Screen events and background events

Alright, some more plumbing to wrap up the week. Aside from the dynamic resize routines, every level has a set of four additional functions: screen init, screen event, background init and background event. The "init" functions, as the name implies, run once at level load. Meanwhile, the "event" functions, much like the resize routines, run once every frame.

Offs_ScreenInit:       dc.l AIZ1_ScreenInit
Offs_BackgroundInit:   dc.l AIZ1_BackgroundInit
                       dc.l AIZ2_ScreenInit
                       dc.l AIZ2_BackgroundInit
Offs_ScreenEvent:      dc.l AIZ1_ScreenEvent
Offs_BackgroundEvent:  dc.l AIZ1_BackgroundEvent
                       dc.l AIZ2_ScreenEvent
                       dc.l AIZ2_BackgroundEvent
                       dc.l HCZ1_ScreenInit
                       dc.l HCZ1_BackgroundInit
                       dc.l HCZ2_ScreenInit
                       dc.l HCZ2_BackgroundInit
                       dc.l HCZ1_ScreenEvent
                       dc.l HCZ1_BackgroundEvent
                       dc.l HCZ2_ScreenEvent
                       dc.l HCZ2_BackgroundEvent
                       ...

The main job of these functions is actually to render the background layers, applying all the stage-specific scroll effects such as distortions and parallax. The "screen" functions are responsible for drawing the foreground plane, whereas the "background" functions draw, well, the background plane. Much like the dynamic resize routines, they're often split into several states, indexed by a value stored in RAM.

AIZ2_ScreenEvent:
    move.w  ($FFFFEECE).w,d0
    add.w   d0,(Camera_Y_pos_copy).w
    move.w  ($FFFFEEC0).w,d0
    jmp     loc_4FF26(pc,d0.w)
; ---------------------------------------------------------------------------

loc_4FF26:
    bra.w   AIZ2SE_Normal
; ---------------------------------------------------------------------------
    bra.w   AIZ2SE_ShipRefresh
; ---------------------------------------------------------------------------
    bra.w   AIZ2SE_ShipDraw
; ---------------------------------------------------------------------------
    bra.w   AIZ2SE_EndRefresh
; ---------------------------------------------------------------------------
    bra.w   AIZ2SE_End
; ---------------------------------------------------------------------------

Moreover, these functions frequently perform the kind of tasks a resize routine would, in levels that don't use them. For instance, Mushroom Hill Zone 1's screen event function is responsible for vertically locking the camera at the end of the level and spawning the act 1 boss object.

MHZ1_ScreenEvent:
    tst.b   ($FFFFEED2).w
    bne.s   loc_54B7C
    tst.w   ($FFFFEEE8).w
    bne.s   loc_54B7C
    jsr     sub_54B80(pc)
    move.w  #$AA0,d0
    cmpi.w  #$4100,($FFFFB010).w
    blo.s   loc_54B40
    move.w  #$710,d0

loc_54B40:
    cmp.w   (Camera_max_Y_pos).w,d0
    beq.s   loc_54B4E
    move.w  d0,(Camera_max_Y_pos).w
    move.w  d0,(Camera_target_max_Y_pos).w

loc_54B4E:
    cmpi.w  #$710,(Camera_Y_pos).w
    blo.s   loc_54B7C
    cmpi.w  #$4298,(Camera_X_pos).w
    blo.s   loc_54B7C
    move.w  #$710,(Camera_min_Y_pos).w
    move.w  #8,($FFFFEEB2).w
    st      ($FFFFEED2).w
    jsr     (Create_New_Sprite).l
    bne.s   loc_54B7C
    move.l  #Obj_MHZ_Miniboss,(a1)

loc_54B7C:
    jmp     DrawTilesAsYouMove(pc)

This kind of stuff can just as easily go on the background event function though, particularly when it involves using the solid background flag. Generally speaking, either function is suitable for executing arbitrary logic throughout the whole stage. There's one thing in particular that the act 1 background event functions are responsible for, though.

HCZ1_BackgroundEvent:
    move.w  ($FFFFEEC2).w,d0
    jmp     loc_50BC6(pc,d0.w)
; ---------------------------------------------------------------------------

loc_50BC6:
    bra.w   HCZ1BGE_Normal
; ---------------------------------------------------------------------------
    bra.w   HCZ1BGE_DoTransition
; ---------------------------------------------------------------------------

That's right, we're finally taking a look at act transitions next. You know, those things that crash Sonic Mania unless you stay perfectly still.