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Elite on the BBC Micro

Maths (Geometry): SNE

[6502 Second Processor version]

Name: SNE [Show more] Type: Variable Category: Maths (Geometry) Summary: Sine/cosine table Deep dive: The sine, cosine and arctan tables Drawing circles Drawing ellipses
Context: See this variable in context in the source code References: This variable is used as follows: * FMLTU2 uses SNE * PLS22 uses SNE

This lookup table contains sine values for the first half of a circle, from 0 to 180 degrees (0 to PI radians). In terms of circle or ellipse line segments, there are 64 segments in a circle, so this contains sine values for segments 0 to 31. In terms of segments, to calculate the sine of the angle at segment x, we look up the value in SNE + x, and to calculate the cosine of the angle we look up the value in SNE + ((x + 16) mod 32). In terms of radians, to calculate the following: sin(theta) * 256 where theta is in radians, we look up the value in: SNE + (theta * 10) To calculate the following: cos(theta) * 256 where theta is in radians, look up the value in: SNE + ((theta * 10) + 16) mod 32 Theta must be between 0 and 3.1 radians, so theta * 10 is between 0 and 31.
.SNE FOR I%, 0, 31 N = ABS(SIN((I% / 64) * 2 * PI)) IF N >= 1 EQUB 255 ELSE EQUB INT(256 * N + 0.5) ENDIF NEXT