'Dixel' scrolling... Ghosts 'n' Goblins

[Lethargeek]

But would it work with graphics like Stormlord? Or Zynaps? Or Robocop?

It would be slower, yes, to push instead of just skipping the solid space. But typical Stormlord screen usually has much more empty space to skip. Zynaps - maybe, with careful level design. Definitely not Robocop.

[Ralf]

But typical Stormlord screen usually has much more empty space to skip

Maybe you missed it from the earlier discussion but Stormlord actually skips empty space.

At address 36132 you have CALL NZ, 36226. Which means draw a tile only if tile nr<>0. And tile nr=0 is empty space.

It works more or less the same way that you are talking about, I believe. Tiles are stored with some empty space at the edges
and when game area scrolls new tiles clear everything behind them.

Raf Cecco was really a clever guy.

[Lethargeek]

Maybe you missed it from the earlier discussion but Stormlord actually skips empty space.

No, i missed nothing. My point was it has more empty space to skip than my example.

It works more or less the same way that you are talking about, I believe. Tiles are stored with some empty space at the edges
and when game area scrolls new tiles clear everything behind them.

Absolutely not the same way. Doesn't scroll anything on-screen and is wasting memory for pre-shifted tiles.

In my example, tiles are stored only once (and rotated 90 degrees to appear at the edge pixel-by-pixel) and later are actually scrolled on-screen.

[catmeows]

This is things in the foreground moving horizontally, rather than the entire background - but you can get pretty fast and furious, and deal with different speeds, when you scroll preshifted stuff. As long as you have the space to store all the different versions. I've gone 128K-only here for that reason!

Eeeee. Does this thing really exist ? On Zx ?

[Vampyre]

Edit: Doh! I can see that the "dixel" term had already been found way earlier in this thread!

[wizbob]

I don't know if you guys have seen this beautifully illustrated review of the dixel scrolling in Ghosts'n'Goblins.

[catmeows]

I don't know if you guys have seen this beautifully illustrated review of the dixel scrolling in Ghosts'n'Goblins.

Thanks.

[Nienn Heskil]

Spends waaay too much time on obvious things like the sprite method, but it was interesting to learn about the scrolling of attributes at least (I've used a similar approach in Mint). Perhaps this is one of the reasons there's no color on levels with vertical scrolling added.

I think the most interesting thing about GnG is definitely the crazy way the backgrounds are put together from different elements. This and Marauder, also maybe Zynaps... I've always wondered what sort of tools they've used to make the level maps (assuming it's not all hardcoded).

[Einar Saukas]

Code: Select all

; your sinclair issue 19 page 74
; it stll needs a full loop

org 32000
        di
        ld bc,6144
        ld hl,22527
loop:   srl (hl)
        call c,sub1
        srl (hl)
        call c,sub2
        dec hl
        dec bc
        ld a,b
        or c
        jr nz,loop
        ei
        ret
sub1    inc hl
        set 6,(hl)
        dec hl
        ret
sub2    inc hl
        set 7,(hl)
        dec hl
        ret

Ouch!

Here's my version of dixel scrolling:

Code: Select all

    ld hl,16384
    ld bc,24
loop:
    rr (hl)
    ex af,af'
    rr (hl)
    ex af,af'
    inc l
    djnz loop
    inc h
    dec c
    jp nz,loop

The same routine using unrolled loop:

Code: Select all

    ld hl,16384
    ld b,192
loop:
REPT 31
    rr (hl)
    ex af,af'
    rr (hl)
    ex af,af'
    inc l
ENDR
    rr (hl)
    ex af,af'
    rr (hl)
    ex af,af'
    inc hl
    dec b
    jp nz,loop

[presh]

Here's my version of dixel scrolling:

Code: Select all

    ld hl,16384
    ld bc,24
loop:
    rr (hl)
    ex af,af'
    rr (hl)
    ex af,af'
    inc l
    djnz loop
    inc h
    dec c
    jp nz,loop

The same routine using unrolled loop:

Code: Select all

    ld hl,16384
    ld b,192
loop:
REPT 31
    rr (hl)
    ex af,af'
    rr (hl)
    ex af,af'
    inc l
ENDR
    rr (hl)
    ex af,af'
    rr (hl)
    ex af,af'
    inc hl
    dec b
    jp nz,loop

Came here after spotting this in the Common machine code sequences thread, and was curious to see how it compares to the version posted in YS which I remembered being mentioned here.

Loving the use of EX AF, AF' to preserve the carry bits. Genius!

[Einar Saukas]

Not really.

Dixel scrolling is not a great idea. If you have enough free memory for the unrolled loop implementation, then running a single pixel scroll twice is actually faster than a single dixel scroll without screen memory contention (although dixel scrolling performs better during memory contention while running the raster scan, if my calculations are correct... I don't have an emulator right now to validate it).

[Joefish]

In this case, isn't it more efficient to fetch a byte from memory, scroll it, then write it back, rather than do two in-place rotates?
Though it's only quick if you're shifting in A, which means you can't do EX AF,AF'... So maybe not...

[Nienn Heskil]

For 2px scrolling, try POP: RL: RL: PUSH. I have one such routine that scrolls some 30x192 area, the resulting speed is about 31T/byte (uncontended), which is close to RL (HL) x2 with nothing in between.

[catmeows]

For 2px scrolling, try POP: RL: RL: PUSH. I have one such routine that scrolls some 30x192 area, the resulting speed is about 31T/byte (uncontended), which is close to RL (HL) x2 with nothing in between.

I don't see how this could work. Could you share your code ?

[Nienn Heskil]

For 2px scrolling, try POP: RL: RL: PUSH. I have one such routine that scrolls some 30x192 area, the resulting speed is about 31T/byte (uncontended), which is close to RL (HL) x2 with nothing in between.

I don't see how this could work. Could you share your code ?

Here

[Einar Saukas]

Nice

[catmeows]

I don't see how this could work. Could you share your code ?

Here

Thanks. That bidirectional shift of A to store 2 bits was the missing part.

[arjun]

if my calculations are correct... I don't have an emulator right now to validate it).

Einar I'm concerned. This is the second post I've seen from you with the above words. Are you okay? Are you being held against your will, somewhere? This is your secret code for "Help. Rescue me!", right?

Please post again with the words "emulation is for n00bs" to confirm. We will immediately create a ticket for this rescue task. You can track the progress being made towards your rescue in the ticket. Please take the time to rate this support on a scale of 1 to 10. Thank you!

[Einar Saukas]

Einar I'm concerned. This is the second post I've seen from you with the above words. Are you okay?

I'm not used to more than a few hours without access to an emulator... but now I'm back home so everything's fine!