Version analysis of PROJ

       Name: PROJ
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Project the current ship or planet onto the screen
  Deep dive: Extended screen coordinates


 Project the current ship's location or the planet onto the screen, either
 returning the screen coordinates of the projection (if it's on-screen), or
 returning an error via the C flag.

 In this context, "on-screen" means that the point is projected into the
 following range:

   centre of screen - 1024 < x < centre of screen + 1024
   centre of screen - 1024 < y < centre of screen + 1024

Cassette, Flight, 6502SP, Master
Electron
 This is to cater for ships (and, more likely, planets and suns) whose centres
 are off-screen but whose edges may still be visible.
 This is to cater for ships (and, more likely, planets) whose centres are
 off-screen but whose edges may still be visible.

 The projection calculation is:

   K3(1 0) = #X + x / z
   K4(1 0) = #Y + y / z

 where #X and #Y are the pixel x-coordinate and y-coordinate of the centre of
 the screen.


 Arguments:

   INWK                 The ship data block for the ship to project on-screen


 Returns:

   K3(1 0)              The x-coordinate of the ship's projection on-screen

   K4(1 0)              The y-coordinate of the ship's projection on-screen

   C flag               Set if the ship's projection doesn't fit on the screen,
                        clear if it does project onto the screen

   A                    Contains K4+1, the high byte of the y-coordinate

Electron
 Other entry points:

   RTS2                 Contains an RTS

.PROJ

 LDA INWK               \ Set P(1 0) = (x_hi x_lo)
 STA P                  \            = x
 LDA INWK+1
 STA P+1

 LDA INWK+2             \ Set A = x_sign

 JSR PLS6               \ Call PLS6 to calculate:
                        \
                        \   (X K) = (A P+1 P) / (z_sign z_hi z_lo)
                        \         = (x_sign x_hi x_lo) / (z_sign z_hi z_lo)
                        \         = x / z

 BCS PL2-1              \ If the C flag is set then the result overflowed and
                        \ the coordinate doesn't fit on the screen, so return
                        \ from the subroutine with the C flag set (as PL2-1
                        \ contains an RTS)

 LDA K                  \ Set K3(1 0) = (X K) + #X
 ADC #X                 \             = #X + x / z
 STA K3                 \
                        \ first doing the low bytes

 TXA                    \ And then the high bytes. #X is the x-coordinate of
 ADC #0                 \ the centre of the space view, so this converts the
 STA K3+1               \ space x-coordinate into a screen x-coordinate

 LDA INWK+3             \ Set P(1 0) = (y_hi y_lo)
 STA P
 LDA INWK+4
 STA P+1

 LDA INWK+5             \ Set A = -y_sign
 EOR #%10000000

 JSR PLS6               \ Call PLS6 to calculate:
                        \
                        \   (X K) = (A P+1 P) / (z_sign z_hi z_lo)
                        \         = -(y_sign y_hi y_lo) / (z_sign z_hi z_lo)
                        \         = -y / z

 BCS PL2-1              \ If the C flag is set then the result overflowed and
                        \ the coordinate doesn't fit on the screen, so return
                        \ from the subroutine with the C flag set (as PL2-1
                        \ contains an RTS)

 LDA K                  \ Set K4(1 0) = (X K) + #Y
 ADC #Y                 \             = #Y - y / z
 STA K4                 \
                        \ first doing the low bytes

 TXA                    \ And then the high bytes. #Y is the y-coordinate of
 ADC #0                 \ the centre of the space view, so this converts the
 STA K4+1               \ space x-coordinate into a screen y-coordinate

 CLC                    \ Clear the C flag to indicate success

Electron
.RTS2

 RTS                    \ Return from the subroutine