Drawing lines: DrawVectorLine (Part 3 of 3)

       Name: DrawVectorLine (Part 3 of 3)                            [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Plot a pixel at (I, J)
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.dvec11

                        \ When we first arrive here:
                        \
                        \   * QQ = 0, where we will tally up the fractional part
                        \          of the move along the shortest axis
                        \   * RR = pixel byte table
                        \   * VV = longest side of delta triangle
                        \   * PP = 0 (shallow horizontal)
                        \          VV (steep vertical)
                        \
                        \ so we now draw a line from (I, J), moving one pixel
                        \ at a time along the longest side of the delta triangle

 LDA I                  \ Set X = I / 4
 LSR A                  \
 LSR A                  \ so X is the number of the character block containing
 TAX                    \ pixel (I, J), as each character block is 4 pixels wide

 LDA J                  \ Set Y = J / 8
 LSR A                  \
 LSR A                  \ so Y is the number of the character row containing
 LSR A                  \ pixel (I, J), as each character row is 8 pixels high
 TAY

 LDA yLookupLo,Y        \ Set P = Y-th byte of yLookupLo
 CLC                    \         + X-th byte of xLookupLo
 ADC xLookupLo,X        \       = LO(screen address) + LO(X * 8)
 STA P

 LDA yLookupHi,Y        \ Set Q = Y-th byte of yLookupHi
 ADC xLookupHi,X        \         + X-th byte of xLookupHi
 STA Q                  \       = HI(screen address) + HI(X * 8)

                        \ So (Q P) is the screen address of the pixel row
                        \ containing pixel (I, J), out by 8 bytes for each row
                        \ above or below the top of the dashboard

 LDA #159               \ Set Y = 159 - J
 SEC                    \
 SBC J                  \ so Y is the number of pixels that (I, J) is above
 TAY                    \ (+ve) or below (-ve) the top of the dashboard, where a
                        \ value of 0 is the bottom pixel inside the canopy, and
                        \ a value of -1 is the white horizontal edge at the
                        \ bottom of the canopy

 LDA I                  \ Set X = bits 0 and 1 of I
 AND #%00000011         \       = I mod 4
 TAX                    \       = pixel number within the 4-pixel byte

 BIT N                  \ If bit 7 of N is set, jump to dvec12 to erase the line
 BMI dvec12             \ with EOR logic instead of drawing it with OR logic

 LDA RR,X               \ Fetch the X-th byte of RR, which is a pixel byte with
                        \ the X-th pixel set to white

 ORA (P),Y              \ OR it with (Q P) + Y, which is the screen address of
                        \ the pixel row containing (I, J)
                        \
                        \ This will keep all pixels the same except the X-th
                        \ pixel, which is set to white, so this will plot a
                        \ pixel at (I, J) when stored in screen memory

 JMP dvec13             \ Jump to dvec13 to skip the following three
                        \ instructions

.dvec12

 LDA RR,X               \ Fetch the X-th byte of RR, which is a pixel byte with
                        \ the X-th pixel set to white

 EOR #%11111111         \ Invert all the bits, so A is now a pixel byte that is
                        \ all white except for the X-th pixel, which is black

 AND (P),Y              \ AND it with (Q P) + Y, which is the screen address of
                        \ pixel (I, J)
                        \
                        \ This will keep all pixels the same except the X-th
                        \ pixel, which is set to black, so this will erase the
                        \ pixel at (I, J) when stored in screen memory

.dvec13

 STA (P),Y              \ Store the byte in A in screen memory at (Q P) + Y,
                        \ which sets all four pixels to the pixel pattern in A,
                        \ which either draws or erases the pixel at (I, J)

 DEC VV                 \ Decrement VV to step one pixel along the longer axis

 BNE dvec7              \ If VV is non-zero, jump up to dvec7 to calculate the
                        \ coordinate of the next pixel in the line

 RTS                    \ Return from the subroutine