Core

This is how I would do it: You may want to switch the positive and negative values if you're using a downward Y axis. In any case, I think it's important to keep the distinction between positive and negative angles, and between acute and obtuse angles. I would think that depends on the engine, not on the programming language. C for example doesn't impose anything; I just prefer to use an upward Y axis because it's similar to the trigonometric circle. But either way it's fine; you just need to make sure all the calculations are consistent.

That's probably because the Japanese word for "echidna" literally means "needle mole" (just like the Japanese word for "hedgehog" literally means "needle mouse").

If you have DeltaX = -15 and DeltaY = -9, then the angle is: angle = atan(-9 / -15) = atan(0.6) = 0.54 radians or 31 degrees (= 0.54 * 180 / pi) I think luksamuk made a mistake; what he calls angleInRadians should be the angle in degrees. An arctan function normally returns the angle in radians, so if you need it in degrees you'll have to convert it. Apparently the ATan2 function did give you the right angle in degrees directly, though it subtracted 180° because you gave a negative DeltaX. Normally you should calculate like this: DeltaX = X2 - X1, rather than X1 - X2. Same with DeltaY. If you give positive numbers to the ATan2 function it should give you the right angle. If you need the angle in radians: angleInRadians = angleInDegrees * pi / 180 I don't know if you will need it; it depends on what you need the angle for. By the way, here you end up with a positive angle for a downward slope. I don't know how Sonic games are made, but I'm used to the opposite: positive angles for upwards slopes and negative angles for a downward slopes, because I use a upward Y axis (basically I would have DeltaX = 15 and DeltaY = -9). Maybe that's normal for Sonic games, but you should check to be sure.

The link to the video doesn't work for me. This one works:

To be honest, the one in the air doesn't really look like a cactus to me.

Sorry about the controller configuration. I initially wanted to have the user assign a button by pressing it, the same way it's done with the keyboard keys. But I couldn't make it work... so instead you have to select the buttons/axes from a list, and if you press a button its number is displayed. It's not ideal, but it should be usable enough...

Thanks a lot for your feedback. I know the camera needs work, but from what I understand in your video, you use the keyboard to jump/roll and the mouse to move the camera. This is strange, because you're supposed to either use the keyboard only (with the C key to move the camera), or use the mouse buttons to jump/roll while moving with the arrow keys. In any case, it is true that the level design needs work. I'm not too good at it, but I'll try to improve. I've already been told that Sonic should have a shadow, but I have no idea how to implement it...

I remember the sounds were stuttering at times, but I think it was related to the slowdowns.

Agreed. I prefer the blue one, but I think something in-between could look better.

Isn't it supposed to be the same ship?

Quick poll: do you think it would be better if I replaced the Insta-Shield with a Homing Attack? It may be too difficult to hit enemies without the Homing Attack, but I fear it would become too easy with it. --- @TailsSena: adding DirectX support is probably something I should do; the problem is that I've never used DirectX and don't know how to use it. I think I'll look into it once the game is finished.

This is what the end of the level looks like. Thoughts?

It looks nice. But maybe you could try adding some lines to connect the ivy leaves so that it looks more like ivy?

@Sparks: it looks very nice. I really like the design. @Delta Hedgehog: it's pretty. I think I like it better with the ivy.

In NiGHTS Journey of Dreams, the rings were a single model (made of many identical blobs) with each blob associated to a different bone. I know nothing about GML though, so I don't know if that's the best way to do it.