iOS 获取图像指定点的颜色 (已修复bug)

百度上iOS获取图像指定点的颜色 的搜索了一份源代码，实际用起来发现有bug。于是乎自己稍微修改了一下。

1. 之前bug：读取的颜色比图片实际低了一个像素点
   修复思路：将 pointY手动+1处理

NSInteger pointY = trunc(point.y + 1); //原错误：NSInteger pointY = trunc(point.y); 

2. 之前bug：2x和3x图片读取错误
   修复思路：将width和height提前乘以比例scale以获取真实像素值

NSUInteger width = self.size.width * self.scale; //原错误：NSUInteger width = self.size.width;
NSUInteger height = self.size.height * self.scale; //原错误：NSUInteger height = self.size. height;

3. 之前bug：带alpha通道的 png图片的颜色会变黑
   修复思路：返回的颜色数值应该是被乘以alpha之后的值，所以再除回去

CGFloat red   = (CGFloat)pixelData[0] / 255.0f / alpha; //原错误：CGFloat red   = (CGFloat)pixelData[0] / 255.0f;
CGFloat green = (CGFloat)pixelData[1] / 255.0f / alpha; //原错误：CGFloat green = (CGFloat)pixelData[1] / 255.0f;
CGFloat blue  = (CGFloat)pixelData[2] / 255.0f / alpha; //原错误：CGFloat blue  = (CGFloat)pixelData[2] / 255.0f;

终于完工，看一下测试效果

#import "UIImage+ColorAtPixel.h"

@implementation UIImage (ColorAtPixel)

//获取图片某一点的颜色
- (UIColor *)colorAtPixel:(CGPoint)point {
    NSUInteger width = self.size.width * self.scale;
    NSUInteger height = self.size.height * self.scale;

    if ( !CGRectContainsPoint( CGRectMake(0.0f, 0.0f, width, height ), point ) ) {
        return nil;
    }
    
    NSInteger pointX = trunc(point.x);
    NSInteger pointY = trunc(point.y + 1);
    CGImageRef cgImage = self.CGImage;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    int bytesPerPixel = 4;
    int bytesPerRow = bytesPerPixel * 1;
    NSUInteger bitsPerComponent = 8;
    unsigned char pixelData[4] = { 0, 0, 0, 0 };
    CGContextRef context = CGBitmapContextCreate(pixelData,
                                                 1,
                                                 1,
                                                 bitsPerComponent,
                                                 bytesPerRow,
                                                 colorSpace,
                                                 kCGImageAlphaPremultipliedLast |     kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);
    CGContextSetBlendMode(context, kCGBlendModeCopy);
    
    CGContextTranslateCTM(context, -pointX, pointY - (CGFloat)height);
    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
    CGContextRelease(context);
    
    CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
    if (alpha == 0) {
        return [UIColor clearColor];
    }
    CGFloat red   = (CGFloat)pixelData[0] / 255.0f / alpha;
    CGFloat green = (CGFloat)pixelData[1] / 255.0f / alpha;
    CGFloat blue  = (CGFloat)pixelData[2] / 255.0f / alpha;
    
    UIColor *resultColor =  [UIColor colorWithRed: red green: green blue: blue alpha: alpha];
    
    return resultColor;
}

@end