海神

如果是截取矩形的区域：

CGImage有函数可以截取一张图片的其中一个区域： CGImageCreateWithImageInRect(originImage, rectWanted);

截取圆形或者不规则形状的方法：

Creating the mask above is really simple using CoreGraphics on the iPhone. The following is a function that takes two images and uses one to mask the other.

- (UIImage*) maskImage:(UIImage *)image withMask:(UIImage *)maskImage {
CGImageRef maskRef = maskImage.CGImage;

CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
CGImageGetHeight(maskRef),
CGImageGetBitsPerComponent(maskRef),
CGImageGetBitsPerPixel(maskRef),
CGImageGetBytesPerRow(maskRef),
CGImageGetDataProvider(maskRef), NULL, false);

CGImageRef masked = CGImageCreateWithMask([image CGImage], mask);
return [UIImage imageWithCGImage:masked];
}

NOTE: The mask image cannot have ANY transparency. Instead, transparent areas must be white or some value between black and white. The more towards black a pixel is the less transparent it becomes.

也就是用灰度图做遮罩，黑色的部分完全显示，白色的部分不显示，灰色的部分为半透明。