Detect tap on curved line? - objective-c

Question

I'm trying to draw a line with a specific width. I searched for examples online, but I only found examples using straight lines. I need curved lines. Also, I need to detect if the user touched within the line. Is it possible to achieve this using Objective C and Sprite Kit? If so can someone provide an example?

Answer 1

You can use UIBezierPath to create bezier curves (nice smooth curves). You can specify this path for a CAShapeLayer and add that as a sublayer to your view:
UIBezierPath *path = [UIBezierPath bezierPath];
[path moveToPoint:CGPointMake(10, 150)];
[path addCurveToPoint:CGPointMake(110, 150) controlPoint1:CGPointMake(40, 100) controlPoint2:CGPointMake(80, 100)];
[path addCurveToPoint:CGPointMake(210, 150) controlPoint1:CGPointMake(140, 200) controlPoint2:CGPointMake(170, 200)];
[path addCurveToPoint:CGPointMake(310, 150) controlPoint1:CGPointMake(250, 100) controlPoint2:CGPointMake(280, 100)];
CAShapeLayer *layer = [CAShapeLayer layer];
layer.lineWidth = 10;
layer.strokeColor = [UIColor redColor].CGColor;
layer.fillColor = [UIColor clearColor].CGColor;
layer.path = path.CGPath;
[self.view.layer addSublayer:layer];
If you want to randomize it a little, you can just randomize some of the curves. If you want some fuzziness, add some shadow. If you want the ends to be round, specify a rounded line cap:
UIBezierPath *path = [UIBezierPath bezierPath];
CGPoint point = CGPointMake(10, 100);
[path moveToPoint:point];
CGPoint controlPoint1;
CGPoint controlPoint2 = CGPointMake(point.x - 5.0 - arc4random_uniform(50), 150.0);
for (NSInteger i = 0; i < 5; i++) {
controlPoint1 = CGPointMake(point.x + (point.x - controlPoint2.x), 50.0);
point.x += 40.0 + arc4random_uniform(20);
controlPoint2 = CGPointMake(point.x - 5.0 - arc4random_uniform(50), 150.0);
[path addCurveToPoint:point controlPoint1:controlPoint1 controlPoint2:controlPoint2];
}
CAShapeLayer *layer = [CAShapeLayer layer];
layer.lineWidth = 5;
layer.strokeColor = [UIColor redColor].CGColor;
layer.fillColor = [UIColor clearColor].CGColor;
layer.path = path.CGPath;
layer.shadowColor = [UIColor redColor].CGColor;
layer.shadowRadius = 2.0;
layer.shadowOpacity = 1.0;
layer.shadowOffset = CGSizeZero;
layer.lineCap = kCALineCapRound;
[self.view.layer addSublayer:layer];
If you want it to be even more irregular, break those beziers into smaller segments, but the idea would be the same. The only trick with conjoined bezier curves is that you want to make sure that the second control point of one curve is in line with the first control point of the next one, or else you end up with sharp discontinuities in the curves.
If you want to detect if and when a user taps on it, that's more complicated. But what you have to do is:
Make a snapshot of the image:
- (UIImage *)captureView:(UIView *)view
{
UIGraphicsBeginImageContextWithOptions(view.bounds.size, NO, 1.0); // usually I'd use 0.0, but we'll use 1.0 here so that the tap point of the gesture matches the pixel of the snapshot
if ([view respondsToSelector:#selector(drawViewHierarchyInRect:afterScreenUpdates:)]) {
BOOL success = [view drawViewHierarchyInRect:view.bounds afterScreenUpdates:YES];
NSAssert(success, #"drawViewHierarchyInRect failed");
} else {
[view.layer renderInContext:UIGraphicsGetCurrentContext()];
}
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}
Get the color of the pixel at the coordinate that the user tapped by identifying the color of the pixel the user tapped.
- (void)handleTap:(UITapGestureRecognizer *)gesture
{
CGPoint point = [gesture locationInView:gesture.view];
CGFloat red, green, blue, alpha;
UIColor *color = [self image:self.image colorAtPoint:point];
[color getRed:&red green:&green blue:&blue alpha:&alpha];
if (green < 0.9 && blue < 0.9 && red > 0.9)
NSLog(#"tapped on curve");
else
NSLog(#"didn't tap on curve");
}
Where I adapted Apple's code for getting the pixel buffer in order to determine the color of the pixel the user tapped on was.
// adapted from https://developer.apple.com/library/mac/qa/qa1509/_index.html
- (UIColor *)image:(UIImage *)image colorAtPoint:(CGPoint)point
{
UIColor *color;
CGImageRef imageRef = image.CGImage;
// Create the bitmap context
CGContextRef context = [self createARGBBitmapContextForImage:imageRef];
NSAssert(context, #"error creating context");
// Get image width, height. We'll use the entire image.
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
CGRect rect = {{0,0},{width,height}};
// Draw the image to the bitmap context. Once we draw, the memory
// allocated for the context for rendering will then contain the
// raw image data in the specified color space.
CGContextDrawImage(context, rect, imageRef);
// Now we can get a pointer to the image data associated with the bitmap
// context.
uint8_t *data = CGBitmapContextGetData (context);
if (data != NULL) {
size_t offset = (NSInteger) point.y * 4 * width + (NSInteger) point.x * 4;
uint8_t alpha = data[offset];
uint8_t red = data[offset+1];
uint8_t green = data[offset+2];
uint8_t blue = data[offset+3];
color = [UIColor colorWithRed:red / 255.0 green:green / 255.0 blue:blue / 255.0 alpha:alpha / 255.0];
}
// When finished, release the context
CGContextRelease(context);
// Free image data memory for the context
if (data) {
free(data); // we used malloc in createARGBBitmapContextForImage, so free it
}
return color;
}
- (CGContextRef) createARGBBitmapContextForImage:(CGImageRef) inImage
{
CGContextRef context = NULL;
CGColorSpaceRef colorSpace;
void * bitmapData;
size_t bitmapByteCount;
size_t bitmapBytesPerRow;
// Get image width, height. We'll use the entire image.
size_t pixelsWide = CGImageGetWidth(inImage);
size_t pixelsHigh = CGImageGetHeight(inImage);
// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow = (pixelsWide * 4);
bitmapByteCount = (bitmapBytesPerRow * pixelsHigh);
// Use the generic RGB color space.
colorSpace = CGColorSpaceCreateDeviceRGB(); // CGColorSpaceCreateDeviceWithName(kCGColorSpaceGenericRGB);
NSAssert(colorSpace, #"Error allocating color space");
// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
bitmapData = malloc(bitmapByteCount);
NSAssert(bitmapData, #"Unable to allocate bitmap buffer");
// Create the bitmap context. We want pre-multiplied ARGB, 8-bits
// per component. Regardless of what the source image format is
// (CMYK, Grayscale, and so on) it will be converted over to the format
// specified here by CGBitmapContextCreate.
context = CGBitmapContextCreate (bitmapData,
pixelsWide,
pixelsHigh,
8, // bits per component
bitmapBytesPerRow,
colorSpace,
(CGBitmapInfo)kCGImageAlphaPremultipliedFirst);
NSAssert(context, #"Context not created!");
// Make sure and release colorspace before returning
CGColorSpaceRelease( colorSpace );
return context;
}