.loin20 LDY YSAV ; Restore Y from YSAV, so that it's preserved CLC ; Clear the C flag for the routine to return RTS ; Return from the subroutine .loin21 ; If we get here then we are drawing our line in a new ; pattern, so it won't contain any pre-existing content LDX pattBufferHiDiv8 ; Set SC(1 0) = (pattBufferHiDiv8 A) * 8 STX SC+1 ; = (pattBufferHi 0) + A * 8 ASL A ; ROL SC+1 ; So SC(1 0) is the address in the pattern buffer for ASL A ; pattern number A (as each pattern contains 8 bytes of ROL SC+1 ; pattern data), which means SC(1 0) points to the ASL A ; pattern data for the tile containing the line we are ROL SC+1 ; drawing STA SC CLC ; Clear the C flag for the additions below LDX Q ; Set X to the value of the x-axis counter .loin22 LDA R ; Fetch the pixel byte from R STA (SC),Y ; Store R into screen memory at SC(1 0) - we don't need ; to merge it with whatever is there, as we just started ; drawing in a new tile DEX ; Decrement the y-coordinate counter in X BEQ loin20 ; If we have just reached the end of the line along the ; y-axis, jump to loin20 to return from the subroutine LDA S ; Set S = S + P to update the slope error ADC P STA S BCC loin23 ; If the addition didn't overflow, jump to loin23 to ; skip the following LSR R ; Shift the single pixel in R to the right to step along ; the x-axis, so the next pixel we plot will be at the ; next x-coordinate along BCS loin28 ; If the pixel fell out of the right end of R into the ; C flag, then jump to loin28 to rotate it into the left ; end and move right by a character block .loin23 DEY ; Decrement Y to point to move to the pixel line above BPL loin22 ; If Y is still positive then we have not yet gone past ; the top of the character block, so jump to loin22 to ; draw the next pixel ; Otherwise we just gone past the top of the current ; character block, so we need to move up into the ; character block above by setting Y and SC2(1 0) LDY #7 ; Set Y to point to the bottom pixel row of the block ; above ; If we get here then the C flag is clear, as we either ; jumped to loin23 using a BCC, or we passed through a ; BCS to get to loin23, so the SBC #31 below actually ; subtracts 32 LDA SC2 ; Subtract 32 from SC2(1 0) to get the tile number on SBC #31 ; the row above (as there are 32 tiles on each row) STA SC2 BCS loin24 DEC SC2+1 ; Fall through into loin24 to fetch the correct tile ; number for the new character block and continue ; drawing .loin24 ; This is the entry point for this part (we jump here ; from part 5 when the line is steep and X1 <= X2) ; ; We jump here with X containing the y-axis counter, ; i.e. the number of steps we need to take along the ; y-axis when drawing the line STX Q ; Store the updated y-axis counter in Q 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 LDX #0 ; If the nametable buffer entry is non-zero for the tile LDA (SC2,X) ; containing the pixel that we want to draw, then a BNE loin25 ; pattern has already been allocated to this entry, so ; skip the following LDA firstFreePattern ; If firstFreePattern is zero then we have run out of BEQ loin29 ; patterns to use for drawing lines and pixels, so jump ; to loin29 to move on to the next pixel in the line STA (SC2,X) ; Otherwise firstFreePattern contains the number of the ; next available pattern for drawing, so allocate this ; pattern to cover the pixel that we want to draw by ; setting the nametable entry to the pattern number we ; just fetched INC firstFreePattern ; Increment firstFreePattern to point to the next ; available pattern for drawing, so it can be added to ; the nametable the next time we need to draw lines or ; pixels into a pattern JMP loin21 ; Jump to loin21 to calculate the pattern buffer address ; for the new tile and continue drawing .loin25 ; If we get here then we are drawing our line in a ; pattern that was already in the nametable, so it might ; contain pre-existing content LDX pattBufferHiDiv8 ; Set SC(1 0) = (pattBufferHiDiv8 A) * 8 STX SC+1 ; = (pattBufferHi 0) + A * 8 ASL A ; ROL SC+1 ; So SC(1 0) is the address in the pattern buffer for ASL A ; pattern number A (as each pattern contains 8 bytes of ROL SC+1 ; pattern data), which means SC(1 0) points to the ASL A ; pattern data for the tile containing the line we are ROL SC+1 ; drawing STA SC CLC ; Clear the C flag for the additions below LDX Q ; Set X to the value of the x-axis counter .loin26 ; We now loop along the line from left to right, using P ; as a decreasing counter, and at each count we plot a ; single pixel using the pixel mask in R LDA R ; Fetch the pixel byte from R ORA (SC),Y ; Store R into screen memory at SC(1 0), using OR logic STA (SC),Y ; so it merges with whatever is already on-screen DEX ; Decrement the y-coordinate counter in X BEQ loin20 ; If we have just reached the end of the line along the ; y-axis, jump to loin20 to return from the subroutine LDA S ; Set S = S + P to update the slope error ADC P STA S BCC loin27 ; If the addition didn't overflow, jump to loin27 to ; skip the following LSR R ; Shift the single pixel in R to the right to step along ; the x-axis, so the next pixel we plot will be at the ; next x-coordinate along BCS loin28 ; If the pixel fell out of the right end of R into the ; C flag, then jump to loin28 to rotate it into the left ; end and move right by a character block .loin27 DEY ; Decrement Y to point to move to the pixel line above BPL loin26 ; If Y is still positive then we have not yet gone past ; the top of the character block, so jump to loin26 to ; draw the next pixel ; Otherwise we just gone past the top of the current ; character block, so we need to move up into the ; character block above by