[Tutorial] How to create a simple Scrolling HUD in Sonic 1.

[Mildanner]

Original Guide by Mildanner (ft. RetroKoH)

In game development, the developers didn't know how to make a simple scrolling HUD and decided to make the HUD pop in the screen without any doubts. In this guide, you will learn how to make a simple scrolling HUD.

First, you search for this variable v_ssangle in your disassembly _Variables.asm and find that UNUSED line code under it.

            ds.b    $C        ; unused

You can simply create a variable for the HUD scrolling, just replace it with this:

                    ds.b    $A        ; unused
                    ds.b    1        ; Reserved for the level start flag (if you want the S2 HUD and/or S2 Rings)
v_hudscrollpos:        ds.b    1        ; Scrolling x-position for the HUD

If that step didn't work, let's move it into a fewer unused RAM byte.

First, you will go to the _Variables.asm to find that variable with fewer unused bytes. Good example is v_ssbganim, you will find this UNUSED code part.

            ds.b    2        ; unused

Replace it with this

v_hudscrollpos:        ds.b    2        ; Scrolling x-position for the HUD

If you are using only 1 byte for the HUD's scrolling variable, we should go above v_obj31ypos (Being a good example) and find this UNUSED code part.

            ds.b    1        ; unused

Replace it simply with this

v_hudscrollpos:        ds.b    1        ; Scrolling x-position for the HUD

Note: You should include only one variable for the HUD's scrolling, including duplicate will give you a error. Just go in a preference you wish to do, 1 byte or 2 bytes.

After you created a variable, you should go to the _incObj/21 HUD.asm and go at the routine HUD_Main and replace it entirely with this new routine:

HUD_Main:    ; Routine 0
        addq.b    #4,obRoutine(a0)  ; Advances to Routine 4 (HUD_Move) on the next frame
        move.w    #0,obX(a0)        ; Starts the HUD offscreen (X = 0)
        move.w    #$108,obScreenY(a0)
        move.l    #Map_HUD,obMap(a0)
        move.w    #ArtTile_HUD,obGfx(a0)
        move.b    #0,obRender(a0)
        move.b    #0,obPriority(a0)
        rts

HUD_Move:    ; Routine 4
        move.w    obX(a0),d0        ; Loads the current HUD X position
        cmpi.w    #$90,d0           ; Has it reached position $90?
        bge.s    .reached          ; If greater or equal, finalize the movement
 
        addq.w    #5,d0             ; Move 5 pixels to the right
        move.w    d0,obX(a0)        ; Update the object's X coordinate directly
        jmp       DisplaySprite

.reached:
        move.b    #2,obRoutine(a0)
        jmp       DisplaySprite

Optional: You can change the scrolling speed to what you want, just simply replace the 5 with the number you think it's better.

If the code mentioned in particular doesn't work, you did some mistake! Just go ahead in HUD_Index: label and add this line under HUD_Flash-HUD_Index

        dc.w HUD_Move-HUD_Index

Having issues with that scrolling? You can just clear it everytime it goes in Title Screen, go to GM_Title and find this piece of code:

        bsr.w    ClearScreen

Above it, you can just place this code underneath it.

        clr.b    (v_hudscrollpos).w            ; clear scroll counter

Did you like it? Now your hack's HUD is scrolling like how it would be.
(OPTIONAL) Adding sound effect if HUD is in the position
So, if you want a sound effect that simply plays when the HUD is moved at the correct position, just go to the ".reached" label and add this below the label:

        move.b    #sfx_Cash,d0            ; set cash sound to be played
        jsr    QueueSound2            ; play it

NOTE: Not all implementations in code like this can work and be aware before using this code.​