3D geometry: ProjectPoint (Part 3 of 3)

       Name: ProjectPoint (Part 3 of 3)                              [Show more]
       Type: Subroutine
   Category: 3D geometry
    Summary: Move the projected coordinates to the centre of the screen and
             update the point's status byte
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


                        \ By this point we have the following projected
                        \ coordinates for the point whose ID is in GG:
                        \
                        \   (SS QQ) = screen x-coordinate of 3D point projected
                        \             onto the screen
                        \
                        \   (RR Q) = screen y-coordinate of 3D point projected
                        \             onto the screen
                        \
                        \ We now move the projected coordinate to the correct
                        \ place on screen, as the projected coordinates have the
                        \ origin straight ahead of us, i.e. in the centre of the
                        \ screen, while the screen has the origin in the
                        \ bottom-left corner
                        \
                        \ In other words, we add (80, 96) to the projected
                        \ coordinates so that a projected coordinate of (0, 0),
                        \ which is straight ahead of us, will appear in the
                        \ centre of the canopy view, which is 160 x 192 pixels
                        \ in size
                        \
                        \ We also set the correct sign for the projected
                        \ coordinate

 LDX GG                 \ Set X to the point ID to process

 LDA zPointHi,X         \ If the point's z-coordinate is negative, jump to
 BMI proj14             \ proj14

 LDA xPointHi,X         \ If the point's x-coordinate is positive, jump to
 BPL proj16             \ proj16 as the coordinates have the same sign

 JMP proj15             \ Otherwise jump to proj15 as the coordinates have
                        \ opposite signs

.proj14

                        \ If we get here then the point's z-coordinate is
                        \ negative

 LDA xPointHi,X         \ If the point's x-coordinate is negative, jump to
 BMI proj16             \ proj16 as the coordinates have the same sign

.proj15

                        \ If we get here then either the point's z-coordinate is
                        \ positive and the point's x-coordinate is negative, or
                        \ the point's z-coordinate is negative and the point's
                        \ x-coordinate is positive - in other words, the point's
                        \ x- and z-coordinates have opposite signs

 LDA #80                \ Set (xPointHi xPointLo) = 80 - (SS QQ)
 SEC                    \
 SBC QQ                 \ starting with the low bytes
 STA xPointLo,X

 LDA #0                 \ And then the high bytes
 SBC SS
 STA xPointHi,X

 JMP proj17             \ Jump to proj17 to move on to the y-coordinate

.proj16

                        \ If we get here then either the point's z-coordinate is
                        \ positive and the point's x-coordinate is positive, or
                        \ the point's z-coordinate is negative and the point's
                        \ x-coordinate is negative - in other words, the point's
                        \ x- and z-coordinates have the same sign

 LDA #80                \ Set (xPointHi xPointLo) = 80 + (SS QQ)
 CLC                    \
 ADC QQ                 \ starting with the low bytes
 STA xPointLo,X

 LDA #0                 \ And then the high bytes
 ADC SS
 STA xPointHi,X

.proj17

                        \ We now move the y-coordinate in the projected result
                        \ to the correct place on screen, by adding 96 to the
                        \ result and setting the correct sign in the process

 LDX GG                 \ Set X to the point ID to process

 LDA zPointHi,X         \ If the point's z-coordinate is negative, jump to
 BMI proj18             \ proj18

 LDA yPointHi,X         \ If the point's y-coordinate is positive, jump to
 BPL proj20             \ proj20 as the coordinates have the same sign

 JMP proj19             \ Otherwise jump to proj19 as the coordinates have
                        \ opposite signs

.proj18

                        \ If we get here then the point's z-coordinate is
                        \ negative

 LDA yPointHi,X         \ If the point's y-coordinate is negative, jump to
 BMI proj20             \ proj20 as the coordinates have the same sign

.proj19

                        \ If we get here then either the point's z-coordinate is
                        \ positive and the point's y-coordinate is negative, or
                        \ the point's z-coordinate is negative and the point's
                        \ y-coordinate is positive - in other words, the point's
                        \ y- and z-coordinates have opposite signs

 LDA #96                \ Set (yPointHi yPointLo) = 96 - (RR Q)
 SEC                    \
 SBC Q                  \ starting with the low bytes
 STA yPointLo,X

 LDA #0                 \ And then the high bytes
 SBC RR
 STA yPointHi,X

 JMP proj21             \ Jump to proj17 to move on to the point's status byte

.proj20

 LDA #96                \ Set (yPointHi yPointLo) = 96 + (RR Q)
 CLC                    \
 ADC Q                  \ starting with the low bytes
 STA yPointLo,X

 LDA #0                 \ And then the high bytes
 ADC RR
 STA yPointHi,X

.proj21

 LDA pointStatus,X      \ Apply any set bits from N to the point's status byte
 ORA N
 STA pointStatus,X

 RTS                    \ Return from the subroutine