imageantialias()

(PHP 4 >= 4.3.2, PHP 5, PHP 7)

是否使用抗锯齿（antialias）功能

说明

对线段和多边形启用快速画图抗锯齿方法。不支持 alpha 部分。使用直接混色操作。仅用于真彩色图像。

不支持线宽和风格。

使用抗锯齿和透明背景色可能出现未预期的结果。混色方法把背景色当成任何其它颜色使用。缺乏 alpha 部分的支持导致不允许基于 alpha 抗锯齿方法。

参数

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

是否启用抗锯齿。

返回值

成功时返回TRUE，或者在失败时返回FALSE。

范例

A comparison of two lines, one with anti-aliasing switched on

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。

参见

• imagecreatetruecolor()新建一个真彩色图像

If you can't be bothered creating (or searching for) a full screen antialias function.
You can actually cheat (well a bit of a dirty inefficient hack really!!) 
and perform a fake antialias on an image by using 'imagecopyresampled'...
first create your source image twice the size of what you really want.
Then use 'imagecopyresampled' to shrink it to half the size, the function 
automatically interpolates pixels to create an antialias effect!
I've used this in a pie chart function and it works brilliantly,
not as slow as I thought it might be!
the rough code below should give you the idea... 

So far using PHP 5.0.4 I've managed to get Imageantialias() to work well with:
ImageLine()
ImagePolygon()
but not with:
ImageArc()
ImageEllipse()
ImageFilled*()
You can still draw antialiased filled polygons by drawing a hollow polygon on top of a filled one with the same dimensions: 

The following function draws an AntiAliased (unfilled) Ellipse.
It is used just liked the nomral ImageEllipse function.
The optional parameter sets the number of segments...
function ImageEllipseAA( &$img, $x, $y, $w, $h,$color,$segments=70) 
{
  $w=$w/2;
  $h=$h/2;
  $jump=2*M_PI/$segments;
  $oldx=$x+sin(-$jump)*$w;
  $oldy=$y+cos(-$jump)*$h;
  for($i=0;$i

Having tried long and hard to get a decent antialiased circle function, I finally gave in a wrote something from scratch. Its very very naive - basically examining every single pixel in the enclosing square, but as a result produces very nice antialiasing, with decent control over stroke width, antialiasing amount and tightness of antialiasing. I'm sure it could be optimised considerably, but it was fast enough for what I was doing. Hope its of some use. 

I did a search in google and got following url:
http://www.isocalc.com/tutorials/antialias.htm
With this tutorial I was able to write a function to convert this algorithm into php, the result for a filled circel is this:

An improvement would be to draw the inner rectangle or more rectangles in the circle with the builtin rectangle function to reduce the usage of imagesetpixel() from (2*r)^2 to 2*Pi*(r + epsilon), in other words, the dependency on r would break down from square to linear.
Another improvement would be to determine filled and unfilled triangles in the observed pixel and calculate their areas, so we can get rid of the inner loops for getting the fraction filled/unfilled.
One can easily modify this function to solve other problems like lines, unfilled circles, etc.

The only trick I found to draw an antialiased polygon AND keep it transparent (to use them as overlays in google maps for example)... make two images and merge them. Order of operations is important and the transparency color of the final image must be set after the merge: 

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