Drawing lines: DrawClippedLine (Part 3 of 6)

       Name: DrawClippedLine (Part 3 of 6)                           [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Work out whether the line's end point is on-screen
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


                        \ We now repeat the above process, but for the line's
                        \ end point, whose y-coordinate is in (H G) and
                        \ x-coordinate is in (QQ W). Refer to the previous part
                        \ for a detailed description

 LDA #0                 \ Set A = 0 so we can capture bits 4 to 7 for the end
                        \ point

 LDX G                  \ Set (Y X) to the end point's y-coordinate in (H G)
 LDY H

 BEQ draw7              \ This section sets the following:
 PHP                    \
 LDX #0                 \   * If H < 0, C = 0 and X = 0
 PLP                    \   * If H = 0, C = 1 and X = G
 CLC                    \   * If H > 0, C = 1 and X = 255
 BMI draw8
 DEX

.draw7

 SEC                    \ See above

.draw8

 ROR A                  \ This section sets bits 6 and 7, which will become
 CPX #152               \ bits 4 and 5 of the final result:
 ROR A                  \
 EOR #%01000000         \                             Bit 6 (4)     Bit 7 (5)
                        \   Off top of screen            0             1
                        \   On-screen                    0             0
                        \   Off bottom of screen         1             0

 LDX W                  \ Set (Y X) to the end point's x-coordinate in (QQ W)
 LDY QQ

 BEQ draw9              \ This section sets the following:
 PHP                    \
 LDX #0                 \   * If QQ < 0, X = 0
 PLP                    \   * If QQ = 0, X = W
 BMI draw9              \   * If QQ > 0, X = 255
 DEX

.draw9

 CPX #4                 \ This section sets bits 6 and 7:
 ROR A                  \
 CPX #156               \                             Bit 6     Bit 7
 ROR A                  \   Off right of screen         0         1
 EOR #%01000000         \   On-screen                   0         0
                        \   Off left of screen          1         0

 STA UU                 \ Store A in UU
                        \
                        \ So UU contains the clipping requirements for the end
                        \ point, in bits 4 to 7, as follows:
                        \
                        \                             Bit 4     Bit 5
                        \   Off top of screen           0         1
                        \   On-screen                   0         0
                        \   Off bottom of screen        1         0
                        \
                        \                             Bit 6     Bit 7
                        \   Off right of screen         0         1
                        \   On-screen                   0         0
                        \   Off left of screen          1         0