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Revs on the BBC Micro

Maths (Geometry): GetRotationMatrix (Part 4 of 5)

Name: GetRotationMatrix (Part 4 of 5) [Show more] Type: Subroutine Category: Maths (Geometry) Summary: Loop back to calculate cos instead of sin Deep dive: The core driving model Trigonometry
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
.rotm5 CPX secondAxis \ If we just processed the second axis, then we have BEQ rotm6 \ now set both sinYawAngle and cosYawAngle, so jump to \ rotm6