iOS / GLES2: How to achieve Glow Effect

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 UIView, so I can manipulate (NeedleView*)needleView as if it was a UIView, just have a couple of extra methods - (void) feedAngle: (float) theta; - (void) feedGlow: (float) glow_01;

doing a bit of research, I have found that the common technique is to take a greyscale copy of the ne开发者_运维技巧edle's image, blur it, save it as a GL texture, and then wrap it onto a quad that sits behind the actual needle's quad.

then I am a bit woolly....

I guess I set the RGBA on the corner points to be (r,g,b) of my desired glow colour, a=glowFactorForThisFrame

and then I set glBlendmode to something appropriate

and then I draw the textured quad for the blur

what I want is something where Alpha is 1 everywhere on the original needle, but it bleeds gradually to complete transparency as we move further away

...

for a start, I would like to find some code that takes care of blurring a greyscale bitmap, I would rather do this from code than in photo shop. (I realize this would take a lot of time on an actual device, so maybe I can calculate it the first time the app is run, save it to file, and subsequently just load from file)

secondly, I'm very sketchy on the precise details of what I have to do.

could someone help me in on either of these points?

If you use drawRect for your drawing your needle you could possible do something like this:

Is there an easy way or library available to let text glow?

You could create the glow around the needle's image in Photoshop. Just replace the image in the app at runtime when glow is turned on.

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