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Elite on the BBC Micro and NES

Drawing lines: LOIN (Part 2 of 7)

[BBC Master version]

Name: LOIN (Part 2 of 7) [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a line: Line has a shallow gradient, step right along x-axis 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: * |delta_y| < |delta_x| * The line is closer to being horizontal than vertical * We are going to step right along the x-axis * We potentially swap coordinates to make sure X1 < X2
.STPX LDX X1 \ Set X = X1 CPX X2 \ If X1 < X2, jump down to LI3, as the coordinates are BCC LI3 \ already in the order that we want DEC SWAP \ Otherwise decrement SWAP from 0 to &FF, to denote that \ we are swapping the coordinates around LDA X2 \ Swap the values of X1 and X2 STA X1 STX X2 TAX \ Set X = X1 LDA Y2 \ Swap the values of Y1 and Y2 LDY Y1 STA Y1 STY Y2 .LI3 \ By this point we know the line is horizontal-ish and \ X1 < X2, so we're going from left to right as we go \ from X1 to X2 LDY Y1 \ Look up the page number of the character row that LDA ylookup,Y \ contains the pixel with the y-coordinate in Y1, and STA SC+1 \ store it in SC+1, so the high byte of SC is set \ correctly for drawing our line LDA Y1 \ Set Y = Y1 mod 8, which is the pixel row within the AND #7 \ character block at which we want to draw the start of TAY \ our line (as each character block has 8 rows) TXA \ Set A = 2 * bits 2-6 of X1 AND #%11111100 \ ASL A \ and shift bit 7 of X1 into the C flag STA SC \ Store this value in SC, so SC(1 0) now contains the \ screen address of the far left end (x-coordinate = 0) \ of the horizontal pixel row that we want to draw the \ start of our line on BCC P%+4 \ If bit 7 of X1 was set, so X1 > 127, increment the INC SC+1 \ high byte of SC(1 0) to point to the second page on \ this screen row, as this page contains the right half \ of the row TXA \ Set R = X1 mod 4, which is the horizontal pixel number AND #3 \ within the character block where the line starts (as STA R \ each pixel line in the character block is 4 pixels \ wide) \ The following section calculates: \ \ Q = Q / P \ = |delta_y| / |delta_x| \ \ using the log tables at logL and log to calculate: \ \ A = log(Q) - log(P) \ = log(|delta_y|) - log(|delta_x|) \ \ by first subtracting the low bytes of the logarithms \ from the table at LogL, and then subtracting the high \ bytes from the table at log, before applying the \ antilog to get the result of the division and putting \ it in Q LDX Q \ Set X = |delta_y| BEQ LIlog7 \ If |delta_y| = 0, jump to LIlog7 to return 0 as the \ result of the division LDA logL,X \ Set A = log(Q) - log(P) LDX P \ = log(|delta_y|) - log(|delta_x|) SEC \ SBC logL,X \ by first subtracting the low bytes of log(Q) - log(P) LDX Q \ And then subtracting the high bytes of log(Q) - log(P) LDA log,X \ so now A contains the high byte of log(Q) - log(P) LDX P SBC log,X BCS LIlog5 \ If the subtraction fitted into one byte and didn't \ underflow, then log(Q) - log(P) < 256, so we jump to \ LIlog5 to return a result of 255 TAX \ Otherwise we set A to the A-th entry from the antilog LDA alogh,X \ table so the result of the division is now in A JMP LIlog6 \ Jump to LIlog6 to return the result .LIlog5 LDA #255 \ The division is very close to 1, so set A to the BNE LIlog6 \ closest possible answer to 256, i.e. 255, and jump to \ LIlog6 to return the result (this BNE is effectively a \ JMP as A is never zero) .LIlog7 LDA #0 \ The numerator in the division is 0, so set A to 0 .LIlog6 STA Q \ Store the result of the division in Q, so we have: \ \ Q = |delta_y| / |delta_x| LDX P \ Set X = P \ = |delta_x| BEQ LIEXS \ If |delta_x| = 0, return from the subroutine, as LIEXS \ contains a BEQ LIEX instruction, and LIEX contains an \ RTS INX \ Set X = P + 1 \ = |delta_x| + 1 \ \ We add 1 so we can skip the first pixel plot if the \ line is being drawn with swapped coordinates LDA Y2 \ If Y2 < Y1 then skip the following instruction CMP Y1 BCC P%+5 JMP DOWN \ Y2 >= Y1, so jump to DOWN, as we need to draw the line \ to the right and down