iOS实现从背景图中取色的代码

void *bitmapData; //内存空间的指针，该内存空间的大小等于图像使用RGB通道所占用的字节数。
 
static CGContextRef CreateRGBABitmapContext (CGImageRef inImage)
{
  CGContextRef context = NULL;
  CGColorSpaceRef colorSpace;
  int bitmapByteCount;
  int bitmapBytesPerRow;
 
  size_t pixelsWide = CGImageGetWidth(inImage); //获取横向的像素点的个数
  size_t pixelsHigh = CGImageGetHeight(inImage);
 
  bitmapBytesPerRow  = (pixelsWide * 4); //每一行的像素点占用的字节数，每个像素点的ARGB四个通道各占8个bit(0-255)的空间
  bitmapByteCount = (bitmapBytesPerRow * pixelsHigh); //计算整张图占用的字节数
 
  colorSpace = CGColorSpaceCreateDeviceRGB();//创建依赖于设备的RGB通道
  //分配足够容纳图片字节数的内存空间
  bitmapData = malloc( bitmapByteCount );
  //创建CoreGraphic的图形上下文，该上下文描述了bitmaData指向的内存空间需要绘制的图像的一些绘制参数
  context = CGBitmapContextCreate (bitmapData,
                   pixelsWide,
                   pixelsHigh,
                   8,
                   bitmapBytesPerRow,
                   colorSpace,
                   kCGImageAlphaPremultipliedLast);
  //Core Foundation中通过含有Create、Alloc的方法名字创建的指针，需要使用CFRelease()函数释放
  CGColorSpaceRelease( colorSpace );
  return context;
}
 
// 返回一个指针，该指针指向一个数组，数组中的每四个元素都是图像上的一个像素点的RGBA的数值(0-255)，用无符号的char是因为它正好的取值范围就是0-255
static unsigned char *RequestImagePixelData(UIImage *inImage)
{
  CGImageRef img = [inImage CGImage];
  CGSize size = [inImage size];
  //使用上面的函数创建上下文
  CGContextRef cgctx = CreateRGBABitmapContext(img);
  CGRect rect = {{0,0},{size.width, size.height}};
  //将目标图像绘制到指定的上下文，实际为上下文内的bitmapData。
  CGContextDrawImage(cgctx, rect, img);
  unsigned char *data = CGBitmapContextGetData (cgctx);
  //释放上面的函数创建的上下文
  CGContextRelease(cgctx);
  return data;
}
 
//设置背景原图片,即取色所用的图片
- (void)setSourceImage:(NSString *)sourceImage ImageWidth:(int)_width ImageHeight:(int)_height {
  //生成指定大小的背景图
  UIImage *im = [UIImage imageNamed:sourceImage];
  UIImage *newImage;
  UIImageView *view = [[UIImageView alloc] initWithImage:im];
  view.frame = CGRectMake(0, 0, _width, _height);
  UIGraphicsBeginImageContext(CGSizeMake(_width, _height)); //size 为CGSize类型，即你所需要的图片尺寸
  [im drawInRect:CGRectMake(0, 0, _width, _height)]; //newImageRect指定了图片绘制区域
  newImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
 
  width = newImage.size.width;
  height = newImage.size.height;
  //将解析背景图为像素，供取色用
  imgPixel = RequestImagePixelData(newImage);
}
 
//计算颜色
-(UIColor*)calColor:(CGPoint)aPoint {
  int i = 4 * width * round(aPoint.y+imageView.frame.size.height/2) + 4 * round(aPoint.x+imageView.frame.size.width/2);
  int _r = (unsigned char)imgPixel[i];
  int _g = (unsigned char)imgPixel[i+1];
  int _b = (unsigned char)imgPixel[i+2];
  NSLog(@"(%f,%f)",aPoint.x,aPoint.y);
  NSLog(@"Red : %f  Green: %f  Blue: %f",_r/255.0,_g/255.0,_b/255.0);
  return [UIColor colorWithRed:_r/255.0f green:_g/255.0f blue:_b/255.0f alpha:1.0];
}  
 
- (void)changColor:(UIColor *)color{
  int width_;
  if (![Util isIpad]) {
    width_ = 30;
  } else {
    width_ = 70;
  }
 
  UIGraphicsBeginImageContext(CGSizeMake(width_, width_));
  CGContextRef ctx = UIGraphicsGetCurrentContext();
  CGContextMoveToPoint(ctx, 20, 20);
  CGContextSetFillColorWithColor(ctx, color.CGColor);
  if (![Util isIpad]) {
    CGContextAddArc(ctx, width_/2, width_/2, 14.5, 0, 6.3, 0);
  } else {
    CGContextAddArc(ctx, width_/2+0.5, width_/2, 31.3, 0, 6.3, 0);
  }
  CGContextFillPath(ctx);
  self->pickedColorImageView.image = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
}