3D objects: GetObjectDistance

       Name: GetObjectDistance                                       [Show more]
       Type: Subroutine
   Category: 3D objects
    Summary: Calculate the distance between an object and the player's car, for
             collision purposes
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * CheckForContact calls GetObjectDistance
             * GetSectionYawAngle calls GetObjectDistance


 This routine is called with the smaller yaw angle of the object, where 0 to
 255 represents 0 to 45 degrees, so 103 = 18.2 degrees. The smaller viewing
 angle is taken from the arctan calculation for the yaw angle calculation in
 GetObjYawAngle, so that's the triangle whose hypotenuse is the line between
 the player and the object, and whose other sides are parallel to the x-axis
 and z-axis.

 If the smaller yaw angle is < 18.2 degrees, the routine does this:

   * Set (L K) = (J I) + (H G) / 8
               = max + min / 8

 If the smaller yaw angle is >= 18.2 degrees, the routine does this:

   * Set (L K) = (J I) * 7/8 + (H G) / 2
               = max * 7/8 + min / 2

 This appears to set the distance between the object and the player's car, for
 the purposes of determining whether contact has been made.

 I suspect the calculation is an approximation of Pythagoras that is much
 faster to calculate, split into small yaw angles (when the objects are
 close to being orthogonal to each other) and larger yaw angles (when their
 relative positions are closer to the diagonal). This is a guess, though.


 Arguments:

   (J I)                max(|x-delta|, |z-delta|)

   (H G)                min(|x-delta|, |z-delta|)

   M                    The smaller yaw angle of the object, where 0 to 255
                        represents 0 to 45 degrees, so 103 = 18.2 degrees


 Returns:

   (L K)                The distance between the object and the player's car

   A                    Contains the high byte of (L K)


.GetObjectDistance

 LDA M                  \ If M >= 103, jump to odis1
 CMP #103
 BCS odis1

 LDA G                  \ Set A = G

 LSR H                  \ Set (H A) = (H A) >> 3
 ROR A                  \           = (H G) >> 3
 LSR H
 ROR A
 LSR H
 ROR A

 CLC                    \ Set (L K) = (J I) + (H A)
 ADC I                  \           = (J I) + (H G) >> 3
 STA K                  \           = (J I) + (H G) / 8
 LDA H
 ADC J
 STA L

 RTS                    \ Return from the subroutine

.odis1

 LSR H                  \ Set (H G) = (H G) >> 1
 ROR G

 LDA J                  \ Set (T A) = (J I)
 STA T
 LDA I

 LSR T                  \ Set (T U) = (T A) >> 3
 ROR A                  \           = (J I) >> 3
 LSR T
 ROR A
 LSR T
 ROR A
 STA U

 LDA G                  \ Set (L K) = (J I) + (H G)
 CLC                    \           = (J I) + (H G) >> 1
 ADC I
 STA K
 LDA H
 ADC J
 STA L

 LDA K                  \ Set (L K) = (L K) - (T U)
 SEC                    \           = (J I) + (H G) >> 1 - (J I) >> 3
 SBC U                  \           = (J I) * 7/8 + (H G) / 2
 STA K
 LDA L
 SBC T
 STA L

 RTS                    \ Return from the subroutine

 EQUB &F1, &0C          \ These bytes appear to be unused
 EQUB &E5, &74
 EQUB &8D, &F6
 EQUB &0C, &60
 EQUB &00, &00
 EQUB &00, &00
 EQUB &00, &00
 EQUB &40