Drawing objects: DrawCarOrSign

       Name: DrawCarOrSign                                           [Show more]
       Type: Subroutine
   Category: Drawing objects
    Summary: Draw a car or sign
  Deep dive: Object definitions
             Drawing a 3D car from 2D parts
             Road signs
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawCars calls DrawCarOrSign
             * MainDrivingLoop (Part 2 of 5) calls DrawCarOrSign


 This routine is used to draw road signs and cars.


 Arguments:

   X                    The car or sign to draw:

                          * 0-19 = Draw the car for this driver number

                          * 20-22 = Draw one of the three extra objects that
                                    make up the four-object car:

                            * 20 = rear wing

                            * 21 = body and helmet

                            * 22 = front tyres

                          * 23 = Draw the road sign

   xStoreDraw           The value to restore into X at the end of the routine


 Returns:

   X                    X is set to xStoreDraw


.DrawCarOrSign

 LDA objectStatus,X     \ Set A to this object's status byte

 BMI dcas3              \ If bit 7 is set then the object is not visible, so
                        \ jump to dcas3 to return from the subroutine without
                        \ drawing anything

 AND #%00001111         \ Extract the object's object type from bits 0-3 and
 STA objectType         \ store it in objectType

 LDA objYawAngleLo,X    \ Set (A T) =   objYawAngle for this object
 SEC                    \             - playerYawAngle
 SBC playerYawAngleLo   \
 STA T                  \ starting with the low bytes

 LDA objYawAngleHi,X    \ And then the high bytes
 SBC playerYawAngleHi   \
                        \ So (A T) now contains the amount that the object we
                        \ are drawing is to the left or right of the player's
                        \ car

 BPL dcas1              \ If the result is positive, jump to dcas1 to perform a
                        \ positive comparison

 CMP #&E0               \ The result is negative, so check to see if A < -32,
 BCC dcas3              \ and if so, jump to dcas3 to return from the subroutine
                        \ without drawing anything

 BCS dcas2              \ Jump to dcas2 (this BCS is effectively a JMP as we
                        \ just passed through a BCC)

.dcas1

 CMP #32                \ The result is positive, so check to see if A >= 32,
 BCS dcas3              \ and if so, jump to dcas3 to return from the subroutine
                        \ without drawing anything

.dcas2

                        \ If we get here then -32 <= A < 32

 ASL T                  \ Set (A T) = (A T) * 4
 ROL A                  \
 ASL T                  \ so -128 <= A < 128
 ROL A

 CLC                    \ Set xPixelCoord = 80 + A
 ADC #80                \
 STA xPixelCoord        \ This moves xPixelCoord so that it is centred on the
                        \ screen, as the centre x-coordinate of the screen is
                        \ at 80 pixels

 LDA objectPitchAngle,X \ Set yPixelCoord to this object's pitch angle
 STA yPixelCoord

 LDA objectSize,X       \ Set scaleUp to this object's size (i.e. the size of
 STA scaleUp            \ the car)

 JSR DrawObject         \ Draw the object on-screen

.dcas3

 LDX xStoreDraw         \ Set X = xStoreDraw so X is unchanged by the routine
                        \ call

 RTS                    \ Return from the subroutine