Drawing lines: LL145 (Part 2 of 4)

       Name: LL145 (Part 2 of 4)                                     [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Clip line: Work out if any part of the line is on-screen
  Deep dive: Line-clipping
             Extended screen coordinates
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 This part does a number of tests to see if the line is on or off the screen.

 If we get here then at least one of (x1, y1) and (x2, y2) is off-screen, with
 XX13 set as follows:

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

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

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

 where "off-screen" is defined as having a non-zero high byte in one of the
 coordinates, or in the case of y-coordinates, having a low byte > Yx2M1, the
 y-coordinate of the bottom of the space view.


.LL83

 LDA XX13               ; If XX13 < 128 then only one of the points is on-screen
 BPL LL115              ; so jump down to LL115 to skip the checks of whether
                        ; both points are in the strips to the right or bottom
                        ; of the screen

                        ; If we get here, both points are off-screen

 LDA XX15+1             ; If both x1_hi and x2_hi have bit 7 set, jump to LL109
 AND XX15+5             ; to return from the subroutine with the C flag set, as
 BMI LL109              ; the entire line is above the top of the screen

 LDA XX15+3             ; If both y1_hi and y2_hi have bit 7 set, jump to LL109
 AND XX12+1             ; to return from the subroutine with the C flag set, as
 BMI LL109              ; the entire line is to the left of the screen

 LDX XX15+1             ; Set A = X = x1_hi - 1
 DEX
 TXA

 LDX XX15+5             ; Set XX12+2 = x2_hi - 1
 DEX
 STX XX12+2

 ORA XX12+2             ; If neither (x1_hi - 1) or (x2_hi - 1) have bit 7 set,
 BPL LL109              ; jump to LL109 to return from the subroutine with the C
                        ; flag set, as the line doesn't fit on-screen

 LDA XX15+2             ; If y1_lo < y-coordinate of screen bottom (which is in
 CMP screenHeight       ; the variable screenHeight), clear the C flag,
                        ; otherwise set it

 LDA XX15+3             ; Set XX12+2 = y1_hi - (1 - C), so:
 SBC #0                 ;
 STA XX12+2             ;  * Set XX12+2 = y1_hi - 1 if y1_lo is on-screen
                        ;  * Set XX12+2 = y1_hi     otherwise
                        ;
                        ; We do this subtraction because we are only interested
                        ; in trying to move the points up by a screen if that
                        ; might move the point into the space view portion of
                        ; the screen, i.e. if y1_lo is on-screen

 LDA XX12               ; If y2_lo < y-coordinate of screen bottom (which is in
 CMP screenHeight       ; the variable screenHeight), clear the C flag,
                        ; otherwise set it

 LDA XX12+1             ; Set XX12+2 = y2_hi - (1 - C), so:
 SBC #0                 ;
                        ;  * Set XX12+1 = y2_hi - 1 if y2_lo is on-screen
                        ;  * Set XX12+1 = y2_hi     otherwise
                        ;
                        ; We do this subtraction because we are only interested
                        ; in trying to move the points up by a screen if that
                        ; might move the point into the space view portion of
                        ; the screen, i.e. if y1_lo is on-screen

 ORA XX12+2             ; If neither XX12+1 or XX12+2 have bit 7 set, jump to
 BPL LL109              ; LL109 to return from the subroutine with the C flag
                        ; set, as the line doesn't fit on-screen