Ipad 更改opcity时图形不正确

我正在使用此代码进行绘图，但绘图路径中出现了一些圆。 我试过不同的颜色 任何人都可以帮助plz

  UITouch *touch = [touches anyObject];
            previousPoint2 = previousPoint1;
            previousPoint1 = [touch previousLocationInView:m_img];
            currentPoint = [touch locationInView:m_img];
                     // calculate mid point
            CGPoint mid1 = [self midPoint:previousPoint1 :previousPoint2];
            CGPoint mid2 =[self midPoint:currentPoint :previousPoint1];
            UIGraphicsBeginImageContext(m_img.frame.size);
            [m_img.image drawInRect:CGRectMake(0, 0, m_img.frame.size.width, m_img.frame.size.height)];
            CGContextRef context = UIGraphicsGetCurrentContext();
            CGContextMoveToPoint(context, mid1.x, mid1.y);
            // Use QuadCurve is the key
            CGContextAddQuadCurveToPoint(context, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y); 
            NSLog(@"%@",color);
            CGBitmapContextGetColorSpace(context);   



            //
            CGContextSetStrokeColorWithColor(context, color.CGColor);
            CGContextSetLineCap(context, kCGLineCapRound);
            CGContextSetLineWidth(context,m_width);
            CGContextStrokePath(context);
            m_img.image = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();

只有这个不见了

CGContextSetBlendMode(context, kCGBlendModeCopy);