.LL118 LDA XX15+1 ; If x1_hi is positive, jump down to LL119 to skip the BPL LL119 ; following STA S ; Otherwise x1_hi is negative, i.e. off the left of the ; screen, so set S = x1_hi JSR LL120 ; Call LL120 to calculate: ; ; (Y X) = (S x1_lo) * XX12+2 if T = 0 ; = x1 * gradient ; ; (Y X) = (S x1_lo) / XX12+2 if T <> 0 ; = x1 / gradient ; ; with the sign of (Y X) set to the opposite of the ; line's direction of slope TXA ; Set y1 = y1 + (Y X) CLC ; ADC XX15+2 ; starting with the low bytes STA XX15+2 TYA ; And then adding the high bytes ADC XX15+3 STA XX15+3 LDA #0 ; Set x1 = 0 STA XX15 STA XX15+1 TAX ; Set X = 0 so the next instruction becomes a JMP .LL119 BEQ LL134 ; If x1_hi = 0 then jump down to LL134 to skip the ; following, as the x-coordinate is already on-screen ; (as 0 <= (x_hi x_lo) <= 255) STA S ; Otherwise x1_hi is positive, i.e. x1 >= 256 and off DEC S ; the right side of the screen, so set S = x1_hi - 1 JSR LL120 ; Call LL120 to calculate: ; ; (Y X) = (S x1_lo) * XX12+2 if T = 0 ; = (x1 - 256) * gradient ; ; (Y X) = (S x1_lo) / XX12+2 if T <> 0 ; = (x1 - 256) / gradient ; ; with the sign of (Y X) set to the opposite of the ; line's direction of slope TXA ; Set y1 = y1 + (Y X) CLC ; ADC XX15+2 ; starting with the low bytes STA XX15+2 TYA ; And then adding the high bytes ADC XX15+3 STA XX15+3 LDX #255 ; Set x1 = 255 STX XX15 INX STX XX15+1 .LL134 ; We have moved the point so the x-coordinate is on ; screen (i.e. in the range 0-255), so now for the ; y-coordinate LDA XX15+3 ; If y1_hi is positive, jump down to LL119 to skip BPL LL135 ; the following STA S ; Otherwise y1_hi is negative, i.e. off the top of the ; screen, so set S = y1_hi LDA XX15+2 ; Set R = y1_lo STA R JSR LL123 ; Call LL123 to calculate: ; ; (Y X) = (S R) / XX12+2 if T = 0 ; = y1 / gradient ; ; (Y X) = (S R) * XX12+2 if T <> 0 ; = y1 * gradient ; ; with the sign of (Y X) set to the opposite of the ; line's direction of slope TXA ; Set x1 = x1 + (Y X) CLC ; ADC XX15 ; starting with the low bytes STA XX15 TYA ; And then adding the high bytes ADC XX15+1 STA XX15+1 LDA #0 ; Set y1 = 0 STA XX15+2 STA XX15+3 .LL135 ;BNE LL139 ; This instruction is commented out in the original ; source LDA XX15+2 ; Set (S R) = (y1_hi y1_lo) - screen height SEC ; SBC #Y*2 ; starting with the low bytes STA R LDA XX15+3 ; And then subtracting the high bytes SBC #0 STA S BCC LL136 ; If the subtraction underflowed, i.e. if y1 < screen ; height, then y1 is already on-screen, so jump to LL136 ; to return from the subroutine, as we are done .LL139 ; If we get here then y1 >= screen height, i.e. off the ; bottom of the screen JSR LL123 ; Call LL123 to calculate: ; ; (Y X) = (S R) / XX12+2 if T = 0 ; = (y1 - screen height) / gradient ; ; (Y X) = (S R) * XX12+2 if T <> 0 ; = (y1 - screen height) * gradient ; ; with the sign of (Y X) set to the opposite of the ; line's direction of slope TXA ; Set x1 = x1 + (Y X) CLC ; ADC XX15 ; starting with the low bytes STA XX15 TYA ; And then adding the high bytes ADC XX15+1 STA XX15+1 LDA #Y*2-1 ; Set y1 = 2 * #Y - 1. The constant #Y is 96, the STA XX15+2 ; y-coordinate of the mid-point of the space view, so LDA #0 ; this sets Y2 to 191, the y-coordinate of the bottom STA XX15+3 ; pixel row of the space view .LL136 RTS ; Return from the subroutineName: LL118 [Show more] Type: Subroutine Category: Drawing lines Summary: Move a point along a line until it is on-screen Deep dive: Line-clippingContext: See this subroutine in context in the source code References: This subroutine is called as follows: * LL145 (Part 4 of 4) calls LL118
Given a point (x1, y1), a gradient and a direction of slope, move the point along the line until it is on-screen, so this effectively clips the (x1, y1) end of a line to be on the screen. See the deep dive on "Line-clipping" for more details.
Arguments: XX15(1 0) x1 as a 16-bit coordinate (x1_hi x1_lo) XX15(3 2) y1 as a 16-bit coordinate (y1_hi y1_lo) XX12+2 The line's gradient * 256 (so 1.0 = 256) XX12+3 The direction of slope: * Positive (bit 7 clear) = top left to bottom right * Negative (bit 7 set) = top right to bottom left T The gradient of slope: * 0 if it's a shallow slope * $FF if it's a steep slope
Returns: XX15 x1 as an 8-bit coordinate XX15+2 y1 as an 8-bit coordinate
Other entry points: LL118-1 Contains an RTS
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Label LL119 is local to this routine
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Subroutine LL120 (category: Maths (Arithmetic))
Calculate (Y X) = (S x1_lo) * XX12+2 or (S x1_lo) / XX12+2
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Subroutine LL123 (category: Maths (Arithmetic))
Calculate (Y X) = (S R) / XX12+2 or (S R) * XX12+2
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Label LL134 is local to this routine
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Label LL135 is local to this routine
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Label LL136 is local to this routine
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Configuration variable Y = 72
The centre y-coordinate of the 256 x 144 space view