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Stardust: FLIP

[BBC Micro disc version, Flight]

Name: FLIP [Show more] Type: Subroutine Category: Stardust Summary: Reflect the stardust particles in the screen diagonal and redraw the stardust field
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: * LOOK1 calls FLIP

Swap the x- and y-coordinates of all the stardust particles and draw the new set of particles. Called by LOOK1 when we switch views. This is a quick way of making the stardust field in the new view feel different without having to generate a whole new field. If you look carefully at the stardust field when you switch views, you can just about see that the new field is a reflection of the previous field in the screen diagonal, i.e. in the line from bottom left to top right. This is the line where x = y when the origin is in the middle of the screen, and positive x and y are right and up, which is the coordinate system we use for stardust).
.FLIP LDY NOSTM \ Set Y to the current number of stardust particles, so \ we can use it as a counter through all the stardust .FLL1 LDX SY,Y \ Copy the Y-th particle's y-coordinate from SY+Y into X LDA SX,Y \ Copy the Y-th particle's x-coordinate from SX+Y into STA Y1 \ both Y1 and the particle's y-coordinate STA SY,Y TXA \ Copy the Y-th particle's original y-coordinate into STA X1 \ both X1 and the particle's x-coordinate, so the x- and STA SX,Y \ y-coordinates are now swapped and (X1, Y1) contains \ the particle's new coordinates LDA SZ,Y \ Fetch the Y-th particle's distance from SZ+Y into ZZ STA ZZ JSR PIXEL2 \ Draw a stardust particle at (X1,Y1) with distance ZZ DEY \ Decrement the counter to point to the next particle of \ stardust BNE FLL1 \ Loop back to FLL1 until we have moved all the stardust \ particles RTS \ Return from the subroutine