.dpol3 \ We jump back here to move on to the next pixel line in \ the polygon LDY yPolygonLine \ Set Y to the number of the polygon pixel line we are \ drawing (which decrements as we draw the polygon from \ top to bottom) CPY yPolygonBottom \ If Y = yPolygonBottom then we have reached the bottom BEQ dpol2 \ of the polygon, so jump to dpol2 to return from the \ subroutine as the whole polygon has now been drawn TYA \ Set A = Y mod 8 AND #%00000111 \ \ So A is the number of the polygon pixel line within \ the current character row BNE dpol11 \ If A is non-zero then the next polygon line (which \ will be on polygon pixel row A - 1) will still be in \ the same character row, so jump to dpol11 to move down \ one pixel line \ If we get here then A = 0, so the next polygon pixel \ line is in a different character row, so we need to \ move on to the next character row in the screen or the \ screen buffer LDA R \ Set (A R) = (S R) + &139 CLC \ = (S R) + 320 - 7 ADC #&39 \ STA R \ We add 320 to move on to the next row down on the LDA S \ screen, but we want to be in the top pixel row within ADC #&01 \ that character row, so we subtract 7 to move up to the \ top of the character block (as each row is eight pixel \ bytes high) CMP #&53 \ If the result of the addition is less than &5300, then BNE dpol4 \ we have not reached the end of row 15 in the screen \ buffer, so jump to dpol4 to skip the following \ If we get here then we need to wrap the screen buffer \ address around so we move from screenBufferRow15 to \ screenBufferRow16 LDA bufferRowAddrLo+16 \ Set (A R) to the address of screenBufferRow16 from the STA R \ bufferRowAddr(Hi Lo) LDA bufferRowAddrHi+16 .dpol4 STA S \ Store the high byte of the result, so we now have: \ \ (S R) = (S R) + 320 - 7 \ \ with the address wrapped around as required BNE dpol12 \ Jump to dpol12 to draw the next polygon pixel line \ (this BNE is effectively a JMP as the high byte of the \ address in A is never zero) .dpol5 \ If we get here then the right edge of the polygon line \ we are drawing extends past the left edge of the \ screen buffer LDA #0 \ Set polygonGoesLeft = 0 to denote that the polygon STA polygonGoesLeft \ extends past the left edge of the buffer, so if we \ are drawing in the right row buffer, this makes us \ draw the rest of the polygon in the left row buffer \ in the DrawPolygon routine BEQ dpol3 \ Jump to dpol3 to move on to the next pixel line in the \ polygon (this BEQ is effectively a JMP as A is always \ zero) .dpol6 \ If we get here then the left edge of the polygon line \ we are drawing extends past the right edge of the \ screen buffer LDA #0 \ Set polygonGoesRight = 0 to denote that the polygon STA polygonGoesRight \ extends past the right edge of the buffer, so if we \ are drawing in the left row buffer, this makes us draw \ the rest of the polygon in the right row buffer in the \ DrawPolygon routine BEQ dpol3 \ Jump to dpol3 to move on to the next pixel line in the \ polygon (this BEQ is effectively a JMP as A is always \ zero) .dpol7 \ If we get here then the right edge of the polygon line \ we are drawing extends past the right edge of the \ screen buffer LDA xBufferWidth \ Set xPolygonRightEdge = xBufferWidth * 2 ASL A \ STA xPolygonRightEdge \ This sets the screen x-coordinate of the right end of \ the polygon line we are drawing to the right edge of \ the screen buffer (we double the value in xBufferWidth \ to get the x-coordinate) STA polygonGoesRight \ Set polygonGoesRight to the value of A, which is at \ least 2 because xBufferWidth is non-zero and we just \ doubled it, so this has the same effect in the \ DrawPolygon routine as setting polygonGoesRight to 0 \ \ In other words, this denotes that the polygon extends \ past the right edge of the buffer, so if we are \ drawing in the left row buffer, this makes us draw the \ rest of the polygon in the right row buffer in the \ DrawPolygon routine BNE dpol15 \ Jump to dpol15 to draw the left edge and fill line for \ this polygon line (this BNE is effectively a JMP as A \ is never zero) .dpol8 \ If we get here then the left edge of the polygon line \ we are drawing extends past the left edge of the \ screen buffer \ \ We therefore need to draw the left end of the line in \ the screen buffer without the edge byte, and with the \ fill portion starting at the buffer's left edge LDA R \ Set (Q P) = (S R) - 8 SEC \ SBC #8 \ This sets (Q P) to the very first byte in the screen STA P \ row, so we don't reserve the first byte for the left LDA S \ edge in part 4 and instead draw the fill portion of SBC #0 \ the polygon line from the left edge of the screen row STA Q LDA #0 \ Set polygonGoesLeft = 0 to denote that the polygon STA polygonGoesLeft \ extends past the left edge of the buffer, so if we \ are drawing in the right row buffer, this makes us \ draw the rest of the polygon in the left row buffer \ in the DrawPolygon routine LDA #&F8 \ Set A = -8 to set the left edge of the line to the \ left edge of the screen (this is the same as setting \ the left edge to 0 and then subtracting 8 for the same \ reason we subtracted 8 from (Q P) above) BNE dpol17 \ Jump to dpol17 to draw the polygon line in part 4 \ (this BNE is effectively a JMP as A is never zero) .dpol9 \ If we get here then the pixel byte offset for the left \ edge is the same as the pixel byte offset for the \ right edge, so we need to draw both the left and right \ edges within the same pixel byte TXA \ Set X to the pixel number within the character block AND #%00000011 \ of the left edge of the polygon (as each character TAX \ block contains four pixels) LDA currentPixelByte \ Set A to the current screen contents for the pixel \ byte containing the right edge (which is the same as \ the pixel byte containing the left edge) \ \ We set this to the result of a LDA (R),Y instruction \ in part 3, so this is the same as fetching the current \ screen