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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