Introducing the image processing library GD in PHP

GD is a very practical image processing library in PHP. Using the GD library, PHP developers can easily process, generate and output images, such as generating verification codes, thumbnails, watermarks, etc. This article will introduce the GD library to you and give some examples of using the GD library in PHP.

The GD library is an open source library, originally designed for C language, and can be used to process various image formats such as JPEG, PNG, and GIF. Since PHP 5.0, the GD library can be used with PHP.

Using the GD library, you can:

• Read and save images
• Adjust image size
• Adjust image quality
• Add text and shapes
• Add watermark
• Convert image format, etc.

In PHP, to use the GD library, you need to first check whether the GD extension is installed. You can check by running the following code:

if (extension_loaded('gd') && function_exists('gd_info')) {
    echo 'GD 库已安装';
} else {
    echo 'GD 库未安装';
}

If the output is GD library installed, it means that the GD library extension has been installed normally.

The following are some practical examples of the GD library.

Generate verification code

Generating verification code is a common task in website development, and the GD library easily implements this function. The specific implementation method is as follows:

// 生成验证码
function generateVerificationCode($length = 4) {
    // 验证码字符集，去掉了容易混淆的字符
    $chars = 'abcdefghkmnprstuvwxyzABCDEFGHKMNPRSTUVWXYZ23456789';
    // 洗牌
    $chars = str_shuffle($chars);
    // 取出前 $length 个字符作为验证码
    $code = substr($chars, 0, $length);
    // 存储验证码
    $_SESSION['verificationCode'] = $code;
    // 创建画布
    $image = imagecreatetruecolor(120, 30);
    // 分配颜色
    $bgColor = imagecolorallocate($image, 255, 255, 255);
    $textColor = imagecolorallocate($image, 0, 0, 0);
    // 填充背景
    imagefill($image, 0, 0, $bgColor);
    // 画上验证码
    imagestring($image, 5, 20, 8, $code, $textColor);
    // 输出图像
    header('Content-type: image/jpeg');
    imagejpeg($image);
    // 销毁图像
    imagedestroy($image);
}

In the above code, we first generate a random verification code and then store it in SESSION. Then create a 120x30 pixel canvas, fill the background with white, draw the verification code onto the canvas, and output it in image format.

Generate thumbnails

In websites, in order to speed up page loading, large images are usually compressed into small thumbnails. The following is the implementation method of using the GD library to generate thumbnails:

// 生成缩略图
function generateThumbnail($src, $dst, $width, $height) {
    // 获取原图信息
    list($srcWidth, $srcHeight, $type) = getimagesize($src);
    // 根据原图类型创建画布
    switch ($type) {
        case IMAGETYPE_JPEG:
            $imageSrc = imagecreatefromjpeg($src);
            break;
        case IMAGETYPE_PNG:
            $imageSrc = imagecreatefrompng($src);
            break;
        case IMAGETYPE_GIF:
            $imageSrc = imagecreatefromgif($src);
            break;
        default:
            throw new Exception('不支持的图像类型');
    }
    // 计算缩略图的宽高比
    $ratio = min($width / $srcWidth, $height / $srcHeight);
    // 计算缩略图的实际宽度和高度
    $thumbWidth = intval($srcWidth * $ratio);
    $thumbHeight = intval($srcHeight * $ratio);
    // 创建目标画布
    $imageDst = imagecreatetruecolor($thumbWidth, $thumbHeight);
    // 复制原图到目标画布，并调整大小
    imagecopyresampled($imageDst, $imageSrc, 0, 0, 0, 0, $thumbWidth, 
        $thumbHeight, $srcWidth, $srcHeight);
    // 保存图片到指定路径
    imagejpeg($imageDst, $dst);
    // 销毁图像
    imagedestroy($imageSrc);
    imagedestroy($imageDst);
}

In the above code, we first obtain the information of the original image and create a canvas based on the original image type. Then calculate the aspect ratio and actual width and height of the thumbnail based on the target width and height of the thumbnail, create the target canvas, and use the imagecopyresampled function to copy the original image to the target canvas and resize it. Finally, save the target canvas to the specified path.

Add watermark

Adding watermarks to your website can protect the copyright of images to a certain extent. The following is the implementation method of adding watermarks to images using the GD library:

// 添加水印
function addWatermark($src, $dst, $watermark) {
    // 获取原图信息
    list($srcWidth, $srcHeight, $type) = getimagesize($src);
    // 根据原图类型创建画布
    switch ($type) {
        case IMAGETYPE_JPEG:
            $imageSrc = imagecreatefromjpeg($src);
            break;
        case IMAGETYPE_PNG:
            $imageSrc = imagecreatefrompng($src);
            break;
        case IMAGETYPE_GIF:
            $imageSrc = imagecreatefromgif($src);
            break;
        default:
            throw new Exception('不支持的图像类型');
    }
    // 获取水印图片信息
    list($watermarkWidth, $watermarkHeight) = getimagesize($watermark);
    // 创建水印画布
    $watermarkImage = imagecreatefrompng($watermark);
    // 定义水印数量和间隔
    $count = 10;
    $padding = 5;
    // 添加水印
    $posX = $padding;
    $posY = $padding;
    for ($i = 0; $i < $count; $i++) {
        imagecopy($imageSrc, $watermarkImage, $posX, $posY, 0, 0, 
            $watermarkWidth, $watermarkHeight);
        $posX += $watermarkWidth + $padding;
    }
    // 保存图片到指定路径
    imagejpeg($imageSrc, $dst);
    // 销毁图像
    imagedestroy($imageSrc);
    imagedestroy($watermarkImage);
}

In the above code, we first obtain the information of the original image and create a canvas based on the original image type. Then get the information of the watermark image and create a watermark canvas. Finally, use the imagecopy function to add the watermark in a loop. Finally, save the original image to the specified path.

The above are several examples of practical applications of the GD library in PHP. By using the GD library, we can easily process, generate and output images, making our website more colorful.

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