Drawing a Speedometer with Core Graphics on OSX in NSView - objective-c

Question

I'm trying draw elements of a Speed Gauge using Core Graphics on OSX. I've almost got it but need a little bit of help on the center ticks inside of the gauge. Here is the image of what I'm trying to do:
Here is an image of what I've got so far:
I know how to draw the circle rings and how to draw segments based around the center of the gauge like this:
- (void)drawOuterGaugeRingsInRect:(CGContextRef)contextRef rect:(NSRect)rect {
CGContextSetLineWidth(contextRef,self.gaugeRingWidth);
CGContextSetStrokeColorWithColor(contextRef, [MyColors SpeedGaugeOuterRingGray].CGColor);
CGFloat startRadians = 0;
CGFloat endRadians = M_PI*2;
CGFloat radius = self.bounds.size.width/2 - 5;
CGContextAddArc(contextRef, CGRectGetMidX(rect),CGRectGetMidY(rect),radius,startRadians,endRadians,YES);
//Render the outer gauge
CGContextStrokePath(contextRef);
//Draw the inner gauge ring.
radius -= self.gaugeRingWidth;
CGContextSetStrokeColorWithColor(contextRef, [MyColors SpeedGaugeInnerRingGray].CGColor);
CGContextAddArc(contextRef, CGRectGetMidX(rect),CGRectGetMidY(rect),radius,startRadians,endRadians,YES);
//Render the inner gauge
CGContextStrokePath(contextRef);
radius -= self.gaugeRingWidth;
//Draw and fill the gauge background
CGContextSetFillColorWithColor(contextRef, [MyColors SpeedGaugeCenterFillBlack ].CGColor);
CGContextSetStrokeColorWithColor(contextRef, [MyColors SpeedGaugeCenterFillBlack].CGColor);
CGContextAddArc(contextRef, CGRectGetMidX(rect),CGRectGetMidY(rect),radius,startRadians,endRadians,YES);
//Render and fill the gauge background
CGContextDrawPath(contextRef, kCGPathFillStroke);
/*BLUE CIRCULAR DIAL */
//Prepare to draw the blue circular dial.
radius -= self.gaugeRingWidth/2;
//Adjust gauge ring width
CGContextSetLineWidth(contextRef,self.gaugeRingWidth/2);
CGContextSetStrokeColorWithColor(contextRef, [MyColors SpeedGaugeBlue].CGColor);
CGFloat startingRadians = [MyMathHelper degressToRadians:135];
CGFloat endingRadians = [MyMathHelper degressToRadians:45];
CGContextAddArc(contextRef, CGRectGetMidX(rect),CGRectGetMidY(rect),radius,startingRadians,endingRadians,NO);
//Render the blue gauge line
CGContextStrokePath(contextRef);
}
The code above is called in the drawRect: method in my NSView
The key section is the code here:
- (void)drawInnerDividerLines:(CGContextRef)context rect:(NSRect)rect {
CGFloat centerX = CGRectGetMidX(rect);
CGFloat centerY = CGRectGetMidY(rect);
CGContextSetLineWidth (context, 3.0);
CGContextSetRGBStrokeColor (context, 37.0/255.0, 204.0/255.0, 227.0/255.0, 0.5);
CGFloat destinationX = centerX + (centerY * (cos((135)*(M_PI/180))));
CGFloat destinationY = centerY + (centerX * (sin((135)*(M_PI/180))));
NSPoint destinationPoint = NSMakePoint(destinationX, destinationY);
CGContextMoveToPoint(context, centerX, centerY);
CGContextAddLineToPoint(context, destinationPoint.x, destinationPoint.y);
CGContextStrokePath(context);
}
I understand what is going on here but the problem I'm trying to solve is drawing the little lines, off of the inner blue line that extend toward the center point of the View, but do not draw all the way to the center. I'm a little unsure on how to modify the math and drawing logic to achieve this. Here is the unit circle I based the angles off of for Core Graphics Drawing.
The main problems I'm trying to solve are:
How to define the proper starting point off of the light blue inner line as a staring point for each gauge tick. Right now, I'm drawing the full line from the center to the edge of the gauge.
How to control the length of the tick gauge as it draws pointed toward the center off of it's origin point on the blue line.
Any tips or advice that would point in me in the right direction to solve this would be appreciated.

Answer 1

I recommend using vectors. You can find a line to any point on the circle given an angle by calculating:
dirX = cos(angle);
dirY = sin(angle);
startPt.x = center.x + innerRadius * dirX;
startPt.y = center.y + innerRadius * dirY;
endPt.x = center.x + outerRadius * dirX;
endPt.y = center.y + outerRadius * dirY;
You can then plot a line between startPt and endPt.

Answer 2

Any tips or advice that would point in me in the right direction to solve this would be appreciated.
Given a point on the circumference of your circle at a certain angle around the centre you can form a right angled triangle, the radius is the hypotenuse, and the other two sides being parallel to the x & y axes (ignore for a moment the degenerate case where the point is at 0, 90, 180 or 270 deg). Given that with the sin & cos formula (remember SOHCAHTOA from school) and some basic math you can calculate the coordinates of the point, and using that draw a radius from the centre to the point.
The end points of a "tick" mark just lie on circles of different radii, so the same math will give you the end points and you can draw the tick. You just need to decide the radii of these circles, i.e. the distance along your original radius the end points of the tick should be.
HTH

Answer 3

Another approach to avoid the trigonometry is to rotate the transformation matrix and just draw a vertical or horizontal line.
// A vertical "line" of width "width" along the y axis at x==0.
NSRect tick = NSMakeRect(-width / 2.0, innerRadius, width, outerRadius - innerRadius);
NSAffineTransform* xform = [NSAffineTransform transform];
// Move the x and y axes to the center of your speedometer so rotation happens around it
[xform translateXBy:center.x yBy:center.y];
// Rotate the coordinate system so that straight up is actually at your speedometer's 0
[xform rotateByDegrees:135];
[xform concat];
// Create a new transform to rotate back around for each tick.
xform = [NSAffineTransform transform];
[xform rotateByDegrees:-270.0 / numTicks];
for (int i = 0; i < numTicks; i++)
{
NSRectFill(tick);
[xform concat];
}
You probably want to wrap this in [NSGraphicsContext saveGraphicsState] and [NSGraphicsContext restoreGraphicsState] so the transformation matrix is restored when you're done.
If you want two different kinds of tick marks (major and minor), then have two different rects and select one based on i % 10 == 0 or whatever. Maybe also toggle the color. Etc.