Particle shader - rotation on global axis instead of local axis?

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:bust_in_silhouette: Asked By MathiasStrohkirch

I’m wondering how I can do a rotation on a global axis rather than the particle’s local axis in a particle shader.

Here are my rotate y and x functions:

mat4 rotate_y(float theta) {
	return mat4(vec4(cos(theta), 0, -sin(theta), 0), vec4(0, 1, 0, 0), vec4(sin(theta), 0, cos(theta), 0), vec4(0, 0, 0, 1));
}

mat4 rotate_x(float theta) {
	return mat4(vec4(1, 0, 0, 0), vec4(0, cos(theta), sin(theta), 0), vec4(0, -sin(theta), cos(theta), 0), vec4(0, 0, 0, 1));
}

But these rotate along the local axis.

If I do this:

void start() {
	TRANSFORM *= rotate_y(PI/2.0);
	TRANSFORM *= rotate_x(PI/2.0);
}

I get something different than if I do the reverse:

void start() {
	TRANSFORM *= rotate_x(PI/2.0);
	TRANSFORM *= rotate_y(PI/2.0);
}

This is because the first rotation changes the local axis direction, and the second rotation then uses that new axis. How can instead do a rotation around the global x and y (and z) axis?

I’m very new to particle shaders and everything 3d. Let me know if there is an easier or better way to do anything. Thank you!