Animating a jellyfish hardcoding?

Question

I have drew a hemi-sphere by myself, and would like to do basic jellyfish animation by animating the vertices of the hemi-sphere. I tried a lot of experiments but it seems I'm doing it wrong. I need hints or idea on about doing it.

This is the code on how to draw a hemi-sphere. I don't know what values actually to play with to animate the sphere as a jellyfish.

for(float phi = 0.0; phi < 1.567; phi += factor) {

        glBegin(GL_QUAD_STRIP);

        for(float theta = 0.0; theta < 2*3.14 + factor; theta += factor) 
        {
            x = rh * sin(phi) * cos(theta);
            z = rh * sin(phi) * sin(theta);
            y = -rh * cos(phi);

            gl::vertex(Vec3f(x, y, z));

            x = rh * sin(phi + factor) * cos(theta);
            z = rh * sin(phi + factor) * sin(theta);
            y = -rh * cos(phi + factor);


            gl::vertex(Vec3f(x, y, z));

        }
        glEnd();
    }

Answer 1

I see. Your update tells me that you are much further from results than I first thought. You're actually looking for an algorithm to define the movement of a jellyfish. That's very complex and is likely thesis level work.

In fact it is thesis level work. See Dave Rudolf's paper (PDF) on the subject. Additionally Marcin Ignac has done some work with the topic.

Perhaps there's some simpler method that I'm not thinking of, but if the linked sites are not suitable for your purposes I think you may be asking for too much. Good luck.

Answer 2

So what you want is a contraction and expansion of the sides in a peristaltic motion to make it look like it's pushing itself. I haven't tried this but the idea should work, it's similar to a cheap waving flag simulation. It is by no means a physical simulation.

First thing you need is a delta Time (dt) to govern how fast the animation will run, and this generates the wave. Second thing you will need is a scale value that you can tweak to make the animation look as good as it can.

What you want to do is modify all your xyz values by a wave based on the values of phi and dt, this will make it change from top to bottom of your hemisphere plus animate over time.

dt is expressed in seconds, a typical frame taking 0.033 to draw. phi ranges from 0 at the top of your hemisphere where you don't want any animation and gets bigger the lower it goes which will make the peristaltic wave larger at the bottom. Scale is needed to tune how big the wave is, a subjective measure of what looks good.

In this example note that x,y and z are all multiplied by the same value. Both sets of your xyz values in the original code would be modified like this each loop.

// setup
    dt = 0.033; // modify to make the wave longer or shorter periodicity
    scale = 1.0;  // modify to make the wave bigger or smaller
    modifier = -cos(dt) * phi * scale;;
// in the loop
    x = rh * sin(phi) * cos(theta);
    x *= modifier;
    z = rh * sin(phi) * sin(theta);
    z *= modifier;
    y = -rh * cos(phi);
    y *= modifier;