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Elite on the BBC Micro

Drawing suns: SUN (Part 2 of 4) (Cassette version)

Name: SUN (Part 2 of 4) [View in context] Type: Subroutine [Compare versions] Category: Drawing suns Summary: Draw the sun: Start from bottom of screen and erase the old sun Deep dive: Drawing the sun
This part erases the old sun, starting at the bottom of the screen and working upwards until we reach the bottom of the new sun.
LDY #2*Y-1 \ Set Y = y-coordinate of the bottom of the screen, \ which we use as a counter in the following routine to \ redraw the old sun LDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the old sun that's currently LDA SUNX+1 \ on-screen STA YY+1 .PLFL2 CPY TGT \ If Y = TGT, we have reached the line where we will BEQ PLFL \ start drawing the new sun, so there is no need to \ keep erasing the old one, so jump down to PLFL LDA LSO,Y \ Fetch the Y-th point from the sun line heap, which \ gives us the half-width of the old sun's line on this \ line of the screen BEQ PLF13 \ If A = 0, skip the following call to HLOIN2 as there \ is no sun line on this line of the screen JSR HLOIN2 \ Call HLOIN2 to draw a horizontal line on pixel line Y, \ with centre point YY(1 0) and half-width A, and remove \ the line from the sun line heap once done .PLF13 DEY \ Decrement the loop counter BNE PLFL2 \ Loop back for the next line in the line heap until \ we have either gone through the entire heap, or \ reached the bottom row of the new sun