iOS 绘制1像素的线

One point does not necessarily correspond to one physical pixel.

例如画1个像素的分割线

1.0f / [UIScreen mainScreen].scale

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.

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。

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.

在非高清屏上，一个Point对应一个像素。为了防止“antialiasing”导致的奇数像素的线渲染时出现失真，你需要设置偏移0.5 Point。
在高清屏幕上，要绘制一个像素的线，需要设置线宽为0.5个Point，同事设置偏移为0.25 Point。
如果线宽为偶数Point的话，则不要去设置偏移，否则线条也会失真。

(1.0f / [UIScreen mainScreen].scale) / 2;

#define SINGLE_LINE_WIDTH           (1 / [UIScreen mainScreen].scale)
#define SINGLE_LINE_ADJUST_OFFSET   ((1 / [UIScreen mainScreen].scale) / 2)

CGFloat xPos = 5;
UIView *view = [[UIView alloc] initWithFrame:CGrect(x - SINGLE_LINE_ADJUST_OFFSET, 0, SINGLE_LINE_WIDTH, 100)];

//
//  SvGridView.h
//  SvSinglePixel
//
//  Created by xiaoyong.cxy on 6/23/15.
//  Copyright (c) 2015 smileEvday. All rights reserved.
//
 
#import <UIKit/UIKit.h>
 
@interface SvGridView : UIView
 
/**
 * @brief 网格间距，默认30
 */
@property (nonatomic, assign) CGFloat   gridSpacing;
 
/**
 * @brief 网格线宽度，默认为1 pixel (1.0f / [UIScreen mainScreen].scale)
 */
@property (nonatomic, assign) CGFloat   gridLineWidth;
 
/**
 * @brief 网格颜色，默认蓝色
 */
@property (nonatomic, strong) UIColor   *gridColor;
 
@end

//
//  SvGridView.m
//  SvSinglePixel
//
//  Created by xiaoyong.cxy on 6/23/15.
//  Copyright (c) 2015 smileEvday. All rights reserved.
//
 
#import "SvGridView.h"
 
#define SINGLE_LINE_WIDTH           (1 / [UIScreen mainScreen].scale)
#define SINGLE_LINE_ADJUST_OFFSET   ((1 / [UIScreen mainScreen].scale) / 2)
 
@implementation SvGridView
 
@synthesize gridColor = _gridColor;
@synthesize gridSpacing = _gridSpacing;
 
- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        self.backgroundColor = [UIColor clearColor];
 
        _gridColor = [UIColor blueColor];
        _gridLineWidth = SINGLE_LINE_WIDTH;
        _gridSpacing = 30;
    }
 
    return self;
}
 
- (void)setGridColor:(UIColor *)gridColor
{
    _gridColor = gridColor;
 
    [self setNeedsDisplay];
}
 
- (void)setGridSpacing:(CGFloat)gridSpacing
{
    _gridSpacing = gridSpacing;
 
    [self setNeedsDisplay];
}
 
- (void)setGridLineWidth:(CGFloat)gridLineWidth
{
    _gridLineWidth = gridLineWidth;
 
    [self setNeedsDisplay];
}
 
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
 
    CGContextBeginPath(context);
    CGFloat lineMargin = self.gridSpacing;
 
    /**
     *  https://developer.apple.com/library/ios/documentation/2DDrawing/Conceptual/DrawingPrintingiOS/GraphicsDrawingOverview/GraphicsDrawingOverview.html
     * 仅当要绘制的线宽为奇数像素时，绘制位置需要调整
     */
    CGFloat pixelAdjustOffset = 0;
    if (((int)(self.gridLineWidth * [UIScreen mainScreen].scale) + 1) % 2 == 0) {
        pixelAdjustOffset = SINGLE_LINE_ADJUST_OFFSET;
    }
 
    CGFloat xPos = lineMargin - pixelAdjustOffset;
    CGFloat yPos = lineMargin - pixelAdjustOffset;
    while (xPos < self.bounds.size.width) {
        CGContextMoveToPoint(context, xPos, 0);
        CGContextAddLineToPoint(context, xPos, self.bounds.size.height);
        xPos += lineMargin;
    }
 
    while (yPos < self.bounds.size.height) {
        CGContextMoveToPoint(context, 0, yPos);
        CGContextAddLineToPoint(context, self.bounds.size.width, yPos);
        yPos += lineMargin;
    }
 
    CGContextSetLineWidth(context, self.gridLineWidth);
    CGContextSetStrokeColorWithColor(context, self.gridColor.CGColor);
    CGContextStrokePath(context);
}
 
@end

SvGridView *gridView = [[SvGridView alloc] initWithFrame:self.view.bounds];
gridView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
gridView.alpha = 0.6;
gridView.gridColor = [UIColor greenColor];
[self.view addSubview:gridView];