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Talk about PHP drawing (1)_PHP tutorial

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Jul 13, 2016 pm 05:35 PM

php one twice Drawing function Library of Talk about it

实在不敢说是在这里“讲”GD库，因为我用GD也才一两次而已，绝大多数的函数还没
有接触到。可是三斑竹小刁热情地向我约稿，我只好硬着头皮写一点自己的心得。希望能
够起到抛砖引玉的效果。

其实，我们在web页面里实现“图”的效果不一定非用GD不可，比较容易解决的是柱状
图——用HTML就可以解决。比如：

/* (1) */

      /* (2) */

          /* (3) */

/* （4） */

是一组数据，数据从哪里来，是无
关大局的，就看你的需要了；代码中需要说两句的地方我都加了注释，现在一一来说明。

(1) 这里要注意的是 valign="bottom"，是为了让单元格的内容底部对齐。为什么加在里
    呢？可以让表格里这一行的内容都遵循这一对齐方式，不必在每一个里指定，这样可
    以使PHP执行结果的HTML页的原代码节约好几十个字节呐！节约浏览者的宝贵时间。

(2) 注意，最关键的东西在这里！

，我们就是利用table的height属性来
    实现不同高度的“柱”的。我这里为了让大家看得清楚，原始数据没有经过按比例的缩放，
    如果你的数据特别大，或者特别小，都不适宜直接赋给table的height属性，而应该根据情
    况按适当比例缩放这些数据。比如你估计你的这组数据的每一个数字都会在3000~8000之间，
    可以考虑将他们缩小25倍，即 height=""

(3) 提一下这一行里的 bgcolor="#xxxxxx"，这是柱体的颜色(RGB)。其实，真正的柱状图应该
    每一个柱体用一种颜色，这里为了代码尽量简单，我用了这个for循环，因此也就没办法给
    每一个柱体指定一种颜色。——其实也是有办法的，我只是实在没有必要为了这个例子再写
    一个抽取颜色的函数来把初学者搞晕。所以，那一部分由你自己去完善吧。

(4) 在这里以与柱体相同的颜色显示真实的数据。当然，你也可以选择把这个数字放在柱体的顶
    上，可能更专业一些。然而我本人还是习惯于把它放在下面。

    借助于HTML的table，我们可以构造出各种柱状图，这个例子讲的是用bgcolor来显示色块，
除此以外，还可以用 background="(图片)" ，图片是带花纹的，于是柱状图的柱体就有了花纹。
而你把真实的数据用反差很大的颜色显示在上面注释(3)所示的那个

里，也是很好的效果。

前面是回避GD的一个有效的方法，但要做复杂的图形，就非用GD不可了。

sadly 的PHP4中文手册里，说GD函数库里有44个函数，但我看最新版的英文PHP4手册里，
GD的函数已经有80余个！由于笔者英文比较差，读英文的手册只能连蒙带猜，所以不能确定
新的GD库是否重新支持GIF了？不管怎样，我认为，既然我们在使用完全免费的PHP，何必要
“冒险”去用有版权的GIF？何不免费到底，用PNG呢？只要你不需用动画，PNG同样可以做出
象GIF一样小的文件！

下面我就结合一段程序，一句代码一句代码地说说常用的这些GD函数。

从开头说起吧。

Header("Content-type: image/png");
// 这是发送一个HTTP头，告诉浏览器：“你听着，这是一个图象，可别当成文字来显示呀！”
// 由于我个人的喜好，用了PNG，当然你也可以用 Header("Content-type: image/gif");
// 或者 Header("Content-type: image/jpeg");
$im = ImageCreate (50, 100);
// 创建图象。注意，图象在创建的时候还没有被指定图象格式。
// ImageCreate函数，两个参数，无庸质疑，这是创建的图象的宽度和高度。
// 它的返回值是一个int数值，这个数值相当重要，你继续绘制这个图象、
// 直到你输出这个图象之前，无处不用到这个数值，我们暂且称之为图象的ID。
// 因为使用的频率相当高，所以，我们把它赋给一个名字比较短的变量。

// 现在我们先画一条线吧。画线的函数是这样的：
// imageline (int im, int x1, int y1, int x2, int y2, int col);
// 第一个参数im，就是图象的ID，后面的 x1,y1,x2,y2，不用说了，
// 是起点(x1,y1) 终点(x2,y2)的坐标呀！（图象的左上角坐标是 (0,0) ）
// 最后一个参数是什么呀？是颜色！GD要求针对图象定义颜色，用定义的这些颜色来作图。
// 为什么要针对图象定义颜色？我猜测，是为了GIF、PNG等图象用之做“调色板”的。
// 这牵扯到图象本身的知识，这里不赘述了。
// 所以，画线之前，我们还要先定义颜色（真麻烦）。

// $col_red = ImageColorAllocate($im, 255,192,192);
// This function has four parameters, the first $im... Do I still need to say it every time? I won’t say it next time!
// The next three parameters are the red (R), green (G), and blue (B) components of the color to be defined, between 0 and 255.
// This again involves physics-optics knowledge. The different light components of the three primary colors of red, green and blue,
// produce ever-changing colors. The color I defined above is 255 red, 192 green, and 192 blue.
// If I'm not mistaken, this is a brighter red. Let's try to draw a line in a moment.
// Why wait a while? Because if a picture has only one color, nothing can be seen!
// Let’s make the background black first!
// Although it is not clearly stated in the manual, I found that the color defined first will be used as the background by default.

$col_black = ImageColorAllocate($im, 0,0,0);
// Defines a color. There is no red light, green light, or blue light. Natural black - black.
// Then define the color for drawing the line:
$col_red = ImageColorAllocate($im, 255,192,192);

// Now you can start drawing the red line:
imageline ($im , 10, 20, 45, 85, $col_red);
// Don’t worry, you won’t be able to see the image after finishing this sentence.

ImagePNG($im);
// This sentence outputs an image. ImagePNG() outputs a png image, ImageJPEG outputs a jpeg image,
// ImageGIF outputs a gif image... …
// Don’t forget there is a parameter here. If it is displayed on the screen instead of saved as a file,
// then omit this parameter - the saved file name. If you want to save it as a file here,
// You should write it like this: ImagePNG($im, "test.png");
// If you don’t specify a path, this file will be saved in your web page. in the directory.
// If it is JPEG, there is one more parameter, which is JPEG quality (0~100).
// If you want to display it on the screen, then ImageJPEG($im,"",80);
// If you want to save it, then ImageJPEG($im,"test.jpg",80);
// Note that if you want to save this image as a file,
// you cannot use Header("Content-type: image/png"); to send the HTTP header that means the image,
// Because once this happens, it means you will output the image.

ImageDestroy($im);
// Destroy the image in memory to free up memory space.
// That’s it: the simplest GD drawing is done.

// Through testing, it was found that the PNG format used to generate this image file only has 131 bytes.
// And the JPEG format, even with the worst quality (0), requires 855 bytes, the image quality is too bad to watch.
// The highest JPEG quality requires 2360 bytes, but the colors are still not as vivid as when using PNG.
// It can be seen that for this kind of image with a small number of colors, using PNG is much more cost-effective than JPEG.
?>

This time I will stop here and I will try to continue writing as soon as possible.

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