Skip to navigation

Elite on the BBC Micro

Drawing ships: SHPPT [Master version]

Name: SHPPT [Show more] Type: Subroutine Category: Drawing ships Summary: Draw a distant ship as a point rather than a full wireframe
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LL9 (Part 2 of 12) calls SHPPT
.SHPPT JSR PROJ \ Project the ship onto the screen, returning: \ \ * K3(1 0) = the screen x-coordinate \ * K4(1 0) = the screen y-coordinate \ * A = K4+1 ORA K3+1 \ If either of the high bytes of the screen coordinates BNE nono \ are non-zero, jump to nono as the ship is off-screen LDA K4 \ Set A = the y-coordinate of the dot CMP #Y*2-2 \ If the y-coordinate is bigger than the y-coordinate of BCS nono \ the bottom of the screen, jump to nono as the ship's \ dot is off the bottom of the space view JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the \ first row of the dot (i.e. a four-pixel dash) LDA K4 \ Set A = y-coordinate of dot + 1 (so this is the second CLC \ row of the two-pixel-high dot) ADC #1 JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the \ first row of the dot (i.e. a four-pixel dash) LDA #%00001000 \ Set bit 3 of the ship's byte #31 to record that we ORA XX1+31 \ have now drawn something on-screen for this ship STA XX1+31 JMP LL155 \ Jump to LL155 to draw any remaining lines that are \ still in the ship line heap and return from the \ subroutine using a tail call .nono LDA #%11110111 \ Clear bit 3 of the ship's byte #31 to record that AND XX1+31 \ nothing is being drawn on-screen for this ship STA XX1+31 JMP LL155 \ Jump to LL155 to draw any remaining lines that are \ still in the ship line heap and return from the \ subroutine using a tail call .Shpt \ This routine draws a horizontal 4-pixel dash, for \ either the top or the bottom of the ship's dot STA Y1 \ Store A in both y-coordinates, as this is a horizontal STA Y2 \ dash at y-coordinate A LDA K3 \ Set A = screen x-coordinate of the ship dot STA X1 \ Store the x-coordinate of the ship dot in X1, as this \ is where the dash starts CLC \ Set A = screen x-coordinate of the ship dot + 3 ADC #3 BCC P%+4 \ If the addition overflowed, set A = 255, the LDA #255 \ x-coordinate of the right edge of the screen STA X2 \ Store the x-coordinate of the ship dot in X1, as this \ is where the dash starts JMP LLX30 \ Draw this edge using flicker-free animation, by first \ drawing the ship's new line and then erasing the \ corresponding old line from the screen, and return \ from the subroutine using a tail call