View source for Needle colour mod

This article refers to a Stunts mod coded by [[Cas]] with quite some work from [[Daniel3D]] by editing the [[Restunts]] source and recompiling it, which allows to configure the instrument needle colour as seen from a car cockpit for each car individually.

== History ==

On 22 August 2021, [[Ryoma]] created [https://forum.stunts.hu/index.php?topic=3810.0 a thread] where he mentioned he had had some success changing the instrument needle colour by creating a mask for one of his cars. [[Daniel3D]] noted that this wasn't a complete solution and decided to look into the [[Restunts]] code to find a better way to do it. He realised the instrument needle colour was actually defined by reference and not hard-coded, as there is a ''needle_meter_color'' pointer in the code.

This called the attention of [[Cas]], who noticed that the pointer actually referenced a constant field, yet, it could be changed for a pointer to a variable. He correctly guessed that there should be a field in memory at a static offset relative to ''needle_meter_color'' that would lay within the car RES information and be currently unused, so changing this pointer to make reference to that field would allow for cars to have a configurable needle colour.

Originally, the idea was to just patch the binary by finding the pointer value and replace it, but this turned out to be challenging, so [[Daniel3D]] helped with recompilation of the source. Red #5 field was chosen because its default value is 16, which is white in Stunts. This worked flawlessly and it's the first version of the mod.

Then, [[Cas]] thought it could be possible to configure different colours for the two instrument needles, but this would require to push new code into the source. Both [[Cas]] and [[Daniel3D]] worked hard to get this done because this would produce artifacts and made the code unstable. With some padding bytes and several tries, they finally got it working. This is the new version and it has so far proved very stable, although it's impossible to confirm it can never lead to errors.

== Technical details ==
The code Daniel3D initially identified in ReStunts is located in the seg005.asm segment source file and it looks like this:

<tt>loc_23456:-
    cmp     [bp+var_6], 0
    jnz     short loc_23485
    mov     ax, si
    shl     ax, 1
    mov     [bp+var_20], ax
    push    meter_needle_color
    mov     bx, ax
    mov     al, (simd_player.spdpoints+1)[bx]
    sub     ah, ah
    push    ax
    mov     al, simd_player.spdpoints[bx]
    push    ax
    push    simd_player.spdcenter.y2
    push    simd_player.spdcenter.x2
    call    preRender_line
    add     sp, 0Ah
loc_23485:
    mov     ax, di
    shl     ax, 1
    mov     [bp+var_20], ax
    push    meter_needle_color
    mov     bx, ax
    mov     al, (simd_player.revpoints+1)[bx]
    sub     ah, ah
    push    ax
    mov     al, simd_player.revpoints[bx]
    push    ax
    push    simd_player.revcenter.y2
    push    simd_player.revcenter.x2
    call    preRender_line
    add     sp, 0Ah
    mov     al, [bp+var_2]
    cbw
    or      ax, ax
    jz      short loc_234BE
    cmp     ax, 2           ; st. whl. position flag
    jz      short loc_234EC
    jmp     short loc_234DE
    ; align 2
    db 144</tt>