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Revs on the BBC Micro

Extra tracks: SetSegmentVector

Name: SetSegmentVector [Show more] Type: Subroutine Category: Extra tracks Summary: Add the yaw angle and height deltas to the yaw angle and height (for curved sections) and calculate the segment vector Deep dive: Dynamic track generation in the extra tracks
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * HookSectionFrom calls SetSegmentVector * HookSegmentVector calls SetSegmentVector

This routine adds the yaw angle and height deltas to the track yaw angle and height, to get the coordinates for the next segment along. It then calls the CalcSegmentVector routine to calculate the segment vectors and store them in the correct tables for the main game code to read. If the current sub-section number has bit 7 set, then this section isn't being generated using the curve tables, but is instead being generated as a straight part of the track. In this case we don't update the yaw angle or track height before calculating the segment vector, as the main game code draws straight segments by simply adding the same track segment vector for each segment in the straight.
.SetSegmentVector STX xStore \ Store X in xStore so we can retrieve it at the end of \ the routine LDY subSection \ Set Y to the number of the current sub-section within \ the current track section BMI sets1 \ If bit 7 of Y is set, then this part of the track is a \ straight section that doesn't use the curve vectors to \ generate the track, so jump to sets1 to skip updating \ the yaw angle and track height, as we simply reuse the \ same track segment vector for each segment within the \ straight \ We start by adding the yaw delta to the yaw angle LDA trackYawDeltaLo,Y \ Set (A T) = (trackYawDeltaHi trackYawDeltaLo) for this STA T \ sub-section LDA trackYawDeltaHi,Y BIT directionFacing \ Set the N flag to the sign of directionFacing, so the \ call to Absolute16Bit sets the sign of (A T) to \ abs(directionFacing) JSR Absolute16Bit \ Set the sign of (A T) to match the sign bit in \ directionFacing, so this negates (A T) if we are \ facing backwards along the track STA U \ Set (U T) = (A T) \ = signed (trackYawDeltaHi trackYawDeltaLo) \ for this sub-section LDA T \ Set yawAngle = yawAngle + (U T) CLC \ = yawAngle + trackYawDelta ADC yawAngleLo \ STA yawAngleLo \ starting with the low bytes LDA U \ And then the high bytes ADC yawAngleHi STA yawAngleHi \ And now we add the track gradient (i.e. the height \ delta) to the track height LDA trackSlopeDelta,Y \ Set A to the change in slope for this sub-section \ (i.e. the change in the gradient over the course of \ each segment in the sub-section) BIT directionFacing \ Set the N flag to the sign of directionFacing, so the \ call to Absolute8Bit sets the sign of A to \ abs(directionFacing) JSR Absolute8Bit \ Set the sign of A to match the sign bit in \ directionFacing, so this negates A if we are facing \ backwards along the track CLC \ Set segmentSlope = segmentSlope + A ADC segmentSlope \ = segmentSlope + trackSlopeDelta STA segmentSlope .sets1 JSR CalcSegmentVector \ Calculate the segment vector for the current segment \ and put it in the xSegmentVectorI, ySegmentVectorI, \ zSegmentVectorI, xSegmentVectorO and zSegmentVectorO \ tables LDX xStore \ Retrieve the value of X we stores above, so we can \ return it unchanged by the routine RTS \ Return from the subroutine EQUB &0D, &0A \ These bytes appear to be unused EQUB &07, &03 EQUB &00, &00 EQUB &00