Can't seem to git rid of line in UIBezierPath - iphone

Question

OK, so I needed a rounded triangle. So what I did was use a technique similar to what I've used in other vector drawing programs. Draw the triangle and use the stroke to create the rounded corners. Worked like a charm too, until I needed to reduce the alpha of the color used to fill and stroke the UIBezierPath. For some reason I keep getting this inset outline that isn't the same color as the Fill and Stroke. Somehow the alpha value isn't being respected. Maybe I'm overlooking something silly here, but try as I might I can't get the triangle all one color with a alpha value lower than 1. This is what I get:
And heres the simple code:
- (void)drawRect:(CGRect)rect
{
UIBezierPath *path = [UIBezierPath bezierPath];
[path moveToPoint: CGPointMake(63.5, 10.5)];
[path addLineToPoint: CGPointMake(4.72, 119.5)];
[path addLineToPoint: CGPointMake(122.28, 119.5)];
[path addLineToPoint: CGPointMake(63.5, 10.5)];
[path closePath];
path.miterLimit = 7;
path.lineCapStyle = kCGLineCapRound;
path.lineJoinStyle = kCGLineJoinRound;
path.lineWidth = 8;
UIColor *whiteAlph5 = [UIColor colorWithWhite:0.6 alpha:0.5];
[whiteAlph5 setFill];
[whiteAlph5 setStroke];
[path fill];
[path stroke];
}
I can't understand why the line would be anything other than the "whiteAlpha5" if that's the only color I've set for both fill and stroke. I suppose I can just draw the rounded triangle out adding the curves to to corners, but I'm just curious as to why this happens.Thanks in advance...

Answer 1

If you must have the stroke, alter your call to [UIBezierPath stroke] like so:
[path fill];
[path strokeWithBlendMode:kCGBlendModeCopy alpha:1.0];
This should achieve the effect you want (I think - haven't been able to test it)
This is a bit of a guess, but I think you're seeing here is essentially two layers of semitransparent white, one drawn on top of the other. When the triangle is just filled in, it would be what you're expecting. When you stroke, it's drawing the same colour - but it's adding it on top of the existing colour, not replacing it, which is the effect you might expect if you've done this before in paint programs or similar. Thus, where the stroke and fill overlap, you're getting a stronger white than you're after. Just using fill by itself could solve this, but might not get the rounded effect you're after.
If you need a visual demonstration of what I mean, you can do this in Photoshop. Create a new image with a black background and create a new layer above it, set to 50% opacity. Draw a white square on it (which will look grey due to the opacity). Then, without changing layers, draw a line through it. You won't see the line, because it's replacing the existing colour - this is what you expected to happen with your code. Then, add another layer above it, also set to 50% opacity. Draw a line on this layer, through the square. You'll see the line as a brighter grey. This is additive, the white overlapping on both layers - the effect that your code is creating.

Answer 2

The line is because your stroke and your fill are drawing to the same pixels. Since both the stroke and the fill are partially transparent, the colors accumulate.
One way to fix this is to just create a path that outlines your rounded triangle, and fill it without stroking it.
Here's the interface for a category that creates a path outlining a rounded polygon:
#interface UIBezierPath (MyRoundedPolygon)
+ (UIBezierPath *)my_roundedPolygonWithSides:(int)sides center:(CGPoint)center
vertexRadius:(CGFloat)vertexRadius cornerRadius:(CGFloat)cornerRadius
rotationOffset:(CGFloat)rotationOffset;
#end
Here's the implementation:
#implementation UIBezierPath (MyRoundedPolygon)
static CGPoint vertexForPolygon(int sides, CGPoint center, CGFloat circumradius, CGFloat index) {
CGFloat angle = index * 2 * M_PI / sides;
return CGPointMake(center.x + circumradius * cosf(angle),
center.y + circumradius * sinf(angle));
}
+ (UIBezierPath *)my_roundedPolygonWithSides:(int)sides center:(CGPoint)center
vertexRadius:(CGFloat)vertexRadius cornerRadius:(CGFloat)cornerRadius
rotationOffset:(CGFloat)rotationOffset
{
CGFloat circumradius = vertexRadius + cornerRadius;
CGPoint veryLastVertex = vertexForPolygon(sides, center, circumradius, rotationOffset - 1);
CGPoint currentVertex = vertexForPolygon(sides, center, circumradius, rotationOffset);
CGMutablePathRef cgpath = CGPathCreateMutable();
CGPathMoveToPoint(cgpath, NULL, (veryLastVertex.x + currentVertex.x) / 2,
(veryLastVertex.y + currentVertex.y) / 2);
for (CGFloat i = 0; i < sides; ++i) {
CGPoint nextVertex = vertexForPolygon(sides, center, circumradius,
i + 1 + rotationOffset);
CGPathAddArcToPoint(cgpath, NULL, currentVertex.x, currentVertex.y,
nextVertex.x, nextVertex.y, cornerRadius);
currentVertex = nextVertex;
}
CGPathCloseSubpath(cgpath);
UIBezierPath *path = [self bezierPathWithCGPath:cgpath];
CGPathRelease(cgpath);
return path;
}
#end
Here's how you use it:
#implementation MyView
- (void)drawRect:(CGRect)rect
{
CGRect bounds = self.bounds;
CGPoint center = CGPointMake(CGRectGetMidX(bounds), CGRectGetMidY(bounds));
UIBezierPath *path = [UIBezierPath my_roundedPolygonWithSides:3 center:center
vertexRadius:70 cornerRadius:8 rotationOffset:0.25];
[[UIColor colorWithWhite:0.6 alpha:0.5] setFill];
[path fill];
}
#end
And here's the result:
Note that setting rotationOffset to 0.25 rotated the triangle one quarter turn. Setting it to zero will give you a right-pointing triangle.