我有一个图像(让我们称之为原始图像),我想在其上为另一个图像添加水印(让我们称之为徽标).
徽标是透明的PNG,而原始图像可以是png,jpg或gif.
我有以下代码：

function watermarkImage($originalFileContents, $originalWidth, $originalHeight) {
    $logoImage = imagecreatefrompng('logo.png');
    imagealphablending($logoImage, true);

    $logoWidth  = imagesx($logoImage);  
    $logoHeight = imagesy($logoImage);

    $originalImage = imagecreatefromstring($originalFileContents);

    $destX = $originalWidth  - $logoWidth;
    $destY = $originalHeight - $logoHeight;

    imagecopy(
        // source
        $originalImage,
        // destination
        $logoImage,
        // destination x and y
        $destX, $destY,
        // source x and y
        0, 0,
        // width and height of the area of the source to copy
        $logoWidth, $logoHeight
    );
    imagepng($originalImage);
}

仅当原始图像是JPG文件时,此代码才能正常工作(良好=保持徽标的透明度).
当原始文件是GIF或PNG时,徽标具有纯白色背景,这意味着透明度不起作用.

为什么？我需要改变什么才能起作用？
谢谢

更新：
这是我的重新编码版本：

function generate_watermarked_image($originalFileContents, $originalWidth, $originalHeight, $paddingFromBottomRight = 0) {
    $watermarkFileLocation = 'watermark.png';
    $watermarkImage = imagecreatefrompng($watermarkFileLocation);
    $watermarkWidth = imagesx($watermarkImage);  
    $watermarkHeight = imagesy($watermarkImage);

    $originalImage = imagecreatefromstring($originalFileContents);

    $destX = $originalWidth - $watermarkWidth - $paddingFromBottomRight;  
    $destY = $originalHeight - $watermarkHeight - $paddingFromBottomRight;

    // creating a cut resource
    $cut = imagecreatetruecolor($watermarkWidth, $watermarkHeight);

    // copying that section of the background to the cut
    imagecopy($cut, $originalImage, 0, 0, $destX, $destY, $watermarkWidth, $watermarkHeight);

    // placing the watermark now
    imagecopy($cut, $watermarkImage, 0, 0, 0, 0, $watermarkWidth, $watermarkHeight);

    // merging both of the images
    imagecopymerge($originalImage, $cut, $destX, $destY, 0, 0, $watermarkWidth, $watermarkHeight, 100);
}

解决方法:

imagecopy不支持使用带有Alpha通道的两个图像.
看看imagecopymerge.

http://php.net/manual/en/function.imagecopymerge.php

用户注释部分有很多示例,并且您可以根据需要完成实现：

http://www.php.net/manual/en/function.imagecopymerge.php#92787