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Elite on the BBC Micro

Drawing lines: LOIN (Part 7 of 7) [Electron version]

Name: LOIN (Part 7 of 7) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a steep line going up and right or down and left Deep dive: Bresenham's line algorithm
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: No direct references to this subroutine in this source file

This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. If we get here, then: * The line is going up and right (no swap) or down and left (swap) * X1 >= X2 and Y1 >= Y2 * Draw from (X1, Y1) at bottom left to (X2, Y2) at top right Other entry points: HL6 Contains an RTS
.LFT LDA SWAP \ If SWAP = 0 then we didn't swap the coordinates above, BEQ LI18 \ jump down to LI18 to skip plotting the first pixel DEX \ Decrement the counter in X because we're about to plot \ the first pixel .LIL6 LDA R \ Fetch the pixel byte from R EOR (SC),Y \ Store R into screen memory at SC(1 0), using EOR STA (SC),Y \ logic so it merges with whatever is already on-screen .LI18 DEY \ Decrement Y to step up along the y-axis BPL LI19 \ If Y is positive we are still within the same \ character block, so skip to LI19 \ We now need to move up into the character block above, \ and each character row in screen memory takes up &140 \ bytes (&100 for the visible part and &20 for each of \ the blank borders on the side of the screen), so \ that's what we need to subtract from SC(1 0) \ \ We also know the C flag is clear, as we call LFT with \ a BCC, so we can subtract &13F in order to get the \ correct result LDA SC \ Set SC(1 0) = SC(1 0) - &140 SBC #&3F \ STA SC \ Starting with the low bytes LDA SC+1 \ And then subtracting the high bytes SBC #&01 STA SC+1 LDY #7 \ Set the pixel line to the last line in that character \ block .LI19 LDA S \ Set S = S + P to update the slope error ADC P STA S BCC LIC6 \ If the addition didn't overflow, jump to LIC6 ASL R \ Otherwise we just overflowed, so shift the single \ pixel in R to the left, so the next pixel we plot \ will be at the previous x-coordinate BCC LIC6 \ If the pixel didn't fall out of the left end of R \ into the C flag, then jump to LIC6 ROL R \ Otherwise we need to move over to the next character \ block, so first rotate R left so the set C flag goes \ back into the right end, giving %0000001 LDA SC \ Subtract 7 from SC, so SC(1 0) now points to the SBC #7 \ previous character along to the left STA SC BCS P%+4 \ If the subtraction of the low bytes of SC underflowed, DEC SC+1 \ decrement the high byte CLC \ Clear the C flag so it doesn't affect the additions \ below .LIC6 DEX \ Decrement the counter in X BNE LIL6 \ If we haven't yet reached the left end of the line, \ loop back to LIL6 to plot the next pixel along LDY YSAV \ Restore Y from YSAV, so that it's preserved .HL6 RTS \ Return from the subroutine