iOS 获取图片某一点的颜色对象（UIColor*）。

- (UIColor *)colorAtPixel:(CGPoint)point {

    // Cancel if point is outside image coordinates

    if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, self.size.width, self.size.height), point)) {

        return nil;

    }

    NSInteger pointX = trunc(point.x);

    NSInteger pointY = trunc(point.y);

    CGImageRef cgImage = self.CGImage;

    NSUInteger width = self.size.width;

    NSUInteger height = self.size.height;

    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);

    // Draw the pixel we are interested in onto the bitmap context

    CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);

    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);

    CGContextRelease(context);

    // Convert color values [0..255] to floats [0.0..1.0]

    CGFloat red   = (CGFloat)pixelData[0] / 255.0f;

    CGFloat green = (CGFloat)pixelData[1] / 255.0f;

    CGFloat blue  = (CGFloat)pixelData[2] / 255.0f;

    CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;

    return [UIColor colorWithRed:red green:green blue:blue alpha:alpha];

}

UIImage+ColorAtPixel.h
 
#import <UIKit/UIKit.h>

/*
 A category on UIImage that enables you to query the color value of arbitrary 
 pixels of the image.
 */
@interface UIImage (ColorAtPixel)

- (UIColor *)colorAtPixel:(CGPoint)point;

@end


#import <CoreGraphics/CoreGraphics.h>

#import "UIImage+ColorAtPixel.h"


@implementation UIImage (ColorAtPixel)

- (UIColor *)colorAtPixel:(CGPoint)point {
    // Cancel if point is outside image coordinates
    if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, self.size.width, self.size.height), point)) {
        return nil;
    }

    NSInteger pointX = trunc(point.x);
    NSInteger pointY = trunc(point.y);
    CGImageRef cgImage = self.CGImage;
    NSUInteger width = self.size.width;
    NSUInteger height = self.size.height;
    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);

    // Draw the pixel we are interested in onto the bitmap context
    CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);
    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
    CGContextRelease(context);
    
    // Convert color values [0..255] to floats [0.0..1.0]
    CGFloat red   = (CGFloat)pixelData[0] / 255.0f;
    CGFloat green = (CGFloat)pixelData[1] / 255.0f;
    CGFloat blue  = (CGFloat)pixelData[2] / 255.0f;
    CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
    return [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
}

@end