iOS 绘制1像素的线

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一、Point Vs Pixel

iOS中当我们使用Quartz，UIKit，CoreAnimation等框架时，所有的坐标系统采用Point来衡量。系统在实际渲染到设置时会帮助我们处理Point到Pixel的转换。

这样做的好处隔离变化，即我们在布局的事后不需要关注当前设备是否为Retina，直接按照一套坐标系统来布局即可。

实际使用中我们需要牢记下面这一点:

1	One point does not necessarily correspond to one physical pixel.

1 Point的线在非Retina屏幕则是一个像素，在Retina屏幕上则可能是2个或者3个，取决于系统设备的DPI。

iOS系统中，UIScreen，UIView，UIImage，CALayer类都提供相关属性来获取scale factor。
原生的绘制技术天然的帮我们处理了scale factor，例如在drawRect:方法中，UIKit自动的根据当前运行的设备设置了正切的scale factor。所以我们在drawRect: 方法中绘制的任何内容都会被自动缩放到设备的物理屏幕上。

基于以上信息可以看出，我们大部分情况下都不需要去关注pixel，然而存在部分情况需要考虑像素的转化。

1	例如画1个像素的分割线

看到这个问题你的第一想法可能是，直接根据当前屏幕的缩放因子计算出1 像素线对应的Point，然后设置线宽即可。
代码如下:

1	1.0f / [UIScreen mainScreen].scale

表面上看着一切正常了，但是通过实际的设备测试你会发现渲染出来的线宽并不是1个像素。

Why?

为了获得良好的视觉效果，绘图系统通常都会采用一个叫“antialiasing(反锯齿)”的技术，iOS也不例外。
显示屏幕有很多小的显示单元组成，可以接单的理解为一个单元就代表一个像素。如果要画一条黑线，条线刚好落在了一列或者一行显示显示单元之内，将会渲染出标准的一个像素的黑线。
但如果线落在了两个行或列的中间时，那么会得到一条“失真”的线，其实是两个像素宽的灰线。

如下图所示:

Positions defined by whole-numbered points fall at the midpoint between pixels. For example, if you draw a one-pixel-wide vertical line from (1.0, 1.0) to (1.0, 10.0), you get a fuzzy grey line. If you draw a two-pixel-wide line, you get a solid black line because it fully covers two pixels (one on either side of the specified point). As a rule, lines that are an odd number of physical pixels wide appear softer than lines with widths measured in even numbers of physical pixels unless you adjust their position to make them cover pixels fully.

官方解释如上，简单翻译一下:

1	规定：奇数像素宽度的线在渲染的时候将会表现为柔和的宽度扩展到向上的整数宽度的线，除非你手动的调整线的位置，使线刚好落在一行或列的显示单元内。

如何对齐呢？

On a low-resolution display (with a scale factor of 1.0), a one-point-wide line is one pixel wide. To avoid antialiasing when you draw a one-point-wide horizontal or vertical line, if the line is an odd number of pixels in width, you must offset the position by 0.5 points to either side of a whole-numbered position. If the line is an even number of points in width, to avoid a fuzzy line, you must not do so.

On a high-resolution display (with a scale factor of 2.0), a line that is one point wide is not antialiased at all because it occupies two full pixels (from -0.5 to +0.5). To draw a line that covers only a single physical pixel, you would need to make it 0.5 points in thickness and offset its position by 0.25 points. A comparison between the two types of screens is shown in Figure 1-4.

翻译一下