.ClipStartOfLine LDA S \ Set (SS S) = (SS S) + 4 CLC \ ADC #4 \ starting with the low bytes STA S BCC clip1 \ If the addition didn't overflow, jump to clip1 to \ skip the following instruction INC SS \ And then we increment the high byte if the addition \ overflowed \ \ So we have now added an extra 4 to the start \ y-coordinate: \ \ (SS S) = (SS S) + 4 \ \ We add this to make the x- and y-coordinate consistent \ in terms of range, as the canopy's x-coordinates start \ at 4, but the y-coordinates start at 0. We remove this \ additional 4 at the end, after clipping the line .clip1 \ We now do some simple checks to weed out lines that \ are entirely off to one side of the screen LDA TT \ Set A = the clipping requirements for the start point BIT V \ If bit 7 of V is clear, jump to clip2 as the x-delta BPL clip2 \ of the line is positive, or to the right \ Bit 7 of V is set, so the x-delta of the line is \ negative, or to the left AND #%01000000 \ If bit 6 of TT is set, then the start point is off the BNE AbortLine \ left of the screen and the line direction is also to \ the left, so jump to AbortLine to stop drawing the \ line as it must be entirely off-screen BEQ clip3 \ Jump to clip3 to move on to the next check (this BEQ \ is effectively a JMP, as A is always zero) .clip2 \ If we get here then the x-delta of the line is \ positive, or to the right AND #%10000000 \ If bit 7 of TT is set, then the start point is off the BNE AbortLine \ right of the screen and the line direction is also to \ the right, so jump to AbortLine to stop drawing the \ line as it must be entirely off-screen BEQ clip3 \ Jump to clip3 to move on to the next check (this BEQ \ is effectively a JMP, as A is always zero) .clip3 LDA TT \ Set A = the clipping requirements for the start point BVC clip4 \ If bit 6 of V is clear, jump to clip4 as the y-delta \ of the line is positive, or up \ Bit 6 of V is set, so the y-delta of the line is \ negative, or down AND #%00010000 \ If bit 4 of TT is set, then the start point is off the BNE AbortLine \ bottom of the screen and the line direction is also \ down, so jump to AbortLine to stop drawing the \ line as it must be entirely off-screen BEQ clip5 \ Jump to clip5 to move on to the next check (this BEQ \ is effectively a JMP, as A is always zero) .clip4 \ If we get here then the y-delta of the line is \ positive, or up AND #%00100000 \ If bit 5 of TT is set, then the start point is off the BNE AbortLine \ top of the screen and the line direction is also up, \ so jump to AbortLine to stop drawing the line as it \ must be entirely off-screen \ If we get here then the line might have an on-screen \ element, so we now move on to the actual clippingName: ClipStartOfLine (Part 1 of 5) [Show more] Type: Subroutine Category: Drawing lines Summary: Check whether the line is completely off-screenContext: See this subroutine in context in the source code References: This subroutine is called as follows: * ClipBestEndOfLine calls ClipStartOfLine * DrawClippedLine (Part 4 of 6) calls ClipStartOfLine
The commentary in this routine is a work in progress.
Arguments: (RR R) The x-coordinate of the line's start point (SS S) The y-coordinate of the line's start point (I T) The line's |x-delta| (J U) The line's |y-delta| TT The clipping requirements for the start point Bit 4 Bit 5 Off top of screen 0 1 On-screen 0 0 Off bottom of screen 1 0 Bit 6 Bit 7 Off right of screen 0 1 On-screen 0 0 Off left of screen 1 0 V Direction of the line: * Bit 7 is the direction of the x-delta * Bit 6 is the direction of the y-delta Direction is like a clock, so positive (clear) is up and right
Returns: R The x-coordinate of the line's start point, clipped to fit on-screen S The y-coordinate of the line's start point, clipped to fit on-screen V Bit 0 is set to indicate the line has been clipped xTemp1Lo If the line has been clipped, contains a copy of R yTemp1Lo If the line has been clipped, contains a copy of S
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Entry point AbortLine in subroutine ClipBestEndOfLine (category: Drawing lines)
Abort drawing this line
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Label clip1 is local to this routine
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Label clip2 is local to this routine
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Label clip3 is local to this routine
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Label clip4 is local to this routine
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Label clip5 in subroutine ClipStartOfLine (Part 2 of 5)