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Elite on the BBC Micro

Drawing circles: HFS2 [Master version]

Name: HFS2 [Show more] Type: Subroutine Category: Drawing circles Summary: Draw the launch or hyperspace tunnel
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: * LL164 calls HFS2 * TT110 calls entry point HFS1

The animation gets drawn like this. First, we draw a circle of radius 8 at the centre, and then double the radius, draw another circle, double the radius again and draw a circle, and we keep doing this until the radius is bigger than 160 (which goes beyond the edge of the screen, which is 256 pixels wide, equivalent to a radius of 128). We then repeat this whole process for an initial circle of radius 9, then radius 10, all the way up to radius 15. This has the effect of making the tunnel appear to be racing towards us as we hurtle out into hyperspace or through the space station's docking tunnel. The hyperspace effect is done in a full mode 2 screen, which makes the rings all coloured and zig-zaggy, while the launch screen is in the normal four-colour mode 1 screen. Arguments: A The step size of the straight lines making up the rings (4 for launch, 8 for hyperspace) Other entry points: HFS1 Don't clear the screen, and draw 8 concentric rings with the step size in STP
.HFS2 STA STP \ Store the step size in A LDA QQ11 \ Store the current view number in QQ11 on the stack PHA LDA #0 \ Clear the top part of the screen, draw a white border, JSR TT66 \ and set the current view type in QQ11 to 0 (the space \ view) PLA \ Restore the view number from the stack STA QQ11 .HFS1 LDX #X \ Set K3 = #X (the x-coordinate of the centre of the STX K3 \ screen) LDX #Y \ Set K4 = #Y (the y-coordinate of the centre of the STX K4 \ screen) LDX #0 \ Set X = 0 STX XX4 \ Set XX4 = 0, which we will use as a counter for \ drawing eight concentric rings STX K3+1 \ Set the high bytes of K3(1 0) and K4(1 0) to 0 STX K4+1 .HFL5 JSR HFL1 \ Call HFL1 below to draw a set of rings, with each one \ twice the radius of the previous one, until they won't \ fit on-screen INC XX4 \ Increment the counter and fetch it into X LDX XX4 CPX #8 \ If we haven't drawn 8 sets of rings yet, loop back to BNE HFL5 \ HFL5 to draw the next ring RTS \ Return from the subroutine .HFL1 LDA XX4 \ Set K to the ring number in XX4 (0-7) + 8, so K has AND #7 \ a value of 8 to 15, which we will use as the starting CLC \ radius for our next set of rings ADC #8 STA K .HFL2 LDA #1 \ Set LSP = 1 to reset the ball line heap STA LSP JSR CIRCLE2 \ Call CIRCLE2 to draw a circle with the centre at \ (K3(1 0), K4(1 0)) and radius K ASL K \ Double the radius in K BCS HF8 \ If the radius had a 1 in bit 7 before the above shift, \ then doubling K will means the circle will no longer \ fit on the screen (which is width 256), so jump to \ HF8 to stop drawing circles LDA K \ If the radius in K <= 160, loop back to HFL2 to draw CMP #160 \ another one BCC HFL2 .HF8 RTS \ Return from the subroutine