.LL170 LDY #3 ; Fetch byte #3 of the ship's blueprint, which contains CLC ; the low byte of the offset to the edges data LDA (XX0),Y ADC XX0 ; Set V = low byte edges offset + XX0 STA V LDY #16 ; Fetch byte #16 of the ship's blueprint, which contains LDA (XX0),Y ; the high byte of the offset to the edges data ADC XX0+1 ; Set V+1 = high byte edges offset + XX0+1 STA V+1 ; ; So V(1 0) now points to the start of the edges data ; for this ship LDY #5 ; Fetch byte #5 of the ship's blueprint, which contains LDA (XX0),Y ; the maximum heap size for plotting the ship (which is STA T1 ; 1 + 4 * the maximum number of visible edges) and store ; it in T1 LDY XX17 ; Set Y to the edge counter in XX17 .LL75 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 LDA (V),Y ; Fetch byte #0 for this edge, which contains the ; visibility distance for this edge, beyond which the ; edge is not shown CMP XX4 ; If XX4 > the visibility distance, where XX4 contains BCC LL79-3 ; the ship's z-distance reduced to 0-31 (which we set in ; part 2), then this edge is too far away to be visible, ; so jump down to LL78 (via LL79-3) to move on to the ; next edge INY ; Increment Y to point to byte #1 LDA (V),Y ; Fetch byte #1 for this edge into A, so: ; ; A = %ffff ffff, where: ; ; * Bits 0-3 = the number of face 1 ; ; * Bits 4-7 = the number of face 2 INY ; Increment Y to point to byte #2 STA P ; Store byte #1 into P AND #%00001111 ; Extract the number of face 1 into X TAX LDA XX2,X ; If XX2+X is non-zero then we decided in part 5 that BNE LL79 ; face 1 is visible, so jump to LL79 LDA P ; Fetch byte #1 for this edge into A LSR A ; Shift right four times to extract the number of face 2 LSR A ; from bits 4-7 into X LSR A LSR A TAX LDA XX2,X ; If XX2+X is non-zero then we decided in part 5 that BNE LL79 ; face 2 is visible, so skip the following instruction JMP LL78 ; Face 2 is hidden, so jump to LL78 .LL79 ; We now build the screen line for this edge, as ; follows: ; ; XX15(1 0) = start x-coordinate ; ; XX15(3 2) = start y-coordinate ; ; XX15(5 4) = end x-coordinate ; ; XX12(1 0) = end y-coordinate ; ; We can then pass this to the line clipping routine ; before storing the resulting line in the ship line ; heap LDA (V),Y ; Fetch byte #2 for this edge into X, which contains TAX ; the number of the vertex at the start of the edge INY ; Increment Y to point to byte #3 LDA (V),Y ; Fetch byte #3 for this edge into Q, which contains STA Q ; the number of the vertex at the end of the edge LDA XX3+1,X ; Fetch the x_hi coordinate of the edge's start vertex STA XX15+1 ; from the XX3 heap into XX15+1 LDA XX3,X ; Fetch the x_lo coordinate of the edge's start vertex STA XX15 ; from the XX3 heap into XX15 LDA XX3+2,X ; Fetch the y_lo coordinate of the edge's start vertex STA XX15+2 ; from the XX3 heap into XX15+2 LDA XX3+3,X ; Fetch the y_hi coordinate of the edge's start vertex STA XX15+3 ; from the XX3 heap into XX15+3 LDX Q ; Set X to the number of the vertex at the end of the ; edge, which we stored in Q LDA XX3,X ; Fetch the x_lo coordinate of the edge's end vertex STA XX15+4 ; from the XX3 heap into XX15+4 LDA XX3+3,X ; Fetch the y_hi coordinate of the edge's end vertex STA XX12+1 ; from the XX3 heap into XX12+1 LDA XX3+2,X ; Fetch the y_lo coordinate of the edge's end vertex STA XX12 ; from the XX3 heap into XX12 LDA XX3+1,X ; Fetch the x_hi coordinate of the edge's end vertex STA XX15+5 ; from the XX3 heap into XX15+5 JSR CLIP2 ; Call CLIP2 to see if the new line segment needs to be ; clipped to fit on-screen, returning the clipped line's ; end-points in (X1, Y1) and (X2, Y2) BCS LL79-3 ; If the C flag is set then the line is not visible on ; screen, so jump to LL78 (via LL79-3) so we don't draw ; this line JSR LOIN ; Draw this edge JMP LL78 ; Jump down to part 11 to skip to the next edgeName: LL9 (Part 10 of 12) [Show more] Type: Subroutine Category: Drawing ships Summary: Draw ship: Calculate the visibility of each of the ship's edges Deep dive: Drawing shipsContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
This part calculates which edges are visible - in other words, which lines we should draw - and clips them to fit on the screen. When we get here, the heap at XX3 contains all the visible vertex screen coordinates.
[X]
Entry point CLIP2 in subroutine LL145 (Part 1 of 4) (category: Drawing lines)
Don't initialise the values in SWAP or A
[X]
Label LL78 in subroutine LL9 (Part 11 of 12)
[X]
Label LL79 is local to this routine
[X]
[X]
Subroutine LOIN (Part 1 of 7) (category: Drawing lines)
Draw a line: Calculate the line gradient in the form of deltas
[X]
Macro SETUP_PPU_FOR_ICON_BAR (category: PPU)
If the PPU has started drawing the icon bar, configure the PPU to use nametable 0 and pattern table 0
[X]
Temporary storage, used to store the address of a ship blueprint. For example, it is used when we add a new ship to the local bubble in routine NWSHP, and it contains the address of the current ship's blueprint as we loop through all the nearby ships in the main flight loop
[X]
Workspace XX3 (category: Workspaces)
Temporary storage space for complex calculations