gzdoom/wadsrc/static/shaders/scene/mateffect_warp1.glsl

vec2 GetTexCoord()
{
	vec2 texCoord = vTexCoord.st;

	const float pi = 3.14159265358979323846;
	vec2 offset = vec2(0,0);

	offset.y = sin(pi * 2.0 * (texCoord.x + timer * 0.125)) * 0.1;
	offset.x = sin(pi * 2.0 * (texCoord.y + timer * 0.125)) * 0.1;

	return texCoord + offset;
}