Flight: PLUT

       Name: PLUT                                                    [Show more]
       Type: Subroutine
   Category: Flight
    Summary: Flip the coordinate axes for the four different views
  Deep dive: Flipping axes between space views
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 11 of 16) calls PLUT


 This routine flips the relevant geometric axes in INWK depending on which
 view we are looking through (front, rear, left, right).


.PLUT

 LDX VIEW               ; Load the current view into X:
                        ;
                        ;   0 = front
                        ;   1 = rear
                        ;   2 = left
                        ;   3 = right

 BEQ PU2-1              ; If the current view is the front view, return from the
                        ; subroutine (PU2-1 contains an RTS), as the geometry in
                        ; INWK is already correct

.PU1

 DEX                    ; Decrement the view, so now:
                        ;
                        ;   0 = rear
                        ;   1 = left
                        ;   2 = right

 BNE PU2                ; If the current view is left or right, jump to PU2,
                        ; otherwise this is the rear view, so continue on

 LDA INWK+2             ; Flip the sign of x_sign
 EOR #%10000000
 STA INWK+2

 LDA INWK+8             ; Flip the sign of z_sign
 EOR #%10000000
 STA INWK+8

 LDA INWK+10            ; Flip the sign of nosev_x_hi
 EOR #%10000000
 STA INWK+10

 LDA INWK+14            ; Flip the sign of nosev_z_hi
 EOR #%10000000
 STA INWK+14

 LDA INWK+16            ; Flip the sign of roofv_x_hi
 EOR #%10000000
 STA INWK+16

 LDA INWK+20            ; Flip the sign of roofv_z_hi
 EOR #%10000000
 STA INWK+20

 LDA INWK+22            ; Flip the sign of sidev_x_hi
 EOR #%10000000
 STA INWK+22

 LDA INWK+26            ; Flip the sign of roofv_z_hi
 EOR #%10000000
 STA INWK+26

 RTS                    ; Return from the subroutine

.PU2

                        ; We enter this with X set to the view, as follows:
                        ;
                        ;   1 = left
                        ;   2 = right

 LDA #0                 ; Set RAT2 = 0 (left view) or -1 (right view)
 CPX #2
 ROR A
 STA RAT2

 EOR #%10000000         ; Set RAT = -1 (left view) or 0 (right view)
 STA RAT

 LDA INWK               ; Swap x_lo and z_lo
 LDX INWK+6
 STA INWK+6
 STX INWK

 LDA INWK+1             ; Swap x_hi and z_hi
 LDX INWK+7
 STA INWK+7
 STX INWK+1

 LDA INWK+2             ; Swap x_sign and z_sign
 EOR RAT                ; If left view, flip sign of new z_sign
 TAX                    ; If right view, flip sign of new x_sign
 LDA INWK+8
 EOR RAT2
 STA INWK+2
 STX INWK+8

 LDY #9                 ; Swap nosev_x_lo and nosev_z_lo
 JSR PUS1               ; Swap nosev_x_hi and nosev_z_hi
                        ; If left view, flip sign of new nosev_z_hi
                        ; If right view, flip sign of new nosev_x_hi

 LDY #15                ; Swap roofv_x_lo and roofv_z_lo
 JSR PUS1               ; Swap roofv_x_hi and roofv_z_hi
                        ; If left view, flip sign of new roofv_z_hi
                        ; If right view, flip sign of new roofv_x_hi

 LDY #21                ; Swap sidev_x_lo and sidev_z_lo
                        ; Swap sidev_x_hi and sidev_z_hi
                        ; If left view, flip sign of new sidev_z_hi
                        ; If right view, flip sign of new sidev_x_hi

.PUS1

 LDA INWK,Y             ; Swap the low x and z bytes for the vector in Y:
 LDX INWK+4,Y           ;
 STA INWK+4,Y           ;   * For Y =  9 swap nosev_x_lo and nosev_z_lo
 STX INWK,Y             ;   * For Y = 15 swap roofv_x_lo and roofv_z_lo
                        ;   * For Y = 21 swap sidev_x_lo and sidev_z_lo

 LDA INWK+1,Y           ; Swap the high x and z bytes for the offset in Y:
 EOR RAT                ;
 TAX                    ;   * If left view, flip sign of new z-coordinate
 LDA INWK+5,Y           ;   * If right view, flip sign of new x-coordinate
 EOR RAT2
 STA INWK+1,Y
 STX INWK+5,Y

                        ; Fall through into LOOK1 to return from the subroutine