="hljs-variable">$img</span></span><span> = </span><span><span class="hljs-title function_ invoke__">imagecreatetruecolor</span></span><span>(</span><span><span class="hljs-number">200</span></span><span>, </span><span><span class="hljs-number">200</span></span><span>);
</span></span>這會生成一個200×200 的畫布，用來承載後續的繪製操作。
imagealphablending()函數用來設置混合模式。如果啟用混合模式，在繪製圖像或顏色時，新的像素會與已有的像素進行混合，從而實現半透明的效果。
例如：
<span><span><span class="hljs-title function_ invoke__">imagealphablending</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-literal">true</span></span><span>);
</span></span>當設置為true時，顏色疊加會按照透明度進行混合，而不是直接覆蓋。
在生成帶有透明效果的PNG 時，還需要使用imagesavealpha()來確保透明通道能夠正確保存：
<span><span><span class="hljs-title function_ invoke__">imagesavealpha</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-literal">true</span></span><span>);
</span></span>這樣在保存PNG 文件時，透明部分才會生效。
下面給出一個生成半透明矩形的完整示例：
<span><span><span class="hljs-meta"><?php</span></span><span>
</span><span><span class="hljs-comment">// 創建畫布</span></span><span>
</span><span><span class="hljs-variable">$img</span></span><span> = </span><span><span class="hljs-title function_ invoke__">imagecreatetruecolor</span></span><span>(</span><span><span class="hljs-number">200</span></span><span>, </span><span><span class="hljs-number">200</span></span><span>);
</span><span><span class="hljs-comment">// 開啟混合模式和保存透明通道</span></span><span>
</span><span><span class="hljs-title function_ invoke__">imagealphablending</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-literal">true</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">imagesavealpha</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-literal">true</span></span><span>);
</span><span><span class="hljs-comment">// 分配透明背景</span></span><span>
</span><span><span class="hljs-variable">$transparent</span></span><span> = </span><span><span class="hljs-title function_ invoke__">imagecolorallocatealpha</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">127</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">imagefilledrectangle</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">200</span></span><span>, </span><span><span class="hljs-number">200</span></span><span>, </span><span><span class="hljs-variable">$transparent</span></span><span>);
</span><span><span class="hljs-comment">// 分配半透明紅色</span></span><span>
</span><span><span class="hljs-variable">$red</span></span><span> = </span><span><span class="hljs-title function_ invoke__">imagecolorallocatealpha</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-number">255</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">75</span></span><span>);
</span><span><span class="hljs-comment">// 繪製矩形</span></span><span>
</span><span><span class="hljs-title function_ invoke__">imagefilledrectangle</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>, </span><span><span class="hljs-number">50</span></span><span>, </span><span><span class="hljs-number">50</span></span><span>, </span><span><span class="hljs-number">150</span></span><span>, </span><span><span class="hljs-number">150</span></span><span>, </span><span><span class="hljs-variable">$red</span></span><span>);
</span><span><span class="hljs-comment">// 輸出 PNG</span></span><span>
</span><span><span class="hljs-title function_ invoke__">header</span></span><span>(</span><span><span class="hljs-string">'Content-Type: image/png'</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">imagepng</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">imagedestroy</span></span><span>(</span><span><span class="hljs-variable">$img</span></span><span>);
</span><span><span class="hljs-meta">?></span></span><span>
</span></span>運行上述代碼後，瀏覽器將顯示一個透明背景、中央有半透明紅色矩形的PNG 圖片。
通過imagecreatetruecolor()創建高質量畫布，再結合imagealphablending()和imagesavealpha() ，開發者可以輕鬆實現圖像的半透明效果。這一技巧在生成水印、特效圖像或UI 元素時尤為有用。掌握這兩個函數的配合使用，能讓PHP 的圖像處理更靈活、更強大。
