Drawing lines: LOIN (Part 2 of 7)

       Name: LOIN (Part 2 of 7)                                      [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a line: Line has a shallow gradient, step right along x-axis
  Deep dive: Bresenham's line algorithm
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


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

   * |delta_y| < |delta_x|

   * The line is closer to being horizontal than vertical

   * We are going to step right along the x-axis

   * We potentially swap coordinates to make sure X1 < X2


.STPX

 LDX X1                 \ Set X = X1

 CPX X2                 \ If X1 < X2, jump down to LI3, as the coordinates are
 BCC LI3                \ already in the order that we want

 DEC SWAP               \ Otherwise decrement SWAP from 0 to &FF, to denote that
                        \ we are swapping the coordinates around

 LDA X2                 \ Swap the values of X1 and X2
 STA X1
 STX X2

 TAX                    \ Set X = X1

 LDA Y2                 \ Swap the values of Y1 and Y2
 LDY Y1
 STA Y1
 STY Y2

.LI3

                        \ By this point we know the line is horizontal-ish and
                        \ X1 < X2, so we're going from left to right as we go
                        \ from X1 to X2

 LDA Y1                 \ Set A = Y1 / 8, so A now contains the character row
 LSR A                  \ that will contain our horizontal line
 LSR A
 LSR A

 ORA #&60               \ As A < 32, this effectively adds &60 to A, which gives
                        \ us the screen address of the character row (as each
                        \ character row takes up 256 bytes, and the first
                        \ character row is at screen address &6000, or page &60)

 STA SCH                \ Store the page number of the character row in SCH, so
                        \ the high byte of SC is set correctly for drawing the
                        \ start of our line

 LDA Y1                 \ Set Y = Y1 mod 8, which is the pixel row within the
 AND #7                 \ character block at which we want to draw the start of
 TAY                    \ our line (as each character block has 8 rows)

 TXA                    \ Set A = bits 3-7 of X1
 AND #%11111000

 STA SC                 \ Store this value in SC, so SC(1 0) now contains the
                        \ screen address of the far left end (x-coordinate = 0)
                        \ of the horizontal pixel row that we want to draw the
                        \ start of our line on

 TXA                    \ Set X = X1 mod 8, which is the horizontal pixel number
 AND #7                 \ within the character block where the line starts (as
 TAX                    \ each pixel line in the character block is 8 pixels
                        \ wide)

 LDA TWOS,X             \ Fetch a 1-pixel byte from TWOS where pixel X is set,
 STA R                  \ and store it in R

                        \ The following calculates:
                        \
                        \   Q = Q / P
                        \     = |delta_y| / |delta_x|
                        \
                        \ using the same shift-and-subtract algorithm that's
                        \ documented in TIS2

 LDA Q                  \ Set A = |delta_y|

 LDX #%11111110         \ Set Q to have bits 1-7 set, so we can rotate through 7
 STX Q                  \ loop iterations, getting a 1 each time, and then
                        \ getting a 0 on the 8th iteration... and we can also
                        \ use Q to catch our result bits into bit 0 each time

.LIL1

 ASL A                  \ Shift A to the left

 BCS LI4                \ If bit 7 of A was set, then jump straight to the
                        \ subtraction

 CMP P                  \ If A < P, skip the following subtraction
 BCC LI5

.LI4

 SBC P                  \ A >= P, so set A = A - P

 SEC                    \ Set the C flag to rotate into the result in Q

.LI5

 ROL Q                  \ Rotate the counter in Q to the left, and catch the
                        \ result bit into bit 0 (which will be a 0 if we didn't
                        \ do the subtraction, or 1 if we did)

 BCS LIL1               \ If we still have set bits in Q, loop back to TIL2 to
                        \ do the next iteration of 7

                        \ We now have:
                        \
                        \   Q = A / P
                        \     = |delta_y| / |delta_x|
                        \
                        \ and the C flag is clear

 LDX P                  \ Set X = P + 1
 INX                    \       = |delta_x| + 1
                        \
                        \ We add 1 so we can skip the first pixel plot if the
                        \ line is being drawn with swapped coordinates

 LDA Y2                 \ Set A = Y2 - Y1 - 1 (as the C flag is clear following
 SBC Y1                 \ the above division)

 BCS DOWN               \ If Y2 >= Y1 - 1 then jump to DOWN, as we need to draw
                        \ the line to the right and down