.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 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 * LL9 (Part 12 of 12) calls via LL118-1
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 = 96
The centre y-coordinate of the 256 x 192 space view