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BBC Micro Elite

Drawing lines: LL145 (Part 2 of 4)

Name: LL145 (Part 2 of 4) [View in context] Type: Subroutine Category: Drawing lines Summary: Clip line: Work out if any part of the line is on-screen
This part does a number of tests to see if the line is on or off the screen. If we get here then at least one of (x1, y1) and (x2, y2) is off-screen, with XX13 set as follows: * 0 = (x1, y1) off-screen, (x2, y2) on-screen * 95 = (x1, y1) on-screen, (x2, y2) off-screen * 191 = (x1, y1) off-screen, (x2, y2) off-screen where "off-screen" is defined as having a non-zero high byte in one of the coordinates, or in the case of y-coordinates, having a low byte > 191, the y-coordinate of the bottom of the space view.
.LL83 LDA XX13 \ If XX13 < 128 then only one of the points is on-screen BPL LL115 \ so jump down to LL115 to skip the checks of whether \ both points are in the strips to the right or bottom \ of the screen \ If we get here, both points are off-screen LDA XX15+1 \ If both x1_hi and x2_hi have bit 7 set, jump to LL109 AND XX15+5 \ to return from the subroutine with the C flag set, as BMI LL109 \ the entire line is above the top of the screen LDA XX15+3 \ If both y1_hi and y2_hi have bit 7 set, jump to LL109 AND XX12+1 \ to return from the subroutine with the C flag set, as BMI LL109 \ the entire line is to the left of the screen LDX XX15+1 \ Set A = X = x1_hi - 1 DEX TXA LDX XX15+5 \ Set XX12+2 = x2_hi - 1 DEX STX XX12+2 ORA XX12+2 \ If neither (x1_hi - 1) or (x2_hi - 1) have bit 7 set, BPL LL109 \ jump to LL109 to return from the subroutine with the C \ flag set, as the line doesn't fit on-screen LDA XX15+2 \ If y1_lo < y-coordinate of screen bottom, clear the C CMP #Y*2 \ flag, otherwise set it LDA XX15+3 \ Set XX12+2 = y1_hi - (1 - C), so: SBC #0 \ STA XX12+2 \ * Set XX12+2 = y1_hi - 1 if y1_lo is on-screen \ * Set XX12+2 = y1_hi otherwise \ \ We do this subtraction because we are only interested \ in trying to move the points up by a screen if that \ might move the point into the space view portion of \ the screen, i.e. if y1_lo is on-screen LDA XX12 \ If y2_lo < y-coordinate of screen bottom, clear the C CMP #Y*2 \ flag, otherwise set it LDA XX12+1 \ Set XX12+2 = y2_hi - (1 - C), so: SBC #0 \ \ * Set XX12+1 = y2_hi - 1 if y2_lo is on-screen \ * Set XX12+1 = y2_hi otherwise \ \ We do this subtraction because we are only interested \ in trying to move the points up by a screen if that \ might move the point into the space view portion of \ the screen, i.e. if y1_lo is on-screen ORA XX12+2 \ If neither XX12+1 or XX12+2 have bit 7 set, jump to BPL LL109 \ LL109 to return from the subroutine with the C flag \ set, as the line doesn't fit on-screen