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⌂ ∞ Revs on the BBC Micro

Screen buffer: SetBackground

       Name: SetBackground                                           [Show more]
       Type: Subroutine
   Category: Screen buffer
    Summary: Set the background colour for any track lines that have not yet
             had a background colour set
  Deep dive: Drawing the track view

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MainDrivingLoop (Part 2 of 5) calls SetBackground




.SetBackground

                        \ We start by setting the colour of the track line that
                        \ contains the horizon

 LDX horizonListIndex   \ Set X = horizonListIndex
                        \
                        \ So X is the index in the track verge buffer for the
                        \ horizon line's right verge

 LDY horizonLine        \ Set Y to the track line number of the horizon

 LDA xVergeLeftHi,X     \ Set A = X-th entry in xVergeLeftHi + 20
 CLC
 ADC #20

 BPL bgnd1              \ If A is positive, then the left edge of the verge is
                        \ on-screen, so jump to bgnd1 to set the lines below
                        \ the horizon to the colour of grass

 LDA xVergeRightHi,X    \ Set A = X-th entry in xVergeRightHi + 20
 CLC
 ADC #20

 BMI bgnd1              \ If A is negative, then the right edge of the verge is
                        \ on-screen, jump to bgnd1 to set the lines below
                        \ the horizon to the colour of grass

                        \ If we get to this point, then:
                        \
                        \   * xVergeRightHi + 20 is positive
                        \
                        \   * xVergeLeftHi + 20 is negative
                        \
                        \ Adding 20 degrees to the yaw angles will move them to
                        \ the right by half the screen width, so this is the
                        \ same as moving the angles from the left edge of the
                        \ screen to the middle
                        \
                        \ We then check whether moving the angles to the centre
                        \ pushes the rightmost verge edge in A past the centre
                        \ (i.e. positive), while still leaving the leftmost edge
                        \ in the left half (i.e. negative)
                        \
                        \ If so, then this means the verge at the horizon is
                        \ straddling the left edge of the screen, so we need to
                        \ set the background colour for the horizon track line
                        \ to black

 LDA #%00100000         \ Set A = %00100000 (colour 0, black) for the horizon
                        \ track line, where:
                        \
                        \   * %00 in bits 0-1 is logical colour 0
                        \
                        \   * %001xx0xx denotes that this value was stored in
                        \     the backgroundColour table by the SetBackground
                        \     routine


 BNE bgnd2              \ Jump to bgnd2 (this BNE is effectively a JMP as A is
                        \ never zero)

.bgnd1

 LDA #%00100011         \ Set A = %00100011 (colour 3, green) for the horizon
                        \ track line, where:
                        \
                        \   * %11 in bits 0-1 is logical colour 3
                        \
                        \   * %001xx0xx denotes that this value was stored in
                        \     the backgroundColour table by the SetBackground
                        \     routine

.bgnd2

 STA backgroundColour,Y \ Set the colour of the horizon line to A

                        \ We now work our way through the whole backgroundColour
                        \ table, filling in any entries that are zero (and which
                        \ have therefore not been set yet) with blue

 LDA #%00100001         \ Set A = %00100001 (colour 1, blue) to use as the line
                        \ colour for lines above the horizon, i.e. the sky,
                        \ where:
                        \
                        \   * %01 in bits 0-1 is logical colour 1
                        \
                        \   * %001xx0xx denotes that this value was stored in
                        \     the backgroundColour table by the SetBackground
                        \     routine

 LDY #79                \ We now loop through all the track lines, starting from
                        \ line 79 at the top of the track view down to 0 at the
                        \ bottom, so set a counter in Y
                        \
                        \ If any values are zero, then we set them to blue

.bgnd3

 LDX backgroundColour,Y \ Set X to the background colour for track line Y

 BEQ bgnd4              \ If X is zero, then this track line doesn't currently
                        \ have a background colour set, so jump to bgnd4 to set
                        \ the track line to blue

 TXA                    \ If we get here then X is non-zero, so there is already
                        \ a backgroundColour value for this track line, so copy
                        \ this value to A so the following STA instruction
                        \ doesn't change anything

.bgnd4

 STA backgroundColour,Y \ Set the background colour for track line Y to A

 DEY                    \ Decrement the loop counter to move down to the next
                        \ track line

 BPL bgnd3              \ Loop back until we have set the line colour for all
                        \ 80 track lines

 RTS                    \ Return from the subroutine