[CALayer renderInContext:] 之后 CGBitmapContextCreateImage() ，图片质量变差，方法不对还是什么原因？

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
UIGraphicsBeginImageContext(layer.bounds.size);
CGContextRef _ctx = UIGraphicsGetCurrentContext();
[self.view.layer renderInContext:_ctx];
CGImageRef image = CGBitmapContextCreateImage(_ctx);
UIGraphicsEndImageContext();

CGImageRef flipImage = CGImageCreateWithImageInRect(image, layer.bounds);

CGContextSaveGState(ctx);
CGContextTranslateCTM(ctx, 0, layer.bounds.size.height);
CGContextScaleCTM(ctx, 1.0f, -1.0f);
CGContextDrawImage(ctx, layer.bounds, flipImage);
CGContextRestoreGState(ctx);
LogMessage(@"drawLayer", 1, @"draw layer content");
}


上面这番处理之后，图像中的一些线条边缘出现锯齿。