如何使用PHP中的GD库来调整​​，作物和水印图像？

PHP的GD库支持图像处理操作无需额外依赖。1.调整大小：使用imagecreatefromjpeg()加载图片，创建新尺寸画布，通过imagecopyresampled()缩放并保存；2.裁剪：加载原图后新建目标尺寸画布，复制指定区域；3.添加水印：可用imagettftext()加文字或用imagecopy()叠加透明PNG标志。基本功能简单有效，复杂需求可考虑其他库。

Resizing, cropping, and adding watermarks to images are common tasks when handling image uploads in PHP. The GD library, which is usually enabled by default in most PHP environments, gives you the tools to do all of that without relying on external libraries or services.

Here’s how to do each step with basic code examples and explanations.

Resize an Image Using GD

Resizing is useful for thumbnails or optimizing upload size. You can scale images proportionally or set exact dimensions.

To resize:

1. Load the source image
2. Create a blank canvas of the target size
3. Use imagecopyresampled() to maintain quality
4. Save and free memory

$source = imagecreatefromjpeg('photo.jpg');

$width = imagesx($source);
$height = imagesy($source);

$newWidth = 200;
$newHeight = 200;

$resized = imagecreatetruecolor($newWidth, $newHeight);
imagecopyresampled($resized, $source, 0, 0, 0, 0, $newWidth, $newHeight, $width, $height);

imagejpeg($resized, 'resized_photo.jpg', 90);

imagedestroy($source);
imagedestroy($resized);

You can adjust this for PNGs using imagecreatefrompng() and imagepng() if needed.

Crop an Image to Specific Dimensions

Cropping is often used to generate square thumbnails or focus on a specific part of an image.

The process involves:

• Loading the original image
• Creating a new image resource with the desired crop size
• Copying only the selected portion using imagecopyresampled()

$source = imagecreatefromjpeg('resized_photo.jpg');

$cropWidth = 150;
$cropHeight = 150;

$cropped = imagecreatetruecolor($cropWidth, $cropHeight);
imagecopyresampled($cropped, $source, 0, 0, 50, 50, $cropWidth, $cropHeight, $cropWidth, $cropHeight);

imagejpeg($cropped, 'cropped_photo.jpg', 90);

imagedestroy($source);
imagedestroy($cropped);

In this example, we're cropping starting from (50,50), which cuts off parts of the edges. Adjust those values based on where you want to focus.

Add a Watermark to an Image

Watermarking protects your content or adds branding. You can use text or another image as a watermark.

Text-based watermark:

$im = imagecreatefromjpeg('cropped_photo.jpg');
$black = imagecolorallocate($im, 0, 0, 0);
$text = 'MySite.com';

// Add shadow effect
imagettftext($im, 20, 0, 11, 21, $black, 'arial.ttf', $text);
// Add main text
$white = imagecolorallocate($im, 255, 255, 255);
imagettftext($im, 20, 0, 10, 20, $white, 'arial.ttf', $text);

imagejpeg($im, 'watermarked_photo.jpg', 90);
imagedestroy($im);

Image-based watermark:

If you want to overlay a logo:

$main = imagecreatefromjpeg('photo.jpg');
$watermark = imagecreatefrompng('logo.png');

$wm_width = imagesx($watermark);
$wm_height = imagesy($watermark);

// Place watermark at bottom-right corner
imagecopy($main, $watermark, imagesx($main) - $wm_width - 10, imagesy($main) - $wm_height - 10, 0, 0, $wm_width, $wm_height);

imagejpeg($main, 'watermarked_with_logo.jpg', 90);

imagedestroy($main);
imagedestroy($watermark);

Make sure the watermark PNG has transparency so it blends well.

These techniques work well for simple image manipulation needs. If you’re dealing with many images or need more advanced features like filters or batch processing, consider looking into libraries like Imagine or ImageMagick.

But for resizing, cropping, and basic watermarking? GD gets the job done — no extra setup required in most cases.

基本上就这些。

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