setting Y and SC2(1 0) LDY #7 ; Set Y to point to the bottom pixel row of the block ; above ; If we get here then the C flag is clear, as we either ; jumped to loin27 using a BCC, or we passed through a ; BCS to get to loin27, so the SBC #31 below actually ; subtracts 32 LDA SC2 ; Subtract 32 from SC2(1 0) to get the tile number on SBC #31 ; the row above (as there are 32 tiles on each row) and STA SC2 ; jump to loin24 to fetch the correct tile number for BCS loin24 ; the new character block and continue drawing (this DEC SC2+1 ; BNE is effectively a JMP as the high byte of SC2(1 0) BNE loin24 ; will never be zero (the nametable buffers start at ; address $7000, so the high byte is at least $70) .loin28 ; If we get here, then we just shifted the pixel out of ; the right end of R, so we now need to put it back into ; the left end of R and move to the right by one ; character block ROR R ; We only reach here via a BCS, so this rotates a 1 into ; the left end of R and clears the C flag INC SC2 ; Increment SC2(1 0) to point to the next tile number to BNE P%+4 ; the right in the nametable buffer INC SC2+1 DEY ; Decrement Y to point to move to the pixel line above BPL loin24 ; If Y is still positive then we have not yet gone past ; the top of the character block, so jump to loin24 to ; draw the next pixel LDY #7 ; Set Y to point to the bottom pixel row of the block ; above LDA SC2 ; Subtract 32 from SC2(1 0) to get the tile number on SBC #31 ; the row above (as there are 32 tiles on each row) and STA SC2 ; jump to loin24 to fetch the correct tile number for BCS loin24 ; the new character block and continue drawing DEC SC2+1 JMP loin24 .loin29 ; If we get here then we have run out of tiles to ; allocate to the line drawing, so we continue with the ; same calculations, but don't actually draw anything in ; this character block LDX Q ; Set X to the value of the x-axis counter .loin30 DEX ; Decrement the x-axis counter in X BEQ loin32 ; If we have just reached the end of the line along the ; x-axis, jump to loin32 to return from the subroutine LDA S ; Set S = S + P to update the slope error ADC P STA S BCC loin31 ; If the addition didn't overflow, jump to loin31 to ; skip the following LSR R ; Shift the single pixel in R to the right to step along ; the x-axis, so the next pixel we plot will be at the ; next x-coordinate along BCS loin28 ; If the pixel fell out of the right end of R into the ; C flag, then jump to loin28 to rotate it into the left ; end and move right by a character block .loin31 DEY ; Decrement Y to point to move to the pixel line above BPL loin30 ; If Y is still positive then we have not yet gone past ; the top of the character block, so jump to loin30 to ; draw the next pixel ; Otherwise we just gone past the top of the current ; character block, so we need to move up into the ; character block above by setting Y and SC2(1 0) LDY #7 ; Set Y to point to the bottom pixel row of the block ; above ; If we get here then the C flag is clear, as we either ; jumped to loin31 using a BCC, or we passed through a ; BCS to get to loin31, so the SBC #31 below actually ; subtracts 32 LDA SC2 ; Subtract 32 from SC2(1 0) to get the tile number on SBC #31 ; the row above (as there are 32 tiles on each row) STA SC2 BCS P%+4 DEC SC2+1 JMP loin24 ; Jump to loin24 to fetch the correct tile number for ; the new character block and continue drawing .loin32 LDY YSAV ; Restore Y from YSAV, so that it's preserved CLC ; Clear the C flag for the routine to return RTS ; Return from the subroutineName: LOIN (Part 6 of 7) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a steep line going up and left or down and right Deep dive: Bresenham's line algorithm Drawing lines in the NES versionContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. If we get here, then: * The line is going up and left (no swap) or down and right (swap) * X1 < X2 and Y1 >= Y2 * Draw from (X1, Y1) at top left to (X2, Y2) at bottom right
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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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Variable firstFreePattern in workspace ZP
Contains the number of the first free pattern in the pattern buffer that we can draw into next (or 0 if there are no free patterns)
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Label loin20 is local to this routine
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Label loin21 is local to this routine
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Label loin22 is local to this routine
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Label loin23 is local to this routine
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Label loin24 is local to this routine
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Label loin25 is local to this routine
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Label loin26 is local to this routine
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Label loin27 is local to this routine
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Label loin28 is local to this routine
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Label loin29 is local to this routine
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Label loin30 is local to this routine
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Label loin31 is local to this routine
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Label loin32 is local to this routine
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Variable pattBufferHiDiv8 in workspace ZP
High byte of the address of the current pattern buffer ($60 or $68) divided by 8