iOS / GLES2: How to achieve Glow Effect

Question

I have a compass style needle, free to spin.

I need it to glow when it approaches a certain orientation, say 55 degrees.

I want to have NeedleView subclass UI

Answer 1

I experimented with a lot of approaches here.

basically I neeed two images; one glowing and one not.

then I can draw them as CALayers on top of one another, and hijack CADisplayLink, setting

glowLayer.opacity = glowFac;
dullLayer.opacity = (1 - glowFac);

I also played with fiddling around in GL shaders, passing the glow factor as a uniform, but this is a ton of code to do basically the same thing.

NB I needed to find a graphic artist to 'glow' the image