Drawing lines: LOIN (Part 1 of 7)

       Name: LOIN (Part 1 of 7)                                      [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a line: Calculate the line gradient in the form of deltas
  Deep dive: Bresenham's line algorithm
             Drawing lines in the NES version
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BLINE calls LOIN
             * CLIP_b1 calls LOIN
             * DrawLightning calls LOIN
             * LASLI calls LOIN
             * LL9 (Part 9 of 12) calls LOIN
             * LL9 (Part 10 of 12) calls LOIN
             * SHPPT calls LOIN
             * TT15 calls LOIN


 This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages.
 This stage calculates the line deltas.


 Arguments:

   X1                   The screen x-coordinate of the start of the line

   Y1                   The screen y-coordinate of the start of the line

   X2                   The screen x-coordinate of the end of the line

   Y2                   The screen y-coordinate of the end of the line


.LOIN

 STY YSAV               ; Store Y into YSAV, so we can preserve it across the
                        ; call to this subroutine

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA #128               ; Set S = 128, which is the starting point for the
 STA S                  ; slope error (representing half a pixel)

 ASL A                  ; Set SWAP = 0, as %10000000 << 1 = 0
 STA SWAP

 LDA X2                 ; Set A = X2 - X1
 SBC X1                 ;       = delta_x
                        ;
                        ; This subtraction works as the ASL A above sets the C
                        ; flag

 BCS LI1                ; If X2 > X1 then A is already positive and we can skip
                        ; the next three instructions

 EOR #%11111111         ; Negate the result in A by flipping all the bits and
 ADC #1                 ; adding 1, i.e. using two's complement to make it
                        ; positive

.LI1

 STA P                  ; Store A in P, so P = |X2 - X1|, or |delta_x|

 SEC                    ; Set the C flag, ready for the subtraction below

 LDA Y2                 ; Set A = Y2 - Y1
 SBC Y1                 ;       = delta_y
                        ;
                        ; This subtraction works as we either set the C flag
                        ; above, or we skipped that SEC instruction with a BCS

 BCS LI2                ; If Y2 > Y1 then A is already positive and we can skip
                        ; the next two instructions

 EOR #%11111111         ; Negate the result in A by flipping all the bits and
 ADC #1                 ; adding 1, i.e. using two's complement to make it
                        ; positive

.LI2

 STA Q                  ; Store A in Q, so Q = |Y2 - Y1|, or |delta_y|

 CMP P                  ; If Q < P, jump to STPX to step along the x-axis, as
 BCC STPX               ; the line is closer to being horizontal than vertical

 JMP STPY               ; Otherwise Q >= P so jump to STPY to step along the
                        ; y-axis, as the line is closer to being vertical than
                        ; horizontal