Skip to navigation

BBC Micro Elite

Drawing planets: PL9 (Part 2 of 3)

Name: PL9 (Part 2 of 3) [View in context] Type: Subroutine Category: Drawing planets Summary: Draw the planet's equator and meridian Deep dive: Drawing meridians and equators
Draw the planet's equator and meridian. Arguments: K(1 0) The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet INWK The planet's ship data block
LDA K \ If the planet's radius is less than 6, the planet is CMP #6 \ too small to show a crater, so jump to PL20 to return BCC PL20 \ from the subroutine LDA INWK+14 \ Set P = -nosev_z_hi EOR #%10000000 STA P LDA INWK+20 \ Set A = roofv_z_hi JSR PLS4 \ Call PLS4 to calculate the following: \ \ CNT2 = arctan(P / A) / 4 \ = arctan(-nosev_z_hi / roofv_z_hi) / 4 \ \ and give the result the opposite sign to nosev_z_hi LDX #9 \ Set X to 9 so the call to PLS1 divides nosev_x JSR PLS1 \ Call PLS1 to calculate the following: STA K2 \ STY XX16 \ (XX16 K2) = nosev_x / z \ \ and increment X to point to nosev_y for the next call JSR PLS1 \ Call PLS1 to calculate the following: STA K2+1 \ STY XX16+1 \ (XX16+1 K2+1) = nosev_y / z LDX #15 \ Set X to 15 so the call to PLS5 divides roofv_x JSR PLS5 \ Call PLS5 to calculate the following: \ \ (XX16+2 K2+2) = roofv_x / z \ \ (XX16+3 K2+3) = roofv_y / z JSR PLS2 \ Call PLS2 to draw the first meridian LDA INWK+14 \ Set P = -nosev_z_hi EOR #%10000000 STA P LDA INWK+26 \ Set A = sidev_z_hi, so the second meridian will be at \ 90 degrees to the first JSR PLS4 \ Call PLS4 to calculate the following: \ \ CNT2 = arctan(P / A) / 4 \ = arctan(-nosev_z_hi / sidev_z_hi) / 4 \ \ and give the result the opposite sign to nosev_z_hi LDX #21 \ Set X to 21 so the call to PLS5 divides sidev_x JSR PLS5 \ Call PLS5 to calculate the following: \ \ (XX16+2 K2+2) = sidev_x / z \ \ (XX16+3 K2+3) = sidev_y / z JMP PLS2 \ Jump to PLS2 to draw the second meridian, returning \ from the subroutine with a tail call