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Drawing lines: LOIN (Part 1 of 7)

[NES version, Bank 7]

Name: LOIN (Part 1 of 7) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a line: Calculate the line gradient in the form of deltas Deep dive: Bresenham's line algorithm Drawing lines in the NES version
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * BLINE calls LOIN * CLIP_b1 calls LOIN * DrawLightning calls LOIN * LASLI calls LOIN * LL9 (Part 9 of 12) calls LOIN * LL9 (Part 10 of 12) calls LOIN * SHPPT calls LOIN * TT15 calls LOIN

This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. This stage calculates the line deltas.
Arguments: X1 The screen x-coordinate of the start of the line Y1 The screen y-coordinate of the start of the line X2 The screen x-coordinate of the end of the line Y2 The screen y-coordinate of the end of the line
.LOIN STY YSAV ; Store Y into YSAV, so we can preserve it across the ; call to this subroutine 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 #128 ; Set S = 128, which is the starting point for the STA S ; slope error (representing half a pixel) ASL A ; Set SWAP = 0, as %10000000 << 1 = 0 STA SWAP LDA X2 ; Set A = X2 - X1 SBC X1 ; = delta_x ; ; This subtraction works as the ASL A above sets the C ; flag BCS LI1 ; If X2 > X1 then A is already positive and we can skip ; the next three instructions EOR #%11111111 ; Negate the result in A by flipping all the bits and ADC #1 ; adding 1, i.e. using two's complement to make it ; positive .LI1 STA P ; Store A in P, so P = |X2 - X1|, or |delta_x| SEC ; Set the C flag, ready for the subtraction below LDA Y2 ; Set A = Y2 - Y1 SBC Y1 ; = delta_y ; ; This subtraction works as we either set the C flag ; above, or we skipped that SEC instruction with a BCS BCS LI2 ; If Y2 > Y1 then A is already positive and we can skip ; the next two instructions EOR #%11111111 ; Negate the result in A by flipping all the bits and ADC #1 ; adding 1, i.e. using two's complement to make it ; positive .LI2 STA Q ; Store A in Q, so Q = |Y2 - Y1|, or |delta_y| CMP P ; If Q < P, jump to STPX to step along the x-axis, as BCC STPX ; the line is closer to being horizontal than vertical JMP STPY ; Otherwise Q >= P so jump to STPY to step along the ; y-axis, as the line is closer to being vertical than ; horizontal