Drawing an angle/angular/“Conical”/“Arcing” gradie

Question

I'm trying to draw a "conical"/"arcing" gradient (I don't know what would be the correct term for this) (Photoshop calls it an "angle" gradient —your friendly neighborhood stackoverflow editor) using Objective-C (IOS), pretty much exactly like the image shown in the following thread.

After days of googling and searching the internet to no avail, I've decided to ask for help here.

A little background on what I'm trying to do. My objective is to create a custom UIView, which is circular progress bar, a ring basicly, somewhat similar to the activity indicator as seen in the TweetBot iPhone app (displays when you drag to refresh, which can be seen in action here, around 17-18 seconds into the video, on top of the iphone screen). I want the progress indicator (the fill of the ring) to be a simple two color gradient, which can be set programmatically, and the view to be resizable.

Filling the ring shape with a gradient that "follows" the arc of the ring is where I'm stuck. The answers that I get from googling, reading Apple's Core Graphics documentation on gradients and searching on SO are either about radial gradients or linear/axial gradients, which is not what I'm trying to achieve.

The thread linked above suggests using pre-made images, but this isn't an option because the colors of the gradient should be settable, the view should be resizable and the fill of the progress bar isn't always 100% full obviously (which would be the state of the gradient as shown in the picture in the thread above).

The only solution that I've come up with is to draw the gradient "manually", so without using a CGGradientRef, clipping small slices of the gradient with single solid color fills within a circular path. I don't know exactly how well this will perform when the bar is being animated though, it shouldn't be that bad, but it might be a problem.

So my first question:

Is there an easier/different solution to draw a conical/arcing gradient in Objective-C (IOS) than the solution I've come up with?

Second question:

If I have to draw the gradient manually in my view using the solution I came up with, how can I determine or calculate (if this is even possible) the value (HEX or RGBA) of each color "slice" of the gradient that I'm trying to draw, as illustrated in the image below.

(Can't link image) gradient slice illustration

Answer 1

The thread linked above suggests using pre-made images, but this isn't an option because the colors of the gradient should be settable, the view should be resizable and the fill of the progress bar isn't always 100% full obviously (which would be the state of the gradient as shown in the picture in the thread above).

Not a problem!

Use the very black-to-white image from the other question (or a bigger version if you need one), in the following fashion:

1. Clip to whatever shape you want to draw the gradient in.
2. Fill with the color at the end of the gradient.
3. Use the black-to-white gradient image as a mask.
4. Fill with the color at the start of the gradient.

You can rotate the gradient by rotating the mask image.

This only supports the simplest case of a gradient with a color at each extreme end; it doesn't scale to three or more colors and doesn't support unusual gradient stop positioning.

Answer 2

FYI: here's also a good tutorial for creating a circular progress bar using Quartz drawing.

http://www.turnedondigital.com/blog/quartz-tutorial-how-to-draw-in-quartz/

Answer 3

Looks to me like a job for a pixel shader. I remember seeing a Quartz Composer example that simulated a radar sweep, and that used a pixel shader to produce an effect like you're describing.

Edit:

Found it. This shader was written by Peter Graffignino:

kernel vec4 radarSweep(sampler image, __color color1,__color color2, float angle, vec4 rect)
{
    vec4 val = sample(image, samplerCoord(image));
    vec2 locCart = destCoord();
    float theta, r, frac, angleDist;

    locCart.x = (locCart.x - rect.z/2.0) / (rect.z/2.0);
    locCart.y = (locCart.y - rect.w/2.0) / (rect.w/2.0);
    // locCart is now normalized
    theta = degrees(atan(locCart.y, locCart.x));
    theta = (theta < 0.0) ? theta + 360.0 : theta;
    r = length(locCart);
    angleDist = theta - angle;
    angleDist = (angleDist < 0.0) ? angleDist + 360.0 : angleDist;
    frac = 1.0 - angleDist/360.0;
    // sum up 3 decaying phosphors with different time constants
    val = val*exp2(-frac/.005) + (val+.1)*exp2(-frac/.25)*color1 + val*exp2(-frac/.021)*color2;
    val = r > 1.0 ? vec4(0.0, 0.0,0.0,0.0) : val; // constrain to circle
    return val;
}