draw ios quartz 2d path with a varying alpha component

Question

I'd like to paint some Bezier curves with the alpha channel that is changing during the curve painting. Right now I'm able to draw bezier paths, with a fixed alpha channel. What I'd like to do is to draw a single bezier curve that uses a certain value of the alpha channel for the first n points of the path another, alpha value for the subsequent m points and so on. The code I'm using for drawing bezier path is:

CGContextSetStrokeColorWithColor(context, curva.color.CGColor);
....
CGContextAddCurveToPoint(context, cp1.x, cp1.y, cp2.x, cp2.y, endPoint.x, endPoint.y);
....
CGContextStrokePath(context);

Is there a way to achieve what I'm describing?

Many thanks, Giovanni

Answer 1

There are two ways I imagine you could achieve this effect:

• If you absolutely must compute the alpha at draw time, you could experiment with creating a colored CGPatternRef ( http://developer.apple.com/library/ios/#documentation/GraphicsImaging/Reference/CGPattern/Reference/reference.html ) whose drawing callback inspects the current context translation and draws a color based on the offset. Instead of setting a stroke color, you'd set CGContextSetStrokePattern to your created pattern.

• If appropriate for the drawn object, you could precompute the alpha across the space of the curve and use your CGPathRef as a mask for the alpha image.

I should qualify that I've never attempted the CGPatternRef method, so I can only point that out as a potential, not a certainty.