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⌂ ∞ ▽ Elite on the 6502

Drawing ships: LL9 (Part 3 of 12)

[NES version, Bank 1]

       Name: LL9 (Part 3 of 12)                                      [Show more]
       Type: Subroutine
   Category: Drawing ships
    Summary: Draw ship: Set up orientation vector, ship coordinate variables
  Deep dive: Drawing ships

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




 This part sets up the following variable blocks:

   * XX16 contains the orientation vectors, divided to normalise them

   * XX18 contains the ship's x, y and z coordinates in space



.LL17

 LDX #5                 ; First we copy the three orientation vectors into XX16,
                        ; so set up a counter in X for the 6 bytes in each
                        ; vector

.LL15

 LDA XX1+21,X           ; Copy the X-th byte of sidev to the X-th byte of XX16
 STA XX16,X

 LDA XX1+15,X           ; Copy the X-th byte of roofv to XX16+6 to the X-th byte
 STA XX16+6,X           ; of XX16+6

 LDA XX1+9,X            ; Copy the X-th byte of nosev to XX16+12 to the X-th
 STA XX16+12,X          ; byte of XX16+12

 DEX                    ; Decrement the counter

 BPL LL15               ; Loop back to copy the next byte of each vector, until
                        ; we have the following:
                        ;
                        ;   * XX16(1 0) = sidev_x
                        ;   * XX16(3 2) = sidev_y
                        ;   * XX16(5 4) = sidev_z
                        ;
                        ;   * XX16(7 6) = roofv_x
                        ;   * XX16(9 8) = roofv_y
                        ;   * XX16(11 10) = roofv_z
                        ;
                        ;   * XX16(13 12) = nosev_x
                        ;   * XX16(15 14) = nosev_y
                        ;   * XX16(17 16) = nosev_z

 LDA #197               ; Set Q = 197
 STA Q

 LDY #16                ; Set Y to be a counter that counts down by 2 each time,
                        ; starting with 16, then 14, 12 and so on. We use this
                        ; to work through each of the coordinates in each of the
                        ; orientation vectors

.LL21

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA XX16,Y             ; Set A = the low byte of the vector coordinate, e.g.
                        ; nosev_z_lo when Y = 16

 ASL A                  ; Shift bit 7 into the C flag

 LDA XX16+1,Y           ; Set A = the high byte of the vector coordinate, e.g.
                        ; nosev_z_hi when Y = 16

 ROL A                  ; Rotate A left, incorporating the C flag, so A now
                        ; contains the original high byte, doubled, and without
                        ; a sign bit, e.g. A = |nosev_z_hi| * 2

 JSR LL28               ; Call LL28 to calculate:
                        ;
                        ;   R = 256 * A / Q
                        ;
                        ; so, for nosev, this would be:
                        ;
                        ;   R = 256 * |nosev_z_hi| * 2 / 197
                        ;     = 2.6 * |nosev_z_hi|

 LDX R                  ; Store R in the low byte's location, so we can keep the
 STX XX16,Y             ; old, unscaled high byte intact for the sign

 DEY                    ; Decrement the loop counter twice
 DEY

 BPL LL21               ; Loop back for the next vector coordinate until we have
                        ; divided them all

                        ; By this point, the vectors have been turned into
                        ; scaled magnitudes, so we have the following:
                        ;
                        ;   * XX16   = scaled |sidev_x|
                        ;   * XX16+2 = scaled |sidev_y|
                        ;   * XX16+4 = scaled |sidev_z|
                        ;
                        ;   * XX16+6  = scaled |roofv_x|
                        ;   * XX16+8  = scaled |roofv_y|
                        ;   * XX16+10 = scaled |roofv_z|
                        ;
                        ;   * XX16+12 = scaled |nosev_x|
                        ;   * XX16+14 = scaled |nosev_y|
                        ;   * XX16+16 = scaled |nosev_z|

 LDX #8                 ; Next we copy the ship's coordinates into XX18, so set
                        ; up a counter in X for 9 bytes

.ll91

 LDA XX1,X              ; Copy the X-th byte from XX1 to XX18
 STA XX18,X

 DEX                    ; Decrement the loop counter

 BPL ll91               ; Loop back for the next byte until we have copied all
                        ; three coordinates

                        ; So we now have the following:
                        ;
                        ;   * XX18(2 1 0) = (x_sign x_hi x_lo)
                        ;
                        ;   * XX18(5 4 3) = (y_sign y_hi y_lo)
                        ;
                        ;   * XX18(8 7 6) = (z_sign z_hi z_lo)

 LDA #255               ; Set the 15th byte of XX2 to 255, so that face 15 is
 STA XX2+15             ; always visible. No ship definitions actually have this
                        ; number of faces, but this allows us to force a vertex
                        ; to always be visible by associating it with face 15
                        ; (see the ship blueprints for the Cobra Mk III at
                        ; SHIP_COBRA_MK_3 and the asteroid at SHIP_ASTEROID for
                        ; examples of vertices that are associated with face 15)

 LDY #12                ; Set Y = 12 to point to the ship blueprint byte #12,

 LDA XX1+31             ; If bit 5 of the ship's byte #31 is clear, then the
 AND #%00100000         ; ship is not currently exploding, so jump down to EE29
 BEQ EE29               ; to skip the following

                        ; Otherwise we fall through to set up the visibility
                        ; block for an exploding ship