Drawing lines: LL145 (Part 1 of 4)

       Name: LL145 (Part 1 of 4)                                     [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Clip line: Work out which end-points are on-screen, if any
  Deep dive: Line-clipping
             Extended screen coordinates
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BLINE calls via CLIP
             * CLIP_b1 calls via CLIP
             * LL9 (Part 9 of 12) calls via CLIP
             * LL9 (Part 10 of 12) calls via CLIP2


 This routine clips the line from (x1, y1) to (x2, y2) so it fits on-screen, or
 returns an error if it can't be clipped to fit. The arguments are 16-bit
 coordinates, and the clipped line is returned using 8-bit screen coordinates.

 This part sets XX13 to reflect which of the two points are on-screen and
 off-screen.


 Arguments:

   XX15(1 0)            x1 as a 16-bit coordinate (x1_hi x1_lo)

   XX15(3 2)            y1 as a 16-bit coordinate (y1_hi y1_lo)

   XX15(5 4)            x2 as a 16-bit coordinate (x2_hi x2_lo)

   XX12(1 0)            y2 as a 16-bit coordinate (y2_hi y2_lo)


 Returns:

   (X1, Y1)             Screen coordinate of the start of the clipped line

   (X2, Y2)             Screen coordinate of the end of the clipped line

   C flag               Clear if the clipped line fits on-screen, set if it
                        doesn't

   XX13                 The state of the original coordinates on-screen:

                          * 0   = (x2, y2) on-screen

                          * 127 = (x1, y1) on-screen,  (x2, y2) off-screen

                          * 255 = (x1, y1) off-screen, (x2, y2) off-screen

                        So XX13 is non-zero if the end of the line was clipped,
                        meaning the next line sent to BLINE can't join onto the
                        end but has to start a new segment

   SWAP                 The swap status of the returned coordinates:

                          * $FF if we swapped the values of (x1, y1) and
                            (x2, y2) as part of the clipping process

                          * 0 if the coordinates are still in the same order

   Y                    Y is preserved


 Other entry points:

   CLIP                 Another name for LL145

   CLIP2                Don't initialise the values in SWAP or A


.LL145

.CLIP

 LDA #0                 ; Set SWAP = 0
 STA SWAP

 LDA XX15+5             ; Set A = x2_hi

.CLIP2

 LDX #255               ; Set X = 255, the highest y-coordinate possible, beyond
                        ; the bottom of the screen

 ORA XX12+1             ; If one or both of x2_hi and y2_hi are non-zero, jump
 BNE LL107              ; to LL107 to skip the following, leaving X at 255

 LDA Yx2M1              ; If y2_lo > the y-coordinate of the bottom of screen
 CMP XX12               ; (which is in the variable Yx2M1), then (x2, y2) is off
 BCC LL107              ; the bottom of the screen, so skip the following
                        ; instruction, leaving X at 255

 LDX #0                 ; Set X = 0

.LL107

 STX XX13               ; Set XX13 = X, so we have:
                        ;
                        ;   * XX13 = 0 if x2_hi = y2_hi = 0, y2_lo is on-screen
                        ;
                        ;   * XX13 = 255 if x2_hi or y2_hi are non-zero or y2_lo
                        ;            is off the bottom of the screen
                        ;
                        ; In other words, XX13 is 255 if (x2, y2) is off-screen,
                        ; otherwise it is 0

 LDA XX15+1             ; If one or both of x1_hi and y1_hi are non-zero, jump
 ORA XX15+3             ; to LL83
 BNE LL83

 LDA Yx2M1              ; If y1_lo > the y-coordinate of the bottom of screen
 CMP XX15+2             ; (which is in the variable Yx2M1),  then (x1, y1) is
 BCC LL83               ; off the bottom of the screen, so jump to LL83

                        ; If we get here, (x1, y1) is on-screen

 LDA XX13               ; If XX13 is non-zero, i.e. (x2, y2) is off-screen, jump
 BNE LL108              ; to LL108 to halve it before continuing at LL83

                        ; If we get here, the high bytes are all zero, which
                        ; means the x-coordinates are < 256 and therefore fit on
                        ; screen, and neither coordinate is off the bottom of
                        ; the screen. That means both coordinates are already on
                        ; screen, so we don't need to do any clipping, all we
                        ; need to do is move the low bytes into (X1, Y1) and
                        ; X2, Y2) and return

.LL146

                        ; If we get here then we have clipped our line to the
                        ; screen edge (if we had to clip it at all), so we move
                        ; the low bytes from (x1, y1) and (x2, y2) into (X1, Y1)
                        ; and (X2, Y2), remembering that they share locations
                        ; with XX15:
                        ;
                        ;   X1 = XX15
                        ;   Y1 = XX15+1
                        ;   X2 = XX15+2
                        ;   Y2 = XX15+3
                        ;
                        ; X1 already contains x1_lo, so now we do the rest

 LDA XX15+2             ; Set Y1 (aka XX15+1) = y1_lo
 STA XX15+1

 LDA XX15+4             ; Set X2 (aka XX15+2) = x2_lo
 STA XX15+2

 LDA XX12               ; Set Y2 (aka XX15+3) = y2_lo
 STA XX15+3

 CLC                    ; Clear the C flag as the clipped line fits on-screen

 RTS                    ; Return from the subroutine

.LL109

 SEC                    ; Set the C flag to indicate the clipped line does not
                        ; fit on-screen

 RTS                    ; Return from the subroutine

.LL108

 LSR XX13               ; If we get here then (x2, y2) is off-screen and XX13 is
                        ; 255, so shift XX13 right to halve it to 127