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Elite on the BBC Micro

Drawing ships: LL9 (Part 7 of 12) (Disc version)

Name: LL9 (Part 7 of 12) [View in context] Type: Subroutine Category: Drawing ships Summary: Draw ship: Calculate the visibility of each of the ship's vertices Deep dive: Drawing ships Calculating vertex coordinates
This section continues the coordinate adding from part 6 by finishing off the calculation that we started above: [ sidev_x roofv_x nosev_x ] [ x ] [ x ] vector to vertex = [ sidev_y roofv_y nosev_y ] . [ y ] + [ y ] [ sidev_z roofv_z nosev_z ] [ z ] [ z ] The gets stored as follows, in sign-magnitude values with the magnitudes fitting into the low bytes: XX15(2 0) [ x y z ] . [ sidev_x roofv_x nosev_x ] + [ x y z ] XX15(5 3) [ x y z ] . [ sidev_y roofv_y nosev_y ] + [ x y z ] (U T) [ x y z ] . [ sidev_z roofv_z nosev_z ] + [ x y z ] Finally, because this vector is from our ship to the vertex, and we are at the origin, this vector is the same as the coordinates of the vertex. In other words, we have just worked out: XX15(2 0) x-coordinate of the current vertex XX15(5 3) y-coordinate of the current vertex (U T) z-coordinate of the current vertex
.LL56 LDA XX1+6 \ Set (U T) = XX1(7 6) - XX12(5 4) SEC \ = (z_hi z_lo) - vertv_z SBC XX12+4 \ STA T \ Starting with the low bytes LDA XX1+7 \ And then doing the high bytes (we can subtract 0 here SBC #0 \ as we know the sign byte of vertv_z is 0) STA U BCC LL140 \ If the subtraction just underflowed, skip to LL140 to \ set (U T) to the minimum value of 4 BNE LL57 \ If U is non-zero, jump down to LL57 LDA T \ If T >= 4, jump down to LL57 CMP #4 BCS LL57 .LL140 LDA #0 \ If we get here then either (U T) < 4 or the STA U \ subtraction underflowed, so set (U T) = 4 LDA #4 STA T .LL57 \ By this point we have our results, so now to scale \ the 16-bit results down into 8-bit values LDA U \ If the high bytes of the result are all zero, we are ORA XX15+1 \ done, so jump down to LL60 for the next stage ORA XX15+4 BEQ LL60 LSR XX15+1 \ Shift XX15(1 0) to the right ROR XX15 LSR XX15+4 \ Shift XX15(4 3) to the right ROR XX15+3 LSR U \ Shift (U T) to the right ROR T JMP LL57 \ Jump back to LL57 to see if we can shift the result \ any more