Imagination PowerVR SDK Blog

Expected result when overflowing a lowp variable?


#1

I am simply curious about this…





On the iPhone Simulator if I use a lowp variable to accumulate a colour result, in something like a “god ray” or blur shader I get a saturated result, ultimately. No matter how many reasonable iterations I carry out.





On iOS devices, however, I get a ‘0’ result, which I can only assume is because of some kind of overflow situation. Replacing the lowp with a medium solves the problem.





I don’t mind which happens, and can see reasons for either.


I am just curious as to what the “expected” result is from Imagination’s perspective…scratt2011-02-16 09:20:44


#2

You should get a saturated result. Can you post the shader you are using?


#3

Sorry, that particular iteration of the fragment shader went in the bin.





It is fairly easy to replicate, and is basically a lowp vec3 (initially defined outside of a loop) accumulating lots of lowp vec3 texture reads inside that loop with about 20 - 40 iterations.





I have filed a bug report with Apple, and been asked for a reproducer.


When I do that for Apple I will put a copy in this thread also.scratt2011-02-18 04:46:09