.LL116 STA XX12+2 ; Store the gradient in XX12+2 (as the call to LL28 in ; part 3 returns the gradient in both A and R) 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 S ; Store the type of slope in XX12+3, bit 7 clear means STA XX12+3 ; top left to bottom right, bit 7 set means top right to ; bottom left LDA XX13 ; If XX13 = 0, skip the following instruction BEQ LL138 BPL LLX117 ; If XX13 is positive, it must be 127. This means ; (x1, y1) is on-screen but (x2, y2) isn't, so we jump ; to LLX117 to swap the (x1, y1) and (x2, y2) ; coordinates around before doing the actual clipping, ; because we need to clip (x2, y2) but the clipping ; routine at LL118 only clips (x1, y1) .LL138 ; If we get here, XX13 = 0 or 255, so (x1, y1) is ; off-screen and needs clipping JSR LL118 ; Call LL118 to move (x1, y1) along the line onto the ; screen, i.e. clip the line at the (x1, y1) end LDA XX13 ; If XX13 = 255, i.e. (x2, y2) is off-screen, jump down BMI LL117 ; down to LL117 to skip the following PLA ; Restore Y from the stack so it gets preserved through TAY ; the call to this subroutine JMP LL146 ; Jump up to LL146 to move the low bytes of (x1, y1) and ; (x2, y2) into (X1, Y1) and (X2, Y2), and return from ; the subroutine with a successfully clipped line .LL117 ; If we get here, XX13 = 255 (both coordinates are ; off-screen) LDA XX15+1 ; If either of x1_hi or y1_hi are non-zero, jump to ORA XX15+3 ; LL137 to return from the subroutine with the C flag BNE LL137 ; set, as the line doesn't fit on-screen LDA XX15+2 ; If y1_lo > y-coordinate of the bottom of the screen CMP screenHeight ; (which is in the variable screenHeight), jump to LL137 BCS LL137 ; to return from the subroutine with the C flag set, as ; the line doesn't fit on-screen .LLX117 ; If we get here, XX13 = 127 or 255, and in both cases ; (x2, y2) is off-screen, so we now need to swap the ; (x1, y1) and (x2, y2) coordinates around before doing ; the actual clipping, because we need to clip (x2, y2) ; but the clipping routine at LL118 only clips (x1, y1) LDX XX15 ; Swap x1_lo = x2_lo LDA XX15+4 STA XX15 STX XX15+4 LDA XX15+5 ; Swap x2_lo = x1_lo LDX XX15+1 STX XX15+5 STA XX15+1 LDX XX15+2 ; Swap y1_lo = y2_lo LDA XX12 STA XX15+2 STX XX12 LDA XX12+1 ; Swap y2_lo = y1_lo LDX XX15+3 STX XX12+1 STA XX15+3 JSR LL118 ; Call LL118 to move (x1, y1) along the line onto the ; screen, i.e. clip the line at the (x1, y1) end LDA XX15+1 ; If either of x1_hi or y1_hi are non-zero, jump to ORA XX15+3 ; LL137 to return from the subroutine with the C flag BNE LL137 ; set, as the line doesn't fit on-screen DEC SWAP ; Set SWAP = $FF to indicate that we just clipped the ; line at the (x2, y2) end by swapping the coordinates ; (the DEC does this as we set SWAP to 0 at the start of ; this subroutine) .LL124 PLA ; Restore Y from the stack so it gets preserved through TAY ; the call to this subroutine ; If we get here then we have clipped our line to the ; (if we had to clip it at all), so we move the low ; bytes from (x1, y1) and (x2, y2) into (X1, Y1) and ; (X2, Y2), remembering that they share locations with ; XX15: ; ; X1 = XX15 ; Y1 = XX15+1 ; X2 = XX15+2 ; Y2 = XX15+3 ; ; X1 already contains x1_lo, so now we do the rest LDA XX15+2 ; Set A = y1_lo CMP screenHeight ; If A >= screenHeight then jump down to clip2 to clip BCS clip2 ; the coordinate to the screen before jumping back to ; clip1 .clip1 STA XX15+1 ; Set Y1 (aka XX15+1) = y1_lo LDA XX15+4 ; Set X2 (aka XX15+2) = x2_lo STA XX15+2 LDA XX12 ; Set Y2 (aka XX15+3) = y2_lo STA XX15+3 CLC ; Clear the C flag as the clipped line fits on-screen RTS ; Return from the subroutine .clip2 LDA Yx2M1 ; Set A = Yx2M1, which contains the height in pixels of ; the space view BNE clip1 ; Jump to clip1 to continue setting the clipped line's ; coordinates (this BNE is effectively a JMP as A is ; never zero) .LL137 PLA ; Restore Y from the stack so it gets preserved through TAY ; the call to this subroutine SEC ; Set the C flag to indicate the clipped line does not ; fit on-screen RTS ; Return from the subroutineName: LL145 (Part 4 of 4) [Show more] Type: Subroutine Category: Drawing lines Summary: Clip line: Call the routine in LL188 to do the actual clipping Deep dive: Line-clipping Extended screen coordinatesContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
This part sets things up to call the routine in LL188, which does the actual clipping. If we get here, then R has been set to the gradient of the line (x1, y1) to (x2, y2), with T indicating the gradient of slope: * 0 = shallow slope (more horizontal than vertical) * $FF = steep slope (more vertical than horizontal) and XX13 has been set as follows: * 0 = (x1, y1) off-screen, (x2, y2) on-screen * 127 = (x1, y1) on-screen, (x2, y2) off-screen * 255 = (x1, y1) off-screen, (x2, y2) off-screen
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Label LL117 is local to this routine
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Subroutine LL118 (category: Drawing lines)
Move a point along a line until it is on-screen
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Label LL137 is local to this routine
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Label LL138 is local to this routine
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Label LL146 in subroutine LL145 (Part 1 of 4)
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Label LLX117 is local to this routine
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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
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Label clip1 is local to this routine
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Label clip2 is local to this routine
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Variable screenHeight in workspace ZP
The height of the drawable part of the screen in pixels (can be 144, 154 or 208 pixels)