imagerotate()

(PHP 4 >= 4.3.0, PHP 5, PHP 7)

用给定角度旋转图像

说明

将$src_im图像用给定的$angle角度旋转。$bgd_color指定了旋转后没有覆盖到的部分的颜色。

旋转的中心是图像的中心，旋转后的图像会按比例缩小以适合目标图像的大小——边缘不会被剪去。

参数

由图象创建函数(例如imagecreatetruecolor())返回的图象资源。

Rotation angle, in degrees. The rotation angle is interpreted as the number of degrees to rotate the image anticlockwise.

Specifies the color of the uncovered zone after the rotation

如果被设为非零值，则透明色会被忽略（否则会被保留）。

返回值

返回旋转后的图像资源，或者在失败时返回FALSE。

更新日志

范例

将图像旋转 180 度

本例将把一幅图像旋转 180 度——上下颠倒。

以上例程的输出类似于：

注释

Note:此函数仅在与 GD 库捆绑编译的 PHP 版本中可用。

After some INet searches and personal try-and-failures I succeed to rotate PNG images with preserving alpha channel transparency (semi transparency). 

An issue with imagerotate() is it might need more memory than what is available. It was failing in my script so I tried to increase it with ini_set('memory_limit', '256M') 
and everything works fine now.

I've benchmarked three methods described here for rotating an image.
I used a 1600 x 1200 jpeg picture that was duplicated ten times on the disk. I looped through them to apply a CW rotation using either method.
First algorithm (the "imagesetpixel" algorithm):

Second algorithm (the "imagecopy" algorithm):

Third algorithm (the "imagerotate" algorithm):

In each step of the loop, the image is loaded, rotated then a copy is written on the disk.
Here are the results:
imagesetpixel algorithm: the 10 pictures were processed in 29 seconds
imagecopy algorithm: the 10 pictures were processed in 26 seconds
imagerotate algorithm: the 10 pictures were processed in 2 seconds (no typo here)
Writing the pictures on the disk allowed me to control the results. Every resulting picture was the same (tested with winmerge).
Conclusion: imagerotate is way faster than other "home-made" algorithms.
I ran the tests several times to see if the difference between imagesetpixel and imagecopy was introduced by my computer's independent activity, but I always got that 10% difference.
Note: the picture I used was taken from a camera, so no transparency was involved.
I ran the tests on Windows XP using PHP Version 5.3.0

Keep in mind this function rotates counterclockwise. if you want to go 90 degrees clockwise, you have to tell imagerotate to go 270 degrees counter-clockwise.
$angle = 360-$angle;

I was quite surprised to see that no one had done any working rotate function. And since I needed one for my web hotel, I whipped this one up. Someone might want to expand it so that it matches imagerotate.
Also, does imagerotate really rotate things counter clockwise? Otherwise this should also be changed.

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