Drawing lines: DrawVectorLine (Part 2 of 3)

       Name: DrawVectorLine (Part 2 of 3)                            [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Calculate the coordinates of the next pixel as we step along the
             line by one pixel
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.dvec3

                        \ If we get here then we need to step along the x-axis

 LDA QQ                 \ Set A = QQ + U
 CLC                    \
 ADC U                  \ so A contains the cumulative step along the y-axis
                        \ (the shorter axis)

 CMP T                  \ If A < T, then we haven't yet reached a full step of
 BCC dvec5              \ length T along the y-axis, so we don't change the
                        \ y-coordinate and instead jump to dvec5 to do the step
                        \ along the x-axis by one pixel

                        \ We now need to step along the y-axis by one pixel as
                        \ the cumulative step has just crossed over into a new
                        \ multiple of T

 SBC T                  \ Set A = A - T
                        \
                        \ so we keep the cumulative step within the bounds of a
                        \ single byte (as we are only interested in when it
                        \ crosses the boundary into a new multiple of T)

                        \ We now move one pixel along the y-axis in the
                        \ direction given in V

 BIT V                  \ If bit 6 of V is clear, jump to dvec4 to step along
 BVC dvec4              \ the y-axis in a positive direction

 DEC J                  \ Bit 6 of V is set, so decrement the y-coordinate in
                        \ J so we move along the y-axis in a negative direction

 BVS dvec5              \ Jump to dvec5 to do the step along the x-axis by one
                        \ pixel (this BVS is effectively a JMP as we know the V
                        \ flag is set)

.dvec4

 INC J                  \ Bit 6 of V is clear, so increment the y-coordinate in
                        \ J so we move along the y-axis in a positive direction

.dvec5

 STA QQ                 \ Store the updated fractional value in QQ

                        \ We now move one pixel along the x-axis in the
                        \ direction given in V

 BIT V                  \ If bit 7 of V is clear, jump to dvec6 to step along
 BPL dvec6              \ the x-axis in a positive direction

 DEC I                  \ Bit 7 of V is set, so decrement the x-coordinate in
                        \ I so we move along the x-axis in a negative direction

 JMP dvec11             \ Now that we have moved (I, J) to the next pixel in the
                        \ line, jump to dvec11 to plot the next pixel

.dvec6

 INC I                  \ Bit 7 of V is clear, so increment the x-coordinate in
                        \ I so we move along the x-axis in a positive direction

 JMP dvec11             \ Now that we have moved (I, J) to the next pixel in the
                        \ line, jump to dvec11 to plot the next pixel

.dvec7

                        \ We jump here when we need to calculate the coordinates
                        \ of the next pixel in the line when stepping along the
                        \ longer delta axis one pixel at a time

 LDA PP                 \ If PP = 0 then this is a shallow horizontal slope, so
 BEQ dvec3              \ jump up to dvec3 step along the x-axis

                        \ If we get here then this is a steep vertical line, so
                        \ we need to step along the y-axis

 LDA QQ                 \ Set A = QQ + T
 CLC                    \
 ADC T                  \ so A contains the cumulative step along the x-axis
                        \ (the shorter axis)

 CMP U                  \ If A < U, then we haven't yet reached a full step of
 BCC dvec9              \ length U along the x-axis, so we don't change the
                        \ x-coordinate and instead jump to dvec9 to do the step
                        \ along the y-axis by one pixel

                        \ We now need to step along the x-axis by one pixel as
                        \ the cumulative step has just crossed over into a new
                        \ multiple of U

 SBC U                  \ Set A = A - U
                        \
                        \ so we keep the cumulative step within the bounds of a
                        \ single byte (as we are only interested in when it
                        \ crosses the boundary into a new multiple of U)

                        \ We now move one pixel along the x-axis in the
                        \ direction given in V

 BIT V                  \ If bit 7 of V is clear, jump to dvec8 to step along
 BPL dvec8              \ the x-axis in a positive direction

 DEC I                  \ Bit 7 of V is set, so decrement the x-coordinate in
                        \ I so we move along the x-axis in a negative direction,
                        \ i.e. to the left

 JMP dvec9              \ Jump to dvec9 to do the step along the y-axis by one
                        \ pixel

.dvec8

 INC I                  \ Bit 7 of V is clear, so increment the x-coordinate in
                        \ I so we move along the x-axis in a positive direction,
                        \ i.e. to the right

.dvec9

 STA QQ                 \ Store the updated fractional value in QQ

                        \ We now move one pixel along the y-axis in the
                        \ direction given in V

 BIT V                  \ If bit 6 of V is clear, jump to dvec10 to step along
 BVC dvec10             \ the y-axis in a positive direction, i.e. up the screen

 DEC J                  \ Bit 6 of V is set, so decrement the y-coordinate in
                        \ J so we move along the y-axis in a negative direction,
                        \ i.e. down the screen

 BVS dvec11             \ Now that we have moved (I, J) to the next pixel in the
                        \ line, jump to dvec11 to plot the next pixel (this BVS
                        \ is effectively a JMP as we know the V flag is set)

.dvec10

 INC J                  \ Bit 6 of V is clear, so increment the y-coordinate in
                        \ J so we move along the y-axis in a positive direction

                        \ Now that we have moved (I, J) to the next pixel in the
                        \ line, we fall through into part 3 to plot that pixel