Skip to navigation

⌂ ∞ ▽ Aviator on the BBC Micro

Drawing lines: DrawCanopyLine (Part 5 of 9)

       Name: DrawCanopyLine (Part 5 of 9)                            [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw a part of the line, working up the screen

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




 The code in this routine is modified by the ModifyDrawRoutine, and by the
 DrawCanopyLine routine itself.

 The default code (i.e. the unmodified version in the source) is run when:

   * The current colour cycle is drawing in colour 1, using the pixel bitmaps
     at colour1L2R and OR logic for updating the screen



.dlin27

                        \ This routine draws a part of a line (one pixel row,
                        \ i.e. a byte) and moves us up to the previous pixel row
                        \
                        \ We call this subroutine with:
                        \
                        \   * A = the current slope error
                        \
                        \   * X = the index of the pixel byte at colour1R2L

 STA SS                 \ Set SS = A

.dlin28

 LDA colour1R2L,X       \ Fetch the X-th pixel byte from colour1R2L
                        \
                        \ Gets modified by the ModifyDrawRoutine routine:
                        \
                        \   * LDA colour1R2L,X when colourLogic = %10000000
                        \
                        \   * LDA colour2R2L,X when colourLogic = %01000000
                        \
                        \   * LDA colour1Row,X when colourLogic = %00000000
                        \                       and colourCycle = %00001111
                        \
                        \   * LDA colour2Row,X when colourLogic = %00000000
                        \                       and colourCycle = %11110000
                        \
                        \ In other words, this instruction has already been
                        \ modified to implement the current colour cycle

.dlin29

 ORA (P),Y              \ OR the pixel byte with the current screen contents
                        \
                        \ Gets modified by the ModifyDrawRoutine routine:
                        \
                        \   * ORA (P),Y when colourLogic = %01000000
                        \
                        \   * AND (P),Y when colourLogic = %00000000
                        \
                        \ In other words, this instruction has already been
                        \ modified to implement the current drawing logic

 STA (P),Y              \ Update the Y-th byte of (Q P) with the result, which
                        \ sets 4 pixels to the pixel pattern in A

.dlin30

 LDA P                  \ Set (Q P) = (Q P) - 8
 SEC                    \
 SBC #8                 \ starting with the low bytes
 STA P

 BCS dlin31             \ And then the high bytes, so (Q P) now points to the
 DEC Q                  \ previous character block to the left

.dlin31

 DEC QQ                 \ Decrement QQ, which contains the current character
                        \ block number

 LDA QQ                 \ If QQ <> I, then we haven't yet reached the right edge
 CMP I                  \ of the screen (whose character block number we set in
 BNE dlin12             \ I in part 2), so jump back to dlin12 to keep drawing
                        \ the line

 JMP dlin65             \ Otherwise we have reached the edge of the screen, so
                        \ jump to dlin65 to process the clipped part of the
                        \ line, if applicable