Drawing lines: DrawOrthoLine

       Name: DrawOrthoLine                                           [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw an orthogonal line (i.e. vertical or horizontal)
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawIndicatorBar calls DrawOrthoLine
             * DrawJoystickCross calls DrawOrthoLine
             * DrawIndicatorBar calls via EraseOrthoLine


 Arguments:

   S                    Defines the starting coordinate for the line:

                          * 0 = (joyCoord, H + W)

                          * 128 = (H + W, joyCoord)

   H + W                Coordinate of the start of the line (it doesn't matter
                        how this value is split between H and W as only the sum
                        is used)

   joyCoord             Coordinate of the start of the line

   T                    Horizontal width/length of line

   U                    Vertical width/length of line

   N                    Drawing mode:

                          * 0 = Draw (using OR logic)

                          * 128 = Erase (using EOR logic)


 Other entry points:

   EraseOrthoLine       Use the value of G instead of H (so the coordinate is
                        G + W) and always use EOR Logic to draw the line (which
                        will erase it if it is already on-screen)


.DrawOrthoLine

 LDA H                  \ Set A = H

 JMP dort1              \ Jump to dort1 to draw the orthogonal line and skip the
                        \ code for the EraseOrthoLine entry point

.EraseOrthoLine

 LDA #128               \ Set N = 128 so the line is drawn with EOR logic, which
 STA N                  \ erases the line if it is already on-screen

 LDA G                  \ Set A = G

.dort1

 CLC                    \ Set A = A + W
 ADC W

 BIT S                  \ If bit 7 of S is set, jump down to dort2
 BMI dort2

 STA J                  \ Set J = A

 LDA joyCoord           \ Set I = joyCoord
 STA I

                        \ We now have (I, J) = (joyCoord, A + W)

 JMP dort3              \ Jump down to dort3

.dort2

 STA I                  \ Set I = A

 LDA joyCoord           \ Set J = joyCoord
 STA J

                        \ We now have (I, J) = (A + W, joyCoord)

.dort3

 LDA #0                 \ Set V = 0 so the line is drawn in a positive direction
 STA V                  \ for both axes

 JSR DrawVectorLine     \ Draw/erase a line from (I, J) as a vector (T, U) with
                        \ direction V

 RTS                    \ Return from the subroutine