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使用 PHP 5.0创建图形的巧妙方法

Jun 13, 2016 am 10:33 AM
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本文将展示如何使用 PHP 构建面向对象的图形层。使用面向对象的系统可以用来构建复杂的图形,这比使用标准 PHP 库中所提供的基本功能来构建图形简单很多。

  我将图形编辑程序分为两类:一类是绘图程序,利用这种程序可以一个像素一个像素地绘制图像;另外一类是制图程序,这种程序提供了一组对象,例如线、椭圆和矩形,您可以使用这些对象来组合成一幅大图像,例如 JPEG。绘图程序非常适合进行像素级的控制。但是对于业务图形来说,制图程序是比较好的方式,因为大部分图形都是由矩形、线和椭圆组成的。

  PHP 内置的制图基本操作与绘图程序非常类似。它们对于绘制图像来说功能非常强大;但是如果您希望自己的图像是一组对象集合时,这就不太适合了。本文将向您展示如何在 PHP 图形库的基础上构建一个面向对象的图形库。您将使用 PHP V5 中提供的面向对象的扩展。

  具有面向对象的图形支持之后,您的图形代码就非常容易理解和维护了。您可能还需要从一种单一的图形源将图形合成为多种类型的媒介:Flash 电影、SVG 等等。

  目标

  创建一个图形对象库包括 3 个主要的目标:

  从基本操作切换到对象上

  它不使用 imageline、imagefilledrectangle 以及其他图形函数,这个库应该提供一些对象,例如 Line、Rectangle 和 Oval,它们可以用来制作图像。它应该还可以支持构建更大的复杂对象或对对象进行分组的功能。

  可以进行 z 值排序

  制图程序让画家可以在画面表面上上下移动图形对象。这个库应该可以支持将一个对象放到其他对象前后的功能:它使用了一个 z 值,用来定义对象从制图平面开始的高度。z 值越大的对象被画得越晚,也就出现在那些 z 值较小的对象之上。

  提供 viewport 的转换

  通常,数据的坐标空间与图像的坐标空间是不同的。PHP 中的图形基本操作是对图像的坐标平面进行操作的。这个图形库应该支持 viewport 的规范,这样您就可以在一个程序员熟悉的坐标系统中指定图形了,并且可以自动进行伸缩来适应任何图像的大小。

  由于这里有很多特性,您将一步步地编写代码来展示这些代码如何不断增加功能。

  基础知识

  让我们首先来看一个图形环境对象和一个名为 GraphicsObject 的接口,它是使用一个 Line 类实现的,功能就是用来画线。UML 如图 1 所示。

图 1. 图形环境和图形对象接口


  GraphicsEnvironment 类中保存了图形对象和一组颜色,还包括宽度和高度。saveAsPng 方法负责将当前的图像输出到指定的文件中。

  GraphicsObject 是任何图形对象都必须使用的接口。要开始使用这个接口,您所需要做的就是使用 render 方法来画这个对象。它是由一个 Line 类实现的,它利用 4 个坐标:开始和结束的 x 值,开始和结束的 y 值。它还有一个颜色。当调用 render 时,这个对象从 sx,sy 到 ex,ey 画一条由名字指定的颜色的线。

  这个库的代码如清单 1 所示。

  清单 1. 基本的图形库

            <?php class GraphicsEnvironment
            {
            public $width;
            public $height;
            public $gdo;
            public $colors = array();
            public function __construct( $width, $height )
            {
            $this->width = $width;
            $this->height = $height;
            $this->gdo = imagecreatetruecolor( $width, $height );
            $this->addColor( "white", 255, 255, 255 );
            imagefilledrectangle( $this->gdo, 0, 0,
            $width, $height,
            $this->getColor( "white" ) );
            }
            public function width() { return $this->width; }
            public function height() { return $this->height; }
            public function addColor( $name, $r, $g, $b )
            {
            $this->colors[ $name ] = imagecolorallocate(
            $this->gdo,
            $r, $g, $b );
            }
            public function getGraphicObject()
            {
            return $this->gdo;
            }
            public function getColor( $name )
            {
            return $this->colors[ $name ];
            }
            public function saveAsPng( $filename )
            {
            imagepng( $this->gdo, $filename );
            }
            }
            abstract class GraphicsObject
            {
            abstract public function render( $ge );
            }
            class Line extends GraphicsObject
            {
            private $color;
            private $sx;
            private $sy;
            private $ex;
            private $ey;
            public function __construct( $color, $sx, $sy, $ex, $ey )
            {
            $this->color = $color;
            $this->sx = $sx;
            $this->sy = $sy;
            $this->ex = $ex;
            $this->ey = $ey;
            }
            public function render( $ge )
            {
            imageline( $ge->getGraphicObject(),
            $this->sx, $this->sy,
            $this->ex, $this->ey,
            $ge->getColor( $this->color ) );
            }
            }
            ?>
            
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  测试代码如清单 2 所示:

  清单 2. 基本图形库的测试代码

            <?php require_once( "glib.php" );
            $ge = new GraphicsEnvironment( 400, 400 );
            $ge->addColor( "black", 0, 0, 0 );
            $ge->addColor( "red", 255, 0, 0 );
            $ge->addColor( "green", 0, 255, 0 );
            $ge->addColor( "blue", 0, 0, 255 );
            $gobjs = array();
            $gobjs []= new Line( "black", 10, 5, 100, 200 );
            $gobjs []= new Line( "blue", 200, 150, 390, 380 );
            $gobjs []= new Line( "red", 60, 40, 10, 300 );
            $gobjs []= new Line( "green", 5, 390, 390, 10 );
            foreach( $gobjs as $gobj ) { $gobj->render( $ge ); }
            $ge->saveAsPng( "test.png" );
            ?>
            
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  这个测试程序创建了一个图形环境。然后创建几条线,它们指向不同的方向,具有不同的颜色。然后,render 方法可以将它们画到图形平面上。最后,这段代码将这个图像保存为 test.png。

  在本文中,都是使用下面的命令行解释程序来运行这段代码,如下所示:

            % php test.php
            %
            
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  图 2 显示了所生成的 test.png 文件在 Firefox 中的样子。

  图2. 简单的图形对象测试



  这当然不如蒙娜丽莎漂亮,但是可以满足目前的工作需要。
 添加维数

  我们的第一个需求 —— 提供图形对象的能力 —— 已经满足了,现在应该开始满足第二个需求了:可以使用一个 z 值将一个对象放到其他对象的上面或下面。

  我们可以将每个 z 值当作是原始图像的一个面。所画的元素是按照 z 值从最小到最大的顺序来画的。例如,让我们画两个图形元素:一个红色的圆和一个黑色的方框。圆的 z 值是 100,而黑方框的 z 值是 200。这样会将圆放到方框之后,如图 3 所示:

  图3. 不同 z 值的面

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