.DrawHalfHorizon LDX #30 \ Set X = 30, to use as the point ID for point 30, which \ is the start of the horizon line LDY #32 \ Set Y = 32, to use as the point ID for point 32 LDA xPointHi,X \ Set (T A) = x_30 STA T LDA xPointLo,X ASL A \ Set (T A) = (T A) * 2 ROL T \ = x_30 * 2 SEC \ Set x_32 = (T A) - x_31 SBC xPointLo+31 \ = (x_30 * 2) - x_31 STA xPointLo,Y LDA T SBC xPointHi+31 STA xPointHi,Y LDA yPointHi,X \ Set (T A) = y_30 STA T LDA yPointLo,X ASL A \ Set (T A) = (T A) * 2 ROL T \ = y_30 * 2 SEC \ Set y_32 = (T A) - y_31 SBC yPointLo+31 \ = (y_30 * 2) - y_31 STA yPointLo,Y LDA T SBC yPointHi+31 STA yPointHi,Y \ So now we have: \ \ x_32 = (x_30 * 2) - x_31 \ y_32 = (y_30 * 2) - y_31 \ \ which is what we want STX L \ Set L to point 30, so we draw the line from this start \ point STY M \ Set L to point 32, so we draw the line to this end \ point JSR DrawClippedHorizon \ Draw the line from point 30 to 32, which is the first \ half of the horizon line RTS \ Return from the subroutineName: DrawHalfHorizon [Show more] Type: Subroutine Category: Drawing lines Summary: Draw half of the horizon lineContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DrawCanopyView calls DrawHalfHorizon
The horizon line is line ID 0, which is the line from point 31 to point 30. This line is only half the actual horizon, so this routine calculates the other half of the horizon into point 30, and draws the line from point 30 to point 32. Point 30 is therefore the mid-point of the two-segment horizon line, which goes point 31 to point 30 to point 32. The calculation looks like this: + (x_32, y_32) .´ | .´ | h .´ | (x_30, y_30) .´_______| .´| w .´ | .´ | h .´ | (x_31, y_31) ´--------+ w We already know the coordinates of points 30 and 31, and we want to calculate point 32. It's easy to calculate w and h in the bottom triangle: w = x_30 - x_31 h = y_30 - y_31 so to extend the line to point 32, we simply add w and h to the coordinates for point 30, like this: x_32 = x_30 + w = x_30 + x_30 - x_31 = (x_30 * 2) - x_31 y_32 = y_30 + h = y_30 + y_30 - y_31 = (y_30 * 2) - y_31 This routine draws the horizon from point 30 to point 32, while the other half from point 31 to 30 is drawn along with all the other lines in DrawCanopyView.
Entry point DrawClippedHorizon in subroutine DrawClippedLine (Part 1 of 6) (category: Drawing lines)
Set bit 1 of the line direction in V (for the horizon)
High byte of the x-coordinate for a point