Sonic the hedgehog?

[psxphill]

That's a game I'd never seen before. Nice work. Plenty of parallax, but the graphics suffer a little from the 7-colour limitation, they've done well but to me it has a kind of "dirty" look. If they made an AGA version it could have been really nice.

Yeah OCS dual playfield is limiting with only 6 bitplanes. You would have to use the blitter for parallax and I'm not sure you could achieve 50/60 fps with that.
 
AGA would be better, but still not as good as the mega drive because of the sprites. But you might be able to pull it off.

[psxphill]

No blitting is required on the background layer. The copper can be used to scroll each scanline sideways independently.

Not when you want a parallax effect without using dual playfield, which limits you to 7 colours on the foreground.
 
You can use 2 bitplanes for the background and 4 for the foreground in EHB and scroll the whole screen with the scroll registers but you have to shift the background the opposite way to compensate. That gives you 4 background colours, 7 foreground colours but also another 7 that are dimmer. You can also use half brite in the foreground layer to dim the background.
 
You need to set the palette properly or it will look wrong (you need to duplicate all the foreground colours across the background apart from the foreground see through colour). Although you can use this for other tricks.
 
If there were seperate scroll per bitplane then it would be easier, but there are only two (one for odd and one for even).

[psxphill]

This is possible, but did anybody ever do that? I believe Robocod used a single bitplane for a background parallax layer. You can also do transparencies this way if you set the palette up right.
 
It is possible to scroll each bitplane independently vertically. Mr Beanbag's title screen does that.

robocod cheats heavily by having a repeating pattern across the screen.
 
Pick your pattern correctly and you only need one copy of it in memory as well (you just need 16 patterns across the screen each starting on a seperate bit boundary). Then you just need enough blank space above and below so you don't have to be move the copper split & a screens worth to the right so offset 15 wraps to 0 etc.
 
Vertical is easy as you have a pointer for each bitplane, the trick is to use them for horizontal scrolling.