draw embossed arc using core graphics

Question

I am trying to implement a custom slider as shown in figure below.

what I

Answer 1

Unless you are going to be changing the shape dynamically, you would probably be better off just creating the image in an image editor. I know it's easy to create that effect in Photoshop, Illustrator, or Fireworks.

That said, drawing an inner shadow like that with Core Graphics requires several steps:

1. Clip to the shape (using e.g. CGContextClip or CGContextClipToMask).
2. Make a path or mask of everything but the shape.
3. Set your shadow parameters (using CGContextSetShadowWithColor).
4. Fill the path or mask from step 2. This casts a shadow inside the shape, and only the shadow is drawn because you clipped to the shape in step 1.

If you do all of that correctly, you can get a nice result like this:

Here's the code I wrote to draw that. I wrote it in the drawRect: of a custom view subclass, but you can easily use this code to draw into any graphics context.

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

First, I create a path that's just an arc:

    static CGFloat const kArcThickness = 20.0f;
    CGRect arcBounds = CGRectInset(self.bounds, 10.0f, 10.0f);
    CGPoint arcCenter = CGPointMake(CGRectGetMidX(arcBounds), CGRectGetMidY(arcBounds));
    CGFloat arcRadius = 0.5f * (MIN(arcBounds.size.width, arcBounds.size.height) - kArcThickness);
    UIBezierPath *arc = [UIBezierPath bezierPathWithArcCenter:arcCenter radius:arcRadius startAngle:-M_PI / 3.0 endAngle:-2.0 * M_PI / 3.0 clockwise:NO];

Next, I ask Core Graphics to make a new path that is the outline of the arc path. Note how I ask it for a stroke width of kArcThickness and round line caps:

    CGPathRef shape = CGPathCreateCopyByStrokingPath(arc.CGPath, NULL, kArcThickness, kCGLineCapRound, kCGLineJoinRound, 10.0f);

I also need the inverse of that path: a path that includes every point except the points in shape. It so happens (although I don't think it's documented) that CGContextCreateCopyByStrokingPath and CGPathAddRect draw in opposite directions. So if I copy shape and draw an enormous rectangle around it, the non-zero winding rule means that the new path will be the inverse of shape:

    CGMutablePathRef shapeInverse = CGPathCreateMutableCopy(shape);
    CGPathAddRect(shapeInverse, NULL, CGRectInfinite);

Now I can actually start drawing. First, I'll fill in the shape with a light gray color:

    CGContextBeginPath(gc);
    CGContextAddPath(gc, shape);
    CGContextSetFillColorWithColor(gc, [UIColor colorWithWhite:.9 alpha:1].CGColor);
    CGContextFillPath(gc);

Next I actually perform the four steps I listed above. I have to save the graphics state so I can undo the clipping and shadow parameters when I'm done.

    CGContextSaveGState(gc); {

Step 1: clip to the shape:

        CGContextBeginPath(gc);
        CGContextAddPath(gc, shape);
        CGContextClip(gc);

Step 2: Well, I did this step already when I created shapeInverse.

Step 3: I set the shadow parameters:

        CGContextSetShadowWithColor(gc, CGSizeZero, 7, [UIColor colorWithWhite:0 alpha:.25].CGColor);

Step 4: I fill the inverse shape from step 2:

        CGContextBeginPath(gc);
        CGContextAddPath(gc, shapeInverse);
        CGContextFillPath(gc);

Now I restore the graphics state, which specifically restores the clipping path and unsets the shadow parameters.

    } CGContextRestoreGState(gc);

Finally, I'll stroke shape with a light gray to make the edge crisper:

    CGContextSetStrokeColorWithColor(gc, [UIColor colorWithWhite:.75 alpha:1].CGColor);
    CGContextSetLineWidth(gc, 1);
    CGContextSetLineJoin(gc, kCGLineCapRound);
    CGContextBeginPath(gc);
    CGContextAddPath(gc, shape);
    CGContextStrokePath(gc);

Of course I clean up when I'm done:

    CGPathRelease(shape);
    CGPathRelease(shapeInverse);
}

For more complex shapes, you can look at my answer here and my answer here.

Here's all the code together for easy copying:

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

    static CGFloat const kArcThickness = 20.0f;
    CGRect arcBounds = CGRectInset(self.bounds, 10.0f, 10.0f);
    CGPoint arcCenter = CGPointMake(CGRectGetMidX(arcBounds), CGRectGetMidY(arcBounds));
    CGFloat arcRadius = 0.5f * (MIN(arcBounds.size.width, arcBounds.size.height) - kArcThickness);
    UIBezierPath *arc = [UIBezierPath bezierPathWithArcCenter:arcCenter radius:arcRadius startAngle:-M_PI / 3.0 endAngle:-2.0 * M_PI / 3.0 clockwise:NO];
    CGPathRef shape = CGPathCreateCopyByStrokingPath(arc.CGPath, NULL, kArcThickness, kCGLineCapRound, kCGLineJoinRound, 10.0f);
    CGMutablePathRef shapeInverse = CGPathCreateMutableCopy(shape);
    CGPathAddRect(shapeInverse, NULL, CGRectInfinite);

    CGContextBeginPath(gc);
    CGContextAddPath(gc, shape);
    CGContextSetFillColorWithColor(gc, [UIColor colorWithWhite:.9 alpha:1].CGColor);
    CGContextFillPath(gc);

    CGContextSaveGState(gc); {
        CGContextBeginPath(gc);
        CGContextAddPath(gc, shape);
        CGContextClip(gc);
        CGContextSetShadowWithColor(gc, CGSizeZero, 7, [UIColor colorWithWhite:0 alpha:.25].CGColor);
        CGContextBeginPath(gc);
        CGContextAddPath(gc, shapeInverse);
        CGContextFillPath(gc);
    } CGContextRestoreGState(gc);

    CGContextSetStrokeColorWithColor(gc, [UIColor colorWithWhite:.75 alpha:1].CGColor);
    CGContextSetLineWidth(gc, 1);
    CGContextSetLineJoin(gc, kCGLineCapRound);
    CGContextBeginPath(gc);
    CGContextAddPath(gc, shape);
    CGContextStrokePath(gc);

    CGPathRelease(shape);
    CGPathRelease(shapeInverse);
}