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BBC Micro Elite

Drawing ships: LL9 (Part 9 of 11)

Name: LL9 (Part 9 of 11) [View in context] Type: Subroutine Category: Drawing ships Summary: Draw ship: Draw laser beams if the ship is firing its laser at us
This part sets things up so we can loop through the edges in the next part. It also adds a line to the ship line heap, if the ship is firing at us. When we get here, the heap at XX3 contains all the visible vertex screen coordinates.
.LL72 LDA XX1+31 \ If bit 5 of the ship's byte #31 is clear, then the AND #%00100000 \ ship is not currently exploding, so jump down to EE31 BEQ EE31 LDA XX1+31 \ The ship is exploding, so set bit 3 of the ship's byte ORA #8 \ #31 to denote that we are drawing something on-screen STA XX1+31 \ for this ship JMP DOEXP \ Jump to DOEXP to display the explosion cloud, \ returning from the subroutine using a tail call .EE31 LDA #%00001000 \ If bit 3 of the ship's byte #31 is clear, then there BIT XX1+31 \ is nothing already being shown for this ship, so skip BEQ LL74 \ to LL74 as we don't need to erase anything from the \ screen JSR LL155 \ Otherwise call LL155 to draw the existing ship, which \ removes it from the screen LDA #%00001000 \ Set bit 3 of A so the next instruction sets bit 3 of \ the ship's byte #31 to denote that we are drawing \ something on-screen for this ship .LL74 ORA XX1+31 \ Apply bit 3 of A to the ship's byte #31, so if there STA XX1+31 \ was no ship already on screen, the bit is clear, \ otherwise it is set LDY #9 \ Fetch byte #9 of the ship's blueprint, which is the LDA (XX0),Y \ number of edges, and store it in XX20 STA XX20 LDY #0 \ We are about to step through all the edges, using Y \ as a counter STY U \ Set U = 0 (though we increment it to 1 below) STY XX17 \ Set XX17 = 0, which we are going to use as a counter \ for stepping through the ship's edges INC U \ We are going to start calculating the lines we need to \ draw for this ship, and will store them in the ship \ line heap, using U to point to the end of the heap, so \ we start by setting U = 1 BIT XX1+31 \ If bit 6 of the ship's byte #31 is clear, then the BVC LL170 \ ship is not firing its lasers, so jump to LL170 to \ skip the drawing of laser lines \ The ship is firing its laser at us, so we need to draw \ the laser lines LDA XX1+31 \ Clear bit 6 of the ship's byte #31 so the ship doesn't AND #%10111111 \ keep firing endlessly STA XX1+31 LDY #6 \ Fetch byte #6 of the ship's blueprint, which is the LDA (XX0),Y \ number * 4 of the vertex where the ship has its lasers TAY \ Put the vertex number into Y, where it can act as an \ index into list of vertex screen coordinates we added \ to the XX3 heap LDX XX3,Y \ Fetch the x_lo coordinate of the laser vertex from the STX XX15 \ XX3 heap into XX15 INX \ If X = 255 then the laser vertex is not visible, as BEQ LL170 \ the value we stored in part 2 wasn't overwritten by \ the vertex calculation in part 6 and 7, so jump to \ LL170 to skip drawing the laser lines \ We now build a laser beam from the ship's laser vertex \ towards our ship, as follows: \ \ XX15(1 0) = laser vertex x-coordinate \ \ XX15(3 2) = laser vertex y-coordinate \ \ XX15(5 4) = x-coordinate of the end of the beam \ \ XX12(1 0) = y-coordinate of the end of the beam \ \ The end of the laser beam will be set positioned to \ look good, rather than being directly aimed at us, as \ otherwise we would only see a flashing point of light \ as they unleashed their attack LDX XX3+1,Y \ Fetch the x_hi coordinate of the laser vertex from the STX XX15+1 \ XX3 heap into XX15+1 INX \ If X = 255 then the laser vertex is not visible, as BEQ LL170 \ the value we stored in part 2 wasn't overwritten by \ a vertex calculation in part 6 and 7, so jump to LL170 \ to skip drawing the laser beam LDX XX3+2,Y \ Fetch the y_lo coordinate of the laser vertex from the STX XX15+2 \ XX3 heap into XX15+2 LDX XX3+3,Y \ Fetch the y_hi coordinate of the laser vertex from the STX XX15+3 \ XX3 heap into XX15+3 LDA #0 \ Set XX15(5 4) = 0, so their laser beam fires to the STA XX15+4 \ left edge of the screen STA XX15+5 STA XX12+1 \ Set XX12(1 0) = the ship's z_lo coordinate, which will LDA XX1+6 \ effectively make the vertical position of the end of STA XX12 \ the laser beam move around as the ship moves in space LDA XX1+2 \ If the ship's x_sign is positive, skip the next BPL P%+4 \ instruction DEC XX15+4 \ The ship's x_sign is negative (i.e. it's on the left \ side of the screen), so switch the laser beam so it \ goes to the right edge of the screen by decrementing \ XX15(5 4) to 255 JSR LL145 \ Call LL145 to see if the laser beam needs to be \ clipped to fit on-screen, returning the clipped line's \ end-points in (X1, Y1) and (X2, Y2) BCS LL170 \ If the C flag is set then the line is not visible on \ screen, so jump to LL170 so we don't store this line \ in the ship line heap LDY U \ Fetch the ship line heap pointer, which points to the \ next free byte on the heap, into Y LDA XX15 \ Add X1 to the end of the heap STA (XX19),Y INY \ Increment the heap pointer LDA XX15+1 \ Add Y1 to the end of the heap STA (XX19),Y INY \ Increment the heap pointer LDA XX15+2 \ Add X2 to the end of the heap STA (XX19),Y INY \ Increment the heap pointer LDA XX15+3 \ Add Y2 to the end of the heap STA (XX19),Y INY \ Increment the heap pointer STY U \ Store the updated ship line heap pointer in U