contents with LDA (R),Y AND pixelsToLeft,X \ Clear the pixels to the right of the left edge by \ AND'ing the current screen contents with a pixel byte \ with all the pixels to the left of position X set \ \ So A contains the existing screen contents for the \ pixels outside the polygon line, and with the polygon \ line pixels cleared, so now we can OR the polygon line \ pixels into the space we just cleared .dpol10 ORA edgePixelsLeft+&3C,X \ This instruction is modified in part 1 to point to \ the correct entry for the specified polygon \ colour: \ \ ORA edgePixelsLeft + polygonColours,X \ \ So this inserts the left edge of the polygon into \ the right part of the pixel byte in the correct \ colours for the currently configured fill and edge \ colours, with the edge pixels starting at pixel \ number X within the byte and ending at the right \ end of the pixel byte \ \ The original value of edgePixelsLeft+&3C is just \ workspace noise and has no meaning, it just sets \ the high byte to HI(edgePixelsLeft) AND leftPixels,X \ Clear all the pixels to the right of the right edge STA T \ and store the result in T, so this contains the pixel \ byte with the left edge in-place and the pixels for \ the right edge cleared \ \ The leftPixels bit mask is the complementary pixel \ byte to pixelsToRight, which we are about to use to \ fill in the right edge LDA (R),Y \ Set A to the current screen contents of the byte we \ are drawing both edges into AND pixelsToRight,X \ Clear the pixels to the left of the right edge by \ AND'ing the current screen contents with a pixel byte \ with all the pixels to the right of position X set \ \ So A contains the existing screen contents for the \ pixels outside the polygon line, and with the polygon \ line pixels cleared, so now we can OR the polygon line \ pixels into the space we just cleared ORA T \ Insert the right edge of the polygon into the pixel \ byte, with the edge pixels starting at the left end \ of the pixel byte and ending at pixel number X within \ the byte STA (R),Y \ Poke the pixel byte containing the left and right \ edges into screen memory JMP dpol3 \ Jump to dpol3 to move on to the next pixel line in the \ polygon .dpol11 INC R \ Increment the screen address in (S R) to move down one \ pixel row within the character row, ready for drawing \ the next polygon pixel line .dpol12 DEY \ Decrement the polygon line number in yPolygonLine to STY yPolygonLine \ move down one line in the polygon, from the top to the \ bottomName: DrawPolygonLines (Part 2 of 4) [Show more] Type: Subroutine Category: Drawing polygons Summary: Process the line overflowing the sides of the buffer, or fitting into one pixel byte, and loop on to the next polygon pixel lineContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
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Variable bufferRowAddrHi (category: Screen buffer)
Address lookup table for character rows in the screen buffer (high byte)
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Variable bufferRowAddrLo (category: Screen buffer)
Address lookup table for character rows in the screen buffer (low byte)
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Variable currentPixelByte in workspace Zero page
The original contents of the screen behind the right edge of the polygon being drawn
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Label dpol11 is local to this routine
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Label dpol12 is local to this routine
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Label dpol15 in subroutine DrawPolygonLines (Part 3 of 4)
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Label dpol17 in subroutine DrawPolygonLines (Part 4 of 4)
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Label dpol2 in subroutine DrawPolygonLines (Part 1 of 4)
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Label dpol3 is local to this routine
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Label dpol4 is local to this routine
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Variable edgePixelsLeft (category: Drawing polygons)
Table to convert a polygon colours byte and a pixel offset (0-3) into a pixel byte for the left edge of the polygon pixel line
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Variable leftPixels (category: Graphics)
Pixel byte with all the pixels to the right of position X + 1 set, to give the complementary pixel byte to pixelsToRight
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Variable pixelsToLeft (category: Graphics)
Pixel byte with all the pixels to the left of position X set
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Variable pixelsToRight (category: Graphics)
Pixel byte with all the pixels to the right of position X set
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Variable polygonGoesLeft in workspace Zero page
A flag to record whether the polygon we are drawing extends to the left of the screen buffer we are drawing in
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Variable polygonGoesRight in workspace Zero page
A flag to record whether the polygon we are drawing extends to the right of the screen buffer we are drawing in
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Variable xBufferWidth in workspace Zero page
The width in pixels of the screen buffer
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Variable xPolygonRightEdge in workspace Zero page
The screen x-coordinate of the right edge of the polygon being drawn
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Variable yPolygonBottom in workspace Zero page
The y-coordinate of the bottom of the polygon being drawn (where higher y-coordinates are up the screen)
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Variable yPolygonLine in workspace Zero page
The polygon y-coordinate of the pixel line being drawn in the DrawPolygonLines routine