Why is UIBezierPath faster than Core Graphics path

问题:

I was playing around with drawing paths, and I noticed that in at least some cases, UIBezierPath outperforms what I thought would be a Core Graphics equivalent. The -drawRect: method below creates two paths: one UIBezierPath, and one CGPath. The paths are identical except for their locations, but stroking the CGPath takes roughly twice as long as stroking the UIBezierPath.

- (void)drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    // Create the two paths, cgpath and uipath.
    CGMutablePathRef cgpath = CGPathCreateMutable();
    CGPathMoveToPoint(cgpath, NULL, 0, 100);

    UIBezierPath *uipath = [[UIBezierPath alloc] init];
    [uipath moveToPoint:CGPointMake(0, 200)];

    // Add 200 curve segments to each path.
    int iterations = 200;
    CGFloat cgBaseline = 100;
    CGFloat uiBaseline = 200;
    CGFloat xincrement = self.bounds.size.width / iterations;
    for (CGFloat x1 = 0, x2 = xincrement;
         x2 < self.bounds.size.width;
         x1 = x2, x2 += xincrement)
    {
        CGPathAddCurveToPoint(cgpath, NULL, x1, cgBaseline-50, x2, cgBaseline+50, x2, cgBaseline);
        [uipath addCurveToPoint:CGPointMake(x2, uiBaseline)
                  controlPoint1:CGPointMake(x1, uiBaseline-50)
                  controlPoint2:CGPointMake(x2, uiBaseline+50)];
    }
    [[UIColor blackColor] setStroke];
    CGContextAddPath(ctx, cgpath);

    // Stroke each path.
    [self strokeContext:ctx];
    [self strokeUIBezierPath:uipath];

    [uipath release];
    CGPathRelease(cgpath);
}

- (void)strokeContext:(CGContextRef)context
{
    CGContextStrokePath(context);
}

- (void)strokeUIBezierPath:(UIBezierPath*)path
{
    [path stroke];
}

Both paths use CGContextStrokePath(), so I created separate methods to stroke each path so that I can see the time used by each path in Instruments. Below are typical results (call tree inverted); you can see that -strokeContext: takes 9.5 sec., while -strokeUIBezierPath: takes only 5 sec.:

Running (Self)      Symbol Name
14638.0ms   88.2%               CGContextStrokePath
9587.0ms   57.8%                 -[QuartzTestView strokeContext:]
5051.0ms   30.4%                 -[UIBezierPath stroke]
5051.0ms   30.4%                  -[QuartzTestView strokeUIBezierPath:]

It looks like UIBezierPath is somehow optimizing the path that it creates, or I'm creating the CGPath in a naïve way. What can I do to speed up my CGPath drawing?

回答1:

You are correct in that UIBezierPath is simply an objective-c wrapper for Core Graphics, and therefore will perform comparably. The difference (and reason for your performance delta) is your CGContext state when drawing your CGPath directly is quite different to that setup by UIBezierPath. If you look at UIBezierPath, it has settings for:

• lineWidth,
• lineJoinStyle,
• lineCapStyle,
• miterLimit and
• flatness

When examining the call (disassembly) to [path stroke], you will note that it configures the current graphic context based on those previous values before performing the CGContextStrokePath call. If you do the same prior to drawing your CGPath, it will perform the same:

- (void)drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    // Create the two paths, cgpath and uipath.
    CGMutablePathRef cgpath = CGPathCreateMutable();
    CGPathMoveToPoint(cgpath, NULL, 0, 100);

    UIBezierPath *uipath = [[UIBezierPath alloc] init];
    [uipath moveToPoint:CGPointMake(0, 200)];

    // Add 200 curve segments to each path.
    int iterations = 80000;
    CGFloat cgBaseline = 100;
    CGFloat uiBaseline = 200;
    CGFloat xincrement = self.bounds.size.width / iterations;
    for (CGFloat x1 = 0, x2 = xincrement;
         x2 < self.bounds.size.width;
         x1 = x2, x2 += xincrement)
    {
        CGPathAddCurveToPoint(cgpath, NULL, x1, cgBaseline-50, x2, cgBaseline+50, x2, cgBaseline);
        [uipath addCurveToPoint:CGPointMake(x2, uiBaseline)
                  controlPoint1:CGPointMake(x1, uiBaseline-50)
                  controlPoint2:CGPointMake(x2, uiBaseline+50)];
    }
    [[UIColor blackColor] setStroke];
    CGContextAddPath(ctx, cgpath);

    // Stroke each path
    CGContextSaveGState(ctx); {
        // configure context the same as uipath
        CGContextSetLineWidth(ctx, uipath.lineWidth);
        CGContextSetLineJoin(ctx, uipath.lineJoinStyle);
        CGContextSetLineCap(ctx, uipath.lineCapStyle);
        CGContextSetMiterLimit(ctx, uipath.miterLimit);
        CGContextSetFlatness(ctx, uipath.flatness);
        [self strokeContext:ctx];
        CGContextRestoreGState(ctx);
    }
    [self strokeUIBezierPath:uipath];

    [uipath release];
    CGPathRelease(cgpath);
}

- (void)strokeContext:(CGContextRef)context
{
    CGContextStrokePath(context);
}

- (void)strokeUIBezierPath:(UIBezierPath*)path
{
    [path stroke];
}

Snapshot from Instruments: