Homing Attack in Sonic 1 - Step 3 - Distance of Homing Attack

Adding a distance of work

As you may have noticed, Sonic is attacking at any distance, and to corret this,
we need add a distance calculator using the Pythagorean theorem, because the Homing Attack is 360º.

go to HA_calcdistance and you see:

Code:

HA_calcdistance:
      move.w  8(a1),d1   ; move the object x-position to d1
      move.w  $C(a1),d2   ; move the object y-position to d2
      sub.w  8(a0),d1   ; sub sonic x-position of object x-position
      sub.w  $C(a0),d2   ; sub sonic y-position of object y-position

below these lines, add this:

Code:

      muls.w  d1,d1   ; horizontal distance squared
      muls.w  d2,d2   ; vertical distance squared
      add.l  d2,d1   ; add vertical distance to horizontal distance
      cmp.l  #16384,d1      ; is distance of Sonic, greater than or equal 128 pixels of the object? (128^2=16384 // $80^2=$4000)
      bge.s  HA_nextobject   ; if yes, don't execute the homing attack

This is the Pythagorean theorem (a^2=b^2+c^2)
D1 and D2 are the leg b and c, and the "16384" is the predetermined "a^2".
To change the distance by another value, change the #16384 (or $4000) by another number n^2.

But if you compile and test the ROM, you see that Homing Attack don't work correctly.
Why? Because the CalcAngle use the d1 & d2, and in these lines of code, the d1 & d2 are changed (multiplied), what cause the error.
How to correct this? Recalculating the value is a good option.

Below of HA_Move, add this:

Code:


; ---------------------------------------------------------------------------

; Recalculating the distance between the Sonic and the object (d1 = x distance / d2 = y distance)
      move.w  8(a1),d1   ; move the object x-position to d1
      move.w  $C(a1),d2   ; move the object y-position to d2
      sub.w  8(a0),d1   ; sub sonic x-position of object x-position
      sub.w  $C(a0),d2   ; sub sonic y-position of object y-position

Why d1 and d2?

Becuse the CalcAngle uses d1 and d2 as entry points.

Why not d0?

If you want to use the D0, you can use instead of d1, d2, but CalcAngle uses d1 and d2 as entry points.


Homing Attack only work if Sonic is facing the object

As you may have noticed, Sonic is attacking the object without be facing for him.
To correct this, we need test if the Sonic is facing the object

Below this:

Code:

HA_calcdistance:   ; distance calculator
      move.w  8(a1),d0   ; move the object x-position to d0
      move.w  $C(a1),d1   ; move the object y-position to d1
      sub.w  8(a0),d0   ; sub sonic x-position of object x-position
      sub.w  $C(a0),d1   ; sub sonic y-position of object y-position

Add these lines:

Code:


; ---------------------------------------------------------------------------

; test if the Sonic is facing the object
      btst    #0,$22(a0)   ; is sonic facing left?
      beq.s  HA_faceleft   ; if yes, branch
      cmpi.w  #8,d1      ; is distance of Sonic, less than 8 pixels of the object?
      blt.s  HA_calcdistance2   ; if yes, branch
      bra.s  HA_nextobject

HA_faceleft:
      cmpi.w  #-8,d1      ; is distance of Sonic, greater than -8 pixels of the object?
      bgt.s  HA_calcdistance2   ; if yes, branch
      bra.s  HA_nextobject
; end of test

; ---------------------------------------------------------------------------

HA_calcdistance2:      ; continuation of distance calculator

What is the "8" and the "-8"?

Is the maximum distance that the Sonic can attack the object, without be facing for him.


The basic Homig Attack is complete

Just an annual year bump!