Drawing MKMapView Overlay like Google Maps Directions

Question

iOS 5 changed the way the built-in Google Maps App draws routes:

I would now like to replicate the design of the route overlay in my own app but I am currently only able to draw a plain blue line. I would like to add the 3D-effect with the gradient, borders and the glow. Any ideas on how to accomplish this?

Currently I'm using the following code:

CGContextSetFillColorWithColor(context, fillColor.CGColor);
CGContextSetLineJoin(context, kCGLineJoinRound);
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineWidth(context, lineWidth);
CGContextAddPath(context, path);
CGContextReplacePathWithStrokedPath(context);
CGContextFillPath(context);

Resulting in a rather ugly line:

Thanks!

Update: The solution should work on iOS 4.0 and up.

Dight

Are you able to post the code you're using the draw the lines, currently? – Imes

@sudo rm -rf: I've added the drawing code. – Dight

Answer 1

I think that @ChrisMiles is correct in that the segments are probably being drawn individually. (I initially thought that this might have been doable using CGPatternRef but you don't have any access to the CTM or path endpoints inside the pattern drawing callback.)

With this in mind, here is an exceedingly crude, back-of-the-envelope example of how you might begin such an effort (filling the segments individually). Note that:

• gradient colors are guessed
• end caps are nonexistent and will need to be separately implemented
• some aliasing artifacts remain
• not a great deal of attention was paid to performance

Hopefully this can get you started at least (and works through some of the analytic geometry).

-  (CGGradientRef)lineGradient
{
    static CGGradientRef gradient = NULL;
    if (gradient == NULL) {
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        CGColorRef white = [[UIColor colorWithWhite:1.f
                                              alpha:0.7f] CGColor];
        CGColorRef blue = [[UIColor colorWithRed:0.1f
                                           green:0.2f
                                            blue:1.f
                                           alpha:0.7f] CGColor];
        CGColorRef lightBlue = [[UIColor colorWithRed:0.4f
                                                green:0.6f
                                                 blue:1.f
                                                alpha:0.7f] CGColor];
        CFMutableArrayRef colors = CFArrayCreateMutable(kCFAllocatorDefault, 
                                                        8,
                                                        NULL);
        CFArrayAppendValue(colors, blue);
        CFArrayAppendValue(colors, blue);
        CFArrayAppendValue(colors, white);
        CFArrayAppendValue(colors, white);
        CFArrayAppendValue(colors, lightBlue);
        CFArrayAppendValue(colors, lightBlue);
        CFArrayAppendValue(colors, blue);
        CFArrayAppendValue(colors, blue);
        CGFloat locations[8] = {0.f, 0.08f, 0.14f, 0.21f, 0.29f, 0.86f, 0.93f, 1.f};
        gradient = CGGradientCreateWithColors(colorSpace,
                                              colors,
                                              locations);
        CFRelease(colors);
        CGColorSpaceRelease(colorSpace);
    }
    return gradient;
}

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

    CGContextSetAllowsAntialiasing(context, YES);
    CGContextSetShouldAntialias(context, YES);

    // Fill background color
    [[UIColor whiteColor] setFill];
    UIRectFill(rect);

    // Build a path
    CGFloat strokeWidth = 10.f;
    CGContextSetLineWidth(context, strokeWidth);

    CGGradientRef gradient = [self lineGradient];

    CGPoint points[9] = {
        CGPointMake(10.f, 25.f),
        CGPointMake(100.f, 100.f),
        CGPointMake(100.f, 150.f),
        CGPointMake(22.f, 300.f),
        CGPointMake(230.f, 400.f),
        CGPointMake(230.f, 200.f),
        CGPointMake(300.f, 200.f),
        CGPointMake(310.f, 160.f),
        CGPointMake(280.f, 100.f)
    };


    for (NSUInteger i = 1; i < 9; i++) {
        CGPoint start = points[i - 1];
        CGPoint end = points[i];
        CGFloat dy = end.y - start.y;
        CGFloat dx = end.x - start.x;
        CGFloat xOffset, yOffset;
        // Remember that, unlike Cartesian geometry, origin is in *upper* left!
        if (dx == 0) {
            // Vertical to start, gradient is horizontal
            xOffset = 0.5 * strokeWidth;
            yOffset = 0.f;
            if (dy < 0) {
                xOffset *= -1;
            }
        }
        else if (dy == 0) {
            // Horizontal to start, gradient is vertical
            xOffset = 0.f;
            yOffset = 0.5 * strokeWidth;
        }
        else {
            // Sloped
            CGFloat gradientSlope = - dx / dy;
            xOffset = 0.5 * strokeWidth / sqrt(1 + gradientSlope * gradientSlope);
            yOffset = 0.5 * strokeWidth / sqrt(1 + 1 / (gradientSlope * gradientSlope));
            if (dx < 0 && dy > 0) {
                yOffset *= -1;
            }
            else if (dx > 0 && dy < 0) {
                xOffset *= -1;
            }
            else if (dx < 0 && dy < 0) {
                yOffset *= -1;
                xOffset *= -1;
            }
            else {
            }
        }
        CGAffineTransform startTransform = CGAffineTransformMakeTranslation(-xOffset, yOffset);
        CGAffineTransform endTransform = CGAffineTransformMakeTranslation(xOffset, -yOffset);
        CGPoint gradientStart = CGPointApplyAffineTransform(start, startTransform);
        CGPoint gradientEnd = CGPointApplyAffineTransform(start, endTransform);

        CGContextSaveGState(context);
        CGContextMoveToPoint(context, start.x, start.y);
        CGContextAddLineToPoint(context, end.x, end.y);
        CGContextReplacePathWithStrokedPath(context);
        CGContextClip(context);
        CGContextDrawLinearGradient(context, 
                                    gradient, 
                                    gradientStart, 
                                    gradientEnd, 
                                    kCGGradientDrawsAfterEndLocation | kCGGradientDrawsBeforeStartLocation);
        CGContextRestoreGState(context);
    }

    CGContextRestoreGState(context);
}

Misguided

Answer 2

I would say they are drawing a CGPath around the original line, stroking the edges and gradient filling it. The ends are capped by adding a semi circle to the CGPath.

Would be a bit more work than simply drawing a single line and stroking it, but gives them full control over the style of the rendered path.

Carob

That sounds about right Chris, thanks. My problem is how to draw a gradient along a CGPath. Any ideas or code snippets you could share? – Dight

I don't know what the results will look like, but you can use CGContextReplacePathWithStrokedPath() to retrieve an outline you can then fill with a gradient. This previous question might be useful: #2738473 – Tarim

After a bit of experimentation I can't find a way to easily get a gradient fill to follow a path, in the way that they are doing it. They might be breaking down the path and drawing a gradient fill for each segment separately. – Carob