Screen buffer: GetColourSup

       Name: GetColourSup                                            [Show more]
       Type: Subroutine
   Category: Screen buffer
    Summary: Calculate the colour of a specific pixel byte in the screen buffer
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawObjectEdge (Part 4 of 5) calls GetColourSup
             * FillAfterObjectSup calls GetColourSup


 Arguments:

   Y                    The track line number of the pixel byte to check

   blockNumber          The dash data block number of the pixel byte to check


 Returns:

   A                    The colour of this pixel byte in the screen buffer


IF _SUPERIOR OR _REVSPLUS

.GetColourSup

 CPY horizonLine        \ If Y <= horizonLine then the byte we want to check is
 BCC scol1              \ below the horizon, so jump to scol1 to work out the
 BEQ scol1              \ byte's colour

 LDA colourPalette+1    \ Otherwise the byte is in the sky, so set A to logical
                        \ colour 1 (blue) from the colour palette

 RTS                    \ Return from the subroutine

.scol1

 LDA blockNumber        \ Set A to the block number containing the pixel byte
                        \ that we want to check

 CMP rightGrassStart,Y  \ If A >= rightGrassStart for this track line, then the
 BCS scol3              \ pixel byte is in the grass to the right of the track,
                        \ so jump to scol3 to return colour 3 (green)

 CMP rightVergeStart,Y  \ If A >= rightVergeStart for this track line, then the
 BCS scol5              \ pixel byte is on the right track verge, so jump to
                        \ scol5 to work out its colour

 CMP leftTrackStart,Y   \ If A >= leftTrackStart for this track line, then the
 BCS scol2              \ pixel byte is on the track, so jump to scol2 to return
                        \ colour 0 (black)

 CMP leftVergeStart,Y   \ If A >= leftVergeStart for this track line, then the
 BCS scol4              \ pixel byte is on the left track verge, so jump to
                        \ scol4 to work out its colour

 LDA backgroundColour,Y \ If we get here then the byte is to the left of the
                        \ left track verge, so set A to the background colour of
                        \ this track line so we can extract the colour from bits
                        \ 0-1 below

 BCC scol7              \ Jump to scol7 to return the pixel byte for the colour
                        \ in A (this BCC is effectively a JMP as we just passed
                        \ through a BCS)

.scol2

 LDA colourPalette      \ Set A to logical colour 0 (black) from the colour
                        \ palette

 RTS                    \ Return from the subroutine

.scol3

 LDA colourPalette+3    \ Set A to logical colour 3 (green) from the colour
                        \ palette

 RTS                    \ Return from the subroutine

.scol4

                        \ If we get here then the pixel byte is on the left
                        \ track verge

 CPY vergeTopLeft       \ If the track line in Y >= vergeTopLeft, jump to scol3
 BCS scol3              \ to return colour 3 (green)

 LDA leftSegment,Y      \ Set A to the index within the track segment list of
                        \ the segment for the left verge on this track line

 JMP scol6              \ Jump to scol6

.scol5

                        \ If we get here then the pixel byte is on the right
                        \ track verge

 CPY vergeTopRight      \ If the track line in Y >= vergeTopRight, jump to scol3
 BCS scol3              \ to return colour 3 (green)

 LDA rightSegment,Y     \ Set A to the index within the track segment list of
                        \ the segment for the right verge on this track line

.scol6

 AND #%01111111         \ Clear bit 7 of A

 TAX                    \ Set X to A

 LDA vergeDataRight-1,X \ Set A to entry X - 1 from vergeDataRight, which
                        \ contains the colour of the verge mark for the segment
                        \ beyond segment X

.scol7

 AND #%00000011         \ Extract the colour number from bits 0-1 of A into X
 TAX

 LDA colourPalette,X    \ Set A to logical colour X from the colour palette

 RTS                    \ Return from the subroutine

ENDIF