Drawing the track: DrawSegmentEdge (Part 6 of 7)

       Name: DrawSegmentEdge (Part 6 of 7)                           [Show more]
       Type: Subroutine
   Category: Drawing the track
    Summary: Calculate the dash data block for the edge
  Deep dive: Drawing the track verges
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 This part draws the edge for this segment by jumping to the correct drawing
 routine, depending on the gradient and direction of the edge.


 LDA SS                 \ If SS < TT, then the edge has a steep gradient, so
 CMP TT                 \ jump to dver26 to jump to the correct drawing routine
 BCC dver26

 LDA objectPalette      \ Set A to the first verge pixel mask that we stored in
                        \ part 3, which is always a four-pixel, single colour
                        \ pixel byte

 CMP #%11111111         \ If the background colour of the verge pixel mask is
 BEQ dver21             \ green, jump to dver21 to skip the following

 LDA backgroundLeft     \ Extract bits 0-1 from backgroundLeft, which contain
 AND #%00000011         \ the colour from the left side of the verge pixel mask,
                        \ i.e. the pixels on the left side of this verge edge

 CMP #3                 \ If %ab = 3, so the colour to the left of the verge
 BEQ dver21             \ edge is green, jump to dver21 to skip the following

                        \ Otherwise, disable the DrawGrass routines

 LDA #&60               \ Set A to the opcode for the RTS instruction

 STA DrawGrassLeft      \ Modify the DrawGrassLeft routine to start with an RTS,
                        \ so it returns without doing anything (so it's
                        \ effectively disabled)

 STA DrawGrassRight     \ Modify the DrawGrassRight routine to start with an
                        \ RTS, so it returns without doing anything (so it's
                        \ effectively disabled)

 BNE dver23             \ Jump to dver23 (this BNE is effectively a JMP as A is
                        \ never zero)

.dver21

 LDA #&E0               \ Set A to the opcode for the CPX #128 instruction

 STA DrawGrassLeft      \ Revert the DrawGrassLeft routine to start with a CPX
                        \ instruction, so it runs as normal

 STA DrawGrassRight     \ Revert the DrawGrassRight routine start with a CPX
                        \ instruction, so it runs as normal

 LDA vergeType          \ Set A to the type of verge we are drawing

 CMP #2                 \ Set bit 7 of A if vergeType >= 2, so we get:
 ROR A                  \
                        \   * Bit 7 clear when we are drawing leftVergeStart or
                        \     leftTrackStart
                        \
                        \   * Bit 7 set when we are drawing rightVergeStart or
                        \     rightGrassStart

 EOR VV                 \ If bit 7 of VV is the same as bit 7 of A, then:
 BPL dver23             \
                        \   * We are drawing the left track verge and drawing
                        \     the edge from left to right (both bits are clear)
                        \
                        \   * We are drawing the right track verge and drawing
                        \     the edge from right to left (both bits are set)
                        \
                        \ In either case, jump to dver23 to skip modifying the
                        \ DrawVergeByteLeft and DrawVergeByteRight routines

                        \ If we get here, then:
                        \
                        \   * We are drawing the left track verge and drawing
                        \     the edge from right to left
                        \
                        \   * We are drawing the right track verge and drawing
                        \     the edge from left to right
                        \
                        \ We now modify both the DrawVergeByteLeft and
                        \ DrawVergeByteRight routines to increment or decrement
                        \ Y on entry, and to do nothing on exit (which flips the
                        \ modifications we did in part 3, which made them do
                        \ nothing on entry and the update of Y on exit)

 LDA verl5              \ Fetch the instruction that the DrawVergeByteLeft and
                        \ DrawVergeByteRight routines currently execute on exit
                        \ (which will be INY or DEY)

 STA verl1              \ Modify the instruction at verl1, so the
                        \ DrawVergeByteLeft routine now does this action on
                        \ entry instead

 STA verb1              \ Modify the instruction at verb1, so the
                        \ DrawVergeByteRight routine now does this action on
                        \ entry instead

 LDA #&EA               \ Set A to the opcode for the NOP instruction

 STA verl5              \ Modify the instruction at verl5, so the
                        \ DrawVergeByteLeft routine now does nothing on exit

 STA verb5              \ Modify the instruction at verb5, so the
                        \ DrawVergeByteRight routine now does nothing on exit

                        \ We also do the first increment or decrement, to get
                        \ things set up correctly, depending on the sign in WW,
                        \ which determines whether the edge is going up or down
                        \ the screen

 LDA WW                 \ If WW is positive, then the line is heading up the
 BPL dver22             \ screen, to jump to dver22 to decrement Y before
                        \ drawing the edge

 INY                    \ WW is negative, so the line is heading down the
                        \ screen, so increment Y

 JMP dver23             \ Jump to dver23 to draw the edge

.dver22

 DEY                    \ WW is positive, so decrement Y and fall through into
                        \ dver23 to draw the edge

.dver23

                        \ If we get here then SS >= TT, so the edge has a
                        \ gradient shallow

 LDA VV                 \ If VV is positive, jump to dver25 to draw the edge
 BPL dver25             \ from left to right

 JSR DrawShallowToLeft  \ VV is negative, so draw the edge from right to left

.dver24

 JMP dver28             \ Jump to dver28 to clean up and return from the
                        \ subroutine

.dver25

 JSR DrawShallowToRight \ VV is positive, so draw the edge from left to right

 JMP dver28             \ Jump to dver28 to clean up and return from the
                        \ subroutine

.dver26

                        \ If we get here then SS < TT, so the edge has a steep
                        \ gradient

 LDA VV                 \ If VV is positive, jump to dver27 to draw the edge
 BPL dver27             \ from left to right

 JSR DrawSteepToLeft    \ VV is negative, so draw the edge from right to left

 JMP dver28             \ Jump to dver28 to clean up and return from the
                        \ subroutine

.dver27

 JSR DrawSteepToRight   \ VV is positive, so draw the edge from left to right