[BUG] Platform Movement Object Quirk

**pulsecode** · 28th December 2012, 07:39 PM

I've noticed a strange bug with the Platform Movement Object when running on an iOS device (tested on iPod Touch 4th and 5th gen).

It seems that the collision detection is off, but only on the right hand side!
Walls and slopes to the left are not affected. Not all walls to the right are affected (it seems to be related to their physical x position) but all slopes to the right are.
For slopes, it doesn't matter which direction the player moves in, so long as the slope is to the right of the player.

I have taken some snap shots of what happens as well as made a demo which reproduces the error.

So! First snapshot: http://www.absolutecreations.co.uk/p...pmo-bug-01.jpg
That occurs when moving down the slope from right to left. It is intermittently jumpy on the way up the slope too.
It is not caused by the step down value being set too low. You can set that to 10 and it still happens.

Snapshot 2: http://www.absolutecreations.co.uk/p...pmo-bug-02.jpg
That shows what should happen when walking into a wall (and what does happen on some right hand walls... as I said, it appears to be related to x position of obstacles. Andos suggested it was to do with rounding the position, which would make sense).

Snapshot 3: http://www.absolutecreations.co.uk/p...pmo-bug-03.jpg
Same as the first snapshot, just with a different slope angle.

Snapshot 4: http://www.absolutecreations.co.uk/p...pmo-bug-04.jpg
The gap marked with the arrow should not be there... the player is jumping and pushing against that right hand wall, so there should be no gap at all.

Snapshot 5: http://www.absolutecreations.co.uk/p...pmo-bug-05.jpg
Just showing how the slopes when on the left work... flawlessly!

Snapshot 6: http://www.absolutecreations.co.uk/p...pmo-bug-06.jpg
A better example of the bug when on the ground. Again, in this shot the player is pushing against the right hand wall so there should be no gap (it should be as in snapshot 2).

Snapshot 7: http://www.absolutecreations.co.uk/p...pmo-bug-07.jpg
Same as above but in a different place in the demo.

So there you have it! 1 pixel gap on the right (but not on every right hand wall) which causes all kinds of problems if you are trying to do say ledge grabbing, or anything that requires decent accuracy really.
But the worst side effect is that slopes on the right are really choppy and the player can even lift off when coming down them (which plays havoc with running / falling animations), even with a high step down value set (as I mentioned, step down value has no effect on this bug).

And finally, the demo file! pmo-bug-ios.zip
It has PC controls so you can see it all works as normal in windows. Delete the PC Controls group and change build type to xcode to test on iOS.

**arsdigita** · 29th December 2012, 04:26 AM

Great in-depth break down of an issue.
I have noticed an issue but had not yet begun full iOS testing yet on our pmo game (also assumed it was on my end in the code).
Thanks for taking the time to do this, hopefully CT can address the bug.

**pulsecode** · 9th January 2013, 04:42 PM

Any thoughts on this guys?
I'm at a complete stop on my project for the time being because of this, so kinda hoping it might get acknowledged and on the way to getting fixed fairly soon

**DistantJ** · 9th January 2013, 10:05 PM

I'm sure they'll sort it out in an update. In the mean time I'd get around this by shaving one pixel off of your collision box object on the right

**pulsecode** · 9th January 2013, 10:18 PM

Hi DistantJ,
Unfortunately it has nothing to do with the width of the collision box. If the box was wider or narrower the gap still happens between the collision box and the environment, which still messes up slopes to the right of the player and any other fine detections to the right of the player (such as ledge grabbing).
In the screen captures I put in my post you can see that it is actually the collision box I'm using to show the gaps.
If it was just a visual error it would be easily overcome when I overlay my character sprite

But its not.

**DistantJ** · 10th January 2013, 01:27 AM

But what I mean is, shave 1 pixel off if the right of the rectangle but have the character sprite appear normal size over it, so the invisible collision box is slightly smaller than it appears to be.

**pulsecode** · 10th January 2013, 01:38 AM

As I said, its not a visual bug.

Making the rectangle 1 pixel more narrow or 1 pixel wider makes no difference. The problem is in the actual collision handling on the right hand side.
Invisible collision box size affects nothing as far as the bug goes. It still acts glitchy on slopes to the right and still doesn't quite touch all walls on the right (its a rounding issue so some walls it does touch).