In the previous article, I brought you "How to upload files in PHP? You’ll understand after reading it! 》, which introduces you in detail how to upload files in PHP. This article will take a look at the relevant knowledge of image processing in PHP. I hope it will be helpful to everyone!

Image processing is something that is often encountered in PHP work. We encounter many scenarios where images need to be processed, such as image verification codes, image watermarks, etc. In fact, the focus of learning image processing is to learn the functions of the GD system in PHP. Image processing in PHP requires the support of the GD library, so let's first take a look at the relevant knowledge about the GD library.

GD library

The GD library can be understood as a function library for creating graphics images. The GD library actually It is written in C language and can be used in PHP. In addition to PHP, it can also be used in Perl and other languages. The GD library provides many interfaces for processing images.

GD is used in PHP The library can draw various graphics images, statistical charts, and image verification codes on the page, which can also be implemented using the GD library.

The example is as follows:

<?php
phpinfo();
?>

After input, in the output result, if the two options shown below appear, it means that the current machine supports image processing through GD:

Or you can use:

<?php
    print_r(gd_info());
?>

After input, run it. If the output result is as shown in the figure below, it also means that the current machine supports processing through GD image.

If it is not installed, you can also just change the "extension=php_gd2.dll" item in the php.ini configuration file under the Windows system The previous comment deletion method is completed.

In PHP, the operation of processing images through the GD library is generally divided into 4 basic steps:

• Create canvas

• Draw image

• Output image

• Release resources

We will complete it through these four steps Now that you know the basic operations on images, let’s take a look at how to operate them.

Create canvas

All our drawing and image processing need to be completed on a background image, which is the canvas, which can be understood as For images, we have a temporary area in memory as a canvas to store image information. Future image operations will be performed on this area, which is the canvas.

imagecreate() and imagecreatetruecolor() are used to create canvas. Their syntax format is as follows:

imagecreate(int $width, int $height)
imagecreatetruecolor(int $width, int $height)

Among them, $width It means that the width of the created canvas can also be understood as the width of the image. $height means that the height of the created canvas is also the height of the image.

Both of these two functions can be used to create a canvas, but the difference is that they can The range of colors accommodated varies.

The example is as follows:

<?php
    $img = @imagecreatetruecolor(120, 20) or die(&#39;畫布創(chuàng)建失?。?amp;#39;);
?>

Since a canvas is just created and there is nothing on it, the browser will not output the canvas, but it can be passed through imagesx() and imagesy( ) to get the width and height of the image:

<?php
    $img = @imagecreatetruecolor(120, 20) or die(&#39;畫布創(chuàng)建失??！&#39;);
    echo &#39;畫布的寬度為：&#39;.imagesx($img).&#39;像素&#39;;
    echo &#39;<br>畫布的高度為：&#39;.imagesy($img).&#39;像素&#39;;
?>

Output result:

From this we have completed creating a canvas.

In addition to using the above two functions to create a canvas, you can also create images through files or URLs in the following ways:

• ##imagecreatefromgif() : Create a new image through a GIF file or URL

• imagecreatefromjpeg() : Create a new image through a JPEG file or UR

• imagecreatefrompng() : Create a new image through a PNG file or URL

• ##imagecreatefromwbmp()

  : Create a new image through a WBMP file or URL An image

Output imageIn the above example, we have successfully created the canvas, which is different in PHP Format images also require different functions for output. The different functions are as follows:

• imagegif()

  : Output a GIF format image to the browser or File

• imagejpeg()

  : Output a JPEG format image to the browser or file

• imagepng( )

  : Output a PNG format image to a browser or file

Their syntax format is as follows:

imagegif(resource $image[, string $filename])
imagejpeg(resource $image[, string $filename[, int $quality]])
imagepng(resource $image[, string $filename])

其中，$image為創(chuàng)建的一個圖像資源；$filename表示為參數(shù)，用來設(shè)置文件需要保存的路徑。

示例如下：

首先我在根目錄中已經(jīng)保存了一個名為php.png的圖像文件

<?php
    header(&#39;Content-type:image/jpeg&#39;);
    $image= imagecreatefrompng(&#39;php.png&#39;);
    imagepng($image);
?>

輸出結(jié)果：

通過imagecreatefrompng()函數(shù)和imagepng()成功的輸出了一個格式為png的圖片。

釋放圖像資源

通過上述示例已經(jīng)成功的輸出了圖片，在圖片的資源使用完成后，通常需要釋放圖像處理時所占用的內(nèi)存，那應(yīng)該怎樣操作呢？

在PHP中我們通過imagedestroy() 函數(shù)來釋放圖像資源，語法格式如下：

imagedestroy(resource $image)

其中$image表示需要釋放資源的圖像

示例如下：

<?php
    header(&#39;Content-type:image/jpeg&#39;);
    $image= imagecreatefrompng(&#39;php.png&#39;);
    imagepng($image);
    imagedestroy($image);
?>

因為釋放圖像資源的操作是沒有輸出內(nèi)容的，所以就不列出輸出結(jié)果了。

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