Robust atan(y,x) on GLSL for converting XY coordinate to angle

Question

In GLSL (specifically 3.00 that I\'m using), there are two versions of atan(): atan(y_over_x) can only return angles between -PI/2, PI/2, while

Answer 1

Your proposed solution still fails in the case x=y=0. Here both of the atan() functions return NaN.

Further I would not rely on mix to switch between the two cases. I am not sure how this is implemented/compiled, but IEEE float rules for x*NaN and x+NaN result again in NaN. So if your compiler really used mix/interpolation the result should be NaN for x=0 or y=0.

Here is another fix which solved the problem for me:

float atan2(in float y, in float x)
{
    return x == 0.0 ? sign(y)*PI/2 : atan(y, x);
}

When x=0 the angle can be ±π/2. Which of the two depends on y only. If y=0 too, the angle can be arbitrary (vector has length 0). sign(y) returns 0 in that case which is just ok.