Adding spread to velocity Vector3D

Question

I am trying to give an object a velocity spread (e.g. shotgun bullets or particle effect).

//dir is the initial direction
float angle1 = atan(dir.x, dir.z) + spread;
float angle2 = atan(dir.y, abs(dir.z)) + spread;
vec3 xz_dir = vec3(sin(angle1, 0.0, cos(angle2));
vec3 yz_dir = vec3(0.0, sin(angle2), cos(angle2));
vec3 new_dir = vec3(xz_dir.x * yz_dir.z, yz_dir.y, xz_dir.z * yz_dir.z);
new_dir.normalize();

The new_dir spread works fine in the XY and YZ planes but if there is a Y component in the direction, the spread along the XY plane doesn't work.

I am trying to work with Euler angles.
I grab two angles from the direction XZ plane and YZ plane and then add a random spread. Then I convert them back into two vectors. Then I combine the two vectors to get the new direction with the added spread.

If I also add a random spread in the XY plane by first grabbing the angle, add a spread, convert to vector:

vec3 xy_dir vec3(cos(angle1_rad), sin(angle1_rad), 0.0);

How would I add this xy vector to the other two vectors? — codecustard, Jul 06 '19 at 07:24
Euler angles will not give you the results you want here. As illustrated here, stacking ranges of Euler angles tends to make odd shapes that are not what you intended or predicted from looking at your math. I'd strongly recommend throwing away Euler angles and trying a fresh approach for bullet spread. — DMGregory, Jul 06 '19 at 12:28

Adding spread to velocity Vector3D

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