Dev Blog #3

Tuesday April 22, 2014

Here we are, last day before our Alpha build is due and our presentation. The project has come a long way since the beginning. I would estimate that we are about 50-60% done, with most of the important features implemented but in need of bug fixing/more tweaks. I will talk about my physics tweaks that I did last night in a little bit. Everyone, save for a couple, did their work and contributed to the project. I applaud my team members for their hard work and dedication. Now, on to the good stuff.

So, yesterday August informed me that his text drawing for our HUD (that displays time, velocity and height) was crashing because the jumper would accelerate so fast that his speed became NaN almost instantaneously. I pounded my head against the code for awhile, then decided to comment out all of the physics I had put in and just simply make the jumper fall under the force of gravity. This allowed August to make sure his text was working fine, which it did. After that was done, I was not happy with having to scrap all of that nice physics code. So, I went back and looked over everything. I took out the gravity changes I made and put back in the original physics. I went through it line by line, and even did the calculations by hand for a couple of height values to make sure my calculations were correct. Turns out they weren't. Oops. Sign issues everywhere. I guess that's what happens when you are tired and coding at 2 AM, but I digress as there is no excuse for sign errors in the simple physics I used. I went back through it again, line by line, doing calculations by hand. I finally got all of my pre-calculated values correct (those not directly affected by height that needed to only be calculated once, and not on a per update basis. Saved us so many clock cycles and memory.) Then it was time to re-test my physics equations. Again, sign issues everywhere. I even had an arithmetic mistake (put something in the Java function Math.pow() that needed to be outside that function). Hehe, oops again. After re-writing that equation and doing hours of research to make sure that I was doing the physics correctly, I tested it one last time and HUZZAH it worked. But, with one major bug. The jumper accelerates just fine now and his terminal velocity (the real max velocity Felix hit, ~372 m/s or ~834 mph) but never slows back down. Don't know why or how. It will take a little while longer and some testing with our delta time value to see if it is a delta time issue (I think it is). But, for the alpha, it is fine as it shows that the physics model works.

Here is what the code looks like for our physics: (Keep in mind, its still a work in progress. Also, excuse formatting. Blogger doesn't have <code> tags...)

public void update(float dt)
{

//deltaT += dt;

//update the jumper's height based on the y component of position (since we are falling in the y direction)
height = position.y;

//updating our falling physics data
temperatureAtHeight = seaLevelTemp - (tempLapseRate * height);
pressureAtHeight = 1000.0f * (seaLevelPressure * (float)(Math.pow((1.0f - (height * preCalcPressureFraction)), preCalcPressureExponent)));
//pressure has to be in Pa, not kPa. so we multiply by 1000

densityAtHeight = (pressureAtHeight * molarMassOfAir) / (gasConstant * temperatureAtHeight);

//calculate air resistance
airResistance = new Vector3D(0.0f, 0.5f * densityAtHeight * (float)Math.pow(-velocity.y, 2.0f) * 0.579f , 0.0f);

//calculate net force
airResistance.add(new Vector3D(0.0f, -gravity, 0.0f));
//Vector3D netForce = new Vector3D(0.0f, airResistance.y, 0.0f);

Vector3D weight = new Vector3D(0.0f, mass * gravity, 0.0f);

acceleration = weight.subtract(airResistance).scale(1.0f / mass);
//from the formula a = (W-D)/m where W is weight, D in our case is the netForce and m is mass

//v = v0 + at
velocity = velocity.add(acceleration.scale(dt));

//check to see if we are at terminal velocity or not
if(-velocity.y > maxSpeed)
{
//if we are, we have to use -maxSpeed because we are falling in the downward direction, not up
//changing the sign below will cause the height to go back up after you hit maxSpeed
velocity.y = -maxSpeed;
}

//position = p0 + vt + (at^2)/2
position = position.add(velocity.scale(dt)).add(acceleration.scale((float) Math.pow(dt, 2)).scale(0.5f));
}

I was also able to complete the spacepod model and get it into the game. It looks really nice, especially after Matt L fixed some tessellation issues with the models themselves. I was also able to complete the cinematic for Phase 1 which I detailed and documented in my last blog post.

Until next session, thanks for reading.