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Drawing suns: WPLS

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Name: WPLS [Show more] Type: Subroutine Category: Drawing suns Summary: Remove the sun from the screen Deep dive: Drawing the sun
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Main flight loop (Part 14 of 16) calls WPLS * PL2 calls WPLS * SUN (Part 1 of 4) calls WPLS

We do this by redrawing it using the lines stored in the sun line heap when the sun was originally drawn by the SUN routine.
Arguments: SUNX(1 0) The x-coordinate of the vertical centre axis of the sun
Other entry points: WPLS-1 Contains an RTS
.WPLS LDA LSX \ If LSX < 0, the sun line heap is empty, so return from BMI WPLS-1 \ the subroutine (as WPLS-1 contains an RTS) LDA SUNX \ Set YY(1 0) = SUNX(1 0), the x-coordinate of the STA YY \ vertical centre axis of the sun that's currently on LDA SUNX+1 \ screen STA YY+1 LDY #2*Y-1 \ #Y is the y-coordinate of the centre of the space \ view, so this sets Y as a counter for the number of \ lines in the space view (i.e. 191), which is also the \ number of lines in the LSO block .WPL2 LDA LSO,Y \ Fetch the Y-th point from the sun line heap, which \ gives us the half-width of the sun's line on this line \ of the screen BEQ P%+5 \ 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 DEY \ Decrement the loop counter BNE WPL2 \ Loop back for the next line in the line heap until \ we have gone through the entire heap DEY \ This sets Y to &FF, as we end the loop with Y = 0 STY LSX \ Set LSX to &FF to indicate the sun line heap is empty RTS \ Return from the subroutine