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# 2D Platformer, Advanced physics SAT. (Think Sonic)

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10 replies to this topic

### #1Morphex  Members

Posted 02 January 2013 - 06:25 PM

Well, first of all I have done this in different ways but right now I am trying to implement it with SAT. Before I start let me set two points clear:

• Separating Axis Algorithm is working flawless, both Swept method (TOI etc)and Discrete(Normal MTV) (I have written a article on it, with a tutorial , soon to be posted for all to use/read)
• I can implement this in a hacky way with a crap ton of sensors around the player., by this I mean around 8.

So my question is, if I wanted to implement a movement like in the sonic Hedgehogs games for genesis using SAT how you go about it?

I have done this before with using sensors around the player to calculate slopes and all the other collisions, I could do it like this again, but I feel this way is hacky, I mean, using SAT I get somewhat a simplified 3D collision system, meaning I have access to normal of the polygons, their vertices, MTV, estimated time of impact, estimated time the polygon will leave (think bullets), etc.

I though of using the normal of the polygon the character is standing but it doesn't seem to want to work decently. I guess I want to find a aproach without using a sensor for each direction, bot, top, left,right angle, main.

So even if you don't know how to handle slopes like sonic does, SAT collision how would you do it in a simple movement way without using sensors, but still getting enough info to know where the collision is and stuff like that (This might trigger some ideas on my side, sinse I already know how to do this, your way might make see another perspective anyway).

Any insights would be greatly appreciated...

Sincerely Morphex.

Check out my new blog: Morphexe

Posted 02 January 2013 - 07:09 PM

How is your collision-response system set up, and what about it isn't working?

Also, what exactly is "movement like in the sonic Hedgehogs games"? What about it are you trying to emulate?

Inspiration from my tea:

"Never wish life were easier. Wish that you were better" -Jim Rohn

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### #3MarkS  Members

Posted 02 January 2013 - 07:23 PM

Also, what exactly is "movement like in the sonic Hedgehogs games"? What about it are you trying to emulate?

I would assume that he wants to do the loops and curved tiles/landscapes. I'd like to know how they did this myself, but from the view of the older hardware. The physics were pretty impressive considering the hardware they were implemented on.

### #4Khatharr  Members

Posted 02 January 2013 - 07:59 PM

I though of using the normal of the polygon the character is standing but it doesn't seem to want to work decently.

That was my first thought about how they did it, since I remember that Sonic's sprite rotated at specific angles even though the ground sprite looked like a smooth curve.

What's it not doing right?
void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

### #5Morphex  Members

Posted 02 January 2013 - 08:16 PM

It does it ok, but I trying to figure out  how to do it "correctly", yeah I was talking about loops and any angle slopes. I can do it "exactly" like sonic but I use a a lot of sensors and I don't think it feels ok (its not the correct way).  I am actually thinking on how to do the algorithm, I have no "really physics applied  just a way to detect correct collision with the info I stated, I want to see if someone has  a better idea than those of the sensors.

Check out my new blog: Morphexe

### #6Paradigm Shifter  Members

Posted 02 January 2013 - 08:38 PM

Looked like it was just circles of fixed radius for the loop the loop bits... and Newtonian mechanics, it just looked nice as a gimmick/selling point and wasn't a full physics solution to me...
"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

Posted 02 January 2013 - 08:39 PM

I wonder, did Sonic actually do a sort of cable-like contact (causing the player to "swing" around the center point as though attached to a cable (maximum distance but no minimum distance)) while the player was actually inside the loop? That would get pretty perfect results, I should think, because as I recall if you weren't moving fast enough you could just sort of jump through the loop-de-loop.

Inspiration from my tea:

"Never wish life were easier. Wish that you were better" -Jim Rohn

soundcloud.com/herwrathmustbedragons

### #8Paradigm Shifter  Members

Posted 02 January 2013 - 08:41 PM

I think if you went through at sufficient speed you did the loop, otherwise you fell off at a certain point and normal 2d platform physics took over.
"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #9Khatharr  Members

Posted 02 January 2013 - 08:43 PM

There was also bridges that sank when you stepped on them and angled terrain that you could run up the side of. His angle would also change when he was on a 'normal' slope as I recall. I remember finding that particular aspect of the game really irritating, since the gravity was applied a bit too liberally and made it really difficult to climb up even a shallow slope unless you had enough momentum.

Found this:

http://info.sonicretro.org/Sonic_Physics_Guide
void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

### #10Paradigm Shifter  Members

Posted 02 January 2013 - 08:45 PM

Yeah, but applying gravity on a slope is a lot easier than doing a loop the loop, I think it was just a hack (a nice hack though).
"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #11Morphex  Members

Posted 02 January 2013 - 09:07 PM

Hum, so should I go with sensors then? with sensor I can do any loops, any slopes whatever, I just feel that for a "general" aproach using SAT its a bit hacky. I am thinking in checking the slope angle with the terrain normal, but I am not sure how to go about it.

For the physics part  its just moving the player in Xspeed * Cos and in the Y its Sin or something like that, so it ould follow the terrain slope.

I guess I am bit "noob" in using Sat to do platformers, since I can seem to find a way to physics without sensors.

Check out my new blog: Morphexe

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