Fix minor boundary issue in FLIP demo

[mitxela]

In the FLIP demo, when transferring velocities to particles, solid cells should not be considered valid.

Particles in contact with the left wall were constantly pushed downwards, this fixes that. It doesn't really matter except that it means the fluid doesn't come to rest. It still doesn't fully settle as there are some other effects on the other edges, not sure if it's worth trying to fix these though.

PS Thank you for making these tutorials, they're excellent.

[emlyn]

Good find, I was wondering what was causing the movement down the left hand side.

It's true there is another effect on the right hand edge (and also the top, but that's not so noticeable, unless you change the gravity direction) that causes what looks like bubbles/boiling. It seems to be dependent on resolution: if you find the line var res = 100; and change that to 150 it gets really bad, while at lower numbers like 50 it's unnoticeable.

[metarapi]

It's true there is another effect on the right hand edge (and also the top, but that's not so noticeable, unless you change the gravity direction) that causes what looks like bubbles/boiling. It seems to be dependent on resolution: if you find the line var res = 100; and change that to 150 it gets really bad, while at lower numbers like 50 it's unnoticeable.

I can see it looks like chain reactions.

At first I though it could be that the push apart logic is causing this because it's updating the particle positions sequentially. So I tried to accumulate all the positional updates in a buffer and then apply them at the end. Didn't help.

However, reducing the dt or increasing simHeight does help.

The positional/velocity updates are too large with regards to the simulation size it seems - since it's simHeigth which determines the particleRadius through h.

[blueedgetechno]

Adding an extra j == f.fNumY - 1 case at line 744 fixes the boiling effect issue on gravity changes. Essentially adding a top wall.

if (i == 0 || i == f.fNumX-1 || j == 0 || j == f.fNumY-1)