Drawing lines: LOIN (Part 7 of 7)

       Name: LOIN (Part 7 of 7)                                      [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a steep line going up and right or down and left
  Deep dive: Bresenham's line algorithm
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * HLOIN calls via HL6


 This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages.
 If we get here, then:

   * The line is going up and right (no swap) or down and left (swap)

   * X1 >= X2 and Y1 >= Y2

   * Draw from (X1, Y1) at bottom left to (X2, Y2) at top right, omitting the
     first pixel


 Other entry points:

   HL6                  Contains an RTS


.LFT

 LDA SWAP               \ If SWAP = 0 then we didn't swap the coordinates above,
 BEQ LI18               \ so jump down to LI18 to skip plotting the first pixel

 DEX                    \ Decrement the counter in X because we're about to plot
                        \ the first pixel

.LIL6

 LDA R                  \ Fetch the pixel byte from R

 EOR (SC),Y             \ Store R into screen memory at SC(1 0), using EOR
 STA (SC),Y             \ logic so it merges with whatever is already on-screen

.LI18

 DEY                    \ Decrement Y to step up along the y-axis

 BPL LI19               \ If Y is positive we are still within the same
                        \ character block, so skip to LI19

 DEC SCH                \ Otherwise we need to move up into the character block
 LDY #7                 \ above, so decrement the high byte of the screen
                        \ address and set the pixel line to the last line in
                        \ that character block

.LI19

 LDA S                  \ Set S = S + P to update the slope error
 ADC P
 STA S

 BCC LIC6               \ If the addition didn't overflow, jump to LIC6

 ASL R                  \ Otherwise we just overflowed, so shift the single
                        \ pixel in R to the left, so the next pixel we plot
                        \ will be at the previous x-coordinate

 BCC LIC6               \ If the pixel didn't fall out of the left end of R
                        \ into the C flag, then jump to LIC6

 ROL R                  \ Otherwise we need to move over to the next character
                        \ block, so first rotate R left so the set C flag goes
                        \ back into the right end, giving %0000001

 LDA SC                 \ Subtract 7 from SC, so SC(1 0) now points to the
 SBC #7                 \ previous character along to the left
 STA SC

 CLC                    \ Clear the C flag so it doesn't affect the additions
                        \ below

.LIC6

 DEX                    \ Decrement the counter in X

 BNE LIL6               \ If we haven't yet reached the left end of the line,
                        \ loop back to LIL6 to plot the next pixel along

.HL6

 RTS                    \ Return from the subroutine