More Cairo fun. This is really just a variant of my last post on the same subject. It uses a radial gradient in both the alpha channel and the hue spectrum. What is more signficant, is that the code to do this has all been "reified" so that doing it is starting to look like Smalltalk code, instead of a bunch of DLLCC invocations.

diameter := 128.
data := CIntegerType unsignedChar gcCalloc: diameter * 4 * diameter.
surface := ImageSurface 
			format: #alphaColor
			extent: diameter asPoint
			usingBits: data
			stride: diameter * 4.
center := surface extent half.
radius := diameter half asDouble.
colorGradient := RadialGradient 
			from: center
			radius: 0
			to: center
			radius: radius.
0 to: radius
	do: 
		[:n | 
		colorGradient addStopAt: n / radius
			colorValue: (ColorValue hue: n / radius)].
alphaGradient := RadialGradient 
			from: center
			radius: 0
			to: center
			radius: radius.
alphaGradient addStopAt: 0 alpha: 0.
alphaGradient addStopAt: 0.5 alpha: 1.
alphaGradient addStopAt: 1 alpha: 0.
cc := surface context.
cc source: colorGradient.
cc mask: alphaGradient.
surface saveAsPng: '/home/travis/cairo_circle.png'

The one unfortunate aspect currently is that for whatever reason, I have to use an ImageSurface where I've allocated the memory locally. Otherwise, the image comes out with undefined artifacts.