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⌂ ∞ Aviator on the BBC Micro

Drawing lines: DrawClippedLine (Part 4 of 6)

       Name: DrawClippedLine (Part 4 of 6)                           [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Calculate the starting point and direction for our clipped vector
             line

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




                        \ In this part, the end-goal is to calculate the start
                        \ point and direction for an on-screen vector line,
                        \ which we can pass them to the DrawCanopyLine routine
                        \ later on
                        \
                        \ This means we want to calculate a pixel coordinate in
                        \ (R, S) and a direction in V, by clipping the current
                        \ start and end points to fit on-screen, if necessary

 LDX L                  \ Set X to the point ID for the line's start point

 LDY M                  \ Set Y to the point ID for the line's end point

 LDA zPointHi,Y         \ If the z-coordinate for the line's end point is
 BPL draw10             \ positive, then it's in front of us, so jump to draw10

 LDA V                  \ The end point is behind us, so flip bits 6 and 7 in V
 EOR #%11000000         \ to reverse the line direction in both axes
 STA V

                        \ The end point is behind us, so we can't use the end
                        \ point as our vector line's starting point, so now we
                        \ check whether we can use the start point

 LDA TT                 \ If TT is zero then the start point is on-screen, so
 BEQ draw15             \ jump to draw15 to use the current values of (R, S) as
                        \ our pixel coordinate, which works because RR and SS
                        \ have to be zero for the start point to be on-screen,
                        \ so (RR R) = (0 R) = R and (SS S) = (0 S) = S, so we
                        \ can just use the low bytes as the two coordinates,
                        \ i.e. (R, S)

 BNE draw12             \ TT is non-zero, so the start point is off-screen, so
                        \ jump to draw12 to clip the start of the line so it
                        \ fits on-screen (this BNE is effectively a JMP as A is
                        \ never zero)

.draw10

                        \ If we get here then the end point is in front of us

 LDA zPointHi,X         \ If the z-coordinate for the line's start point is
 BPL draw11             \ positive, which is in front of us, jump to draw11

 JSR SwapLinePoints     \ The start point is behind us and the end point is in
                        \ front of us, so copy the end point's coordinates and
                        \ clipping information into the start point, so the
                        \ start point is now in front of us

 LDA TT                 \ If TT is zero then the start point is on-screen, so
 BEQ draw15             \ jump to draw15

 BNE draw12             \ TT is non-zero, so the start point is off-screen, so
                        \ jump to draw12 to clip the start of the line (this BNE
                        \ is effectively a JMP as A is never zero)

.draw11

                        \ If we get here then both the start and end points are
                        \ in front of us

 LDA TT                 \ If TT is non-zero then the start point is off-screen,
 BNE draw14             \ so jump to draw14 to potentially clip from the end of
                        \ the line (i.e. if the end point is off-screen)

 LDA UU                 \ If UU is non-zero then the end point is off-screen, so
 BNE draw15             \ jump to draw13 via draw15 to clip from the end of the
                        \ line

 BEQ draw21             \ Both TT and UU are zero, so both points are on-screen
                        \ and we don't need to do any clipping, so jump to
                        \ draw21

.draw12

                        \ If we get here then the one point is off-screen but
                        \ in front of us, and the other point is behind us and
                        \ can't be used as our vector line starting point, and
                        \ we've set the start point to be the point that is in
                        \ front of us

 JSR ClipStartOfLine    \ Clip the line at the start to move the start point
                        \ on-screen, so we can use it as our vector line's
                        \ starting point

 JMP draw15             \ We now have an on-screen pixel coordinate in (R, S),
                        \ so jump to draw15 to move on to the next stage

.draw13

                        \ If we get here then both the start and end points are
                        \ off-screen, but they are both in front of us, so we
                        \ need to clip both ends of the line

 JSR ClipBestEndOfLine  \ Clip the line at either the start or end point,
                        \ whichever is best, so that it fits on-screen

 JMP draw15             \ We now have an on-screen pixel coordinate in (R, S),
                        \ so jump to draw15 to move on to the next stage

.draw14

                        \ If we get here then both the start and end points are
                        \ in front of us and the start point is off-screen

 LDA UU                 \ If UU is non-zero then the end point is also
 BNE draw13             \ off-screen, so jump to draw13 to clip both ends of the
                        \ line

                        \ If we get here then the start point is off-screen but
                        \ the end point is on-screen, so we now use the end
                        \ point for our vector line's starting point

 LDA V                  \ Flip bits 6 and 7 in V to reverse the line direction
 EOR #%11000000
 STA V

 LDA W                  \ Set (R, S) to the end point's coordinate in (W, G)
 STA R
 LDA G
 STA S