Draw error on retina screen - ios

Question

I am working on "drawing" project.My project works well on screen. But when i test it on retina screen so i got an image like this retina screen!
It works on ipad2 very well.
some part of my code:
CGPoint mid1 = midPoint(previousPoint1, previousPoint2);
CGPoint mid2 = midPoint(currentPoint, previousPoint1);
[curImage drawAtPoint:CGPointMake(0, 0)];
CGContextRef context = UIGraphicsGetCurrentContext();
[self.layer renderInContext:context];
CGContextMoveToPoint(context, mid1.x, mid1.y);
CGContextAddQuadCurveToPoint(context, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y);
if(previousPoint1.x == previousPoint2.x && previousPoint1.y == previousPoint2.y && previousPoint1.x == currentPoint.x && previousPoint1.y == currentPoint.y)
{
CGContextSetLineCap(context, kCGLineCapRound);
}
else
{
CGContextSetLineCap(context, kCGLineCapRound);
}
CGContextSetBlendMode(context, kCGBlendModeNormal);
CGContextSetLineJoin(context, kCGLineJoinRound);
CGContextSetLineWidth(context, self.lineWidth);
CGContextSetStrokeColorWithColor(context, self.lineColor.CGColor);
CGContextSetShouldAntialias(context, YES);
CGContextSetAllowsAntialiasing(context, YES);
CGContextSetFlatness(context, 0.1f);
CGContextSetAlpha(context, self.lineAlpha);
CGContextStrokePath(context);
- (void) calculateMinImageArea:(CGPoint)pp1 :(CGPoint)pp2 :(CGPoint)cp{
CGPoint mid1 = midPoint(pp1, pp2);
CGPoint mid2 = midPoint(cp, pp1);
CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint(path, NULL, mid1.x, mid1.y);
CGPathAddQuadCurveToPoint(path, NULL, pp1.x, pp1.y, mid2.x, mid2.y);
CGRect bounds = CGPathGetBoundingBox(path);
CGPathRelease(path);
CGRect drawBox = bounds;
//Pad our values so the bounding box respects our line width
drawBox.origin.x -= self.lineWidth * 1;
drawBox.origin.y -= self.lineWidth * 1;
drawBox.size.width += self.lineWidth * 2;
drawBox.size.height += self.lineWidth * 2;
UIGraphicsBeginImageContext(drawBox.size);
[self.layer renderInContext:UIGraphicsGetCurrentContext()];
curImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[self setNeedsDisplayInRect:drawBox];
[[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate: [NSDate date]];
CGPoint midPoint(CGPoint p1, CGPoint p2)
{
return CGPointMake((p1.x + p2.x) * 0.5, (p1.y + p2.y) * 0.5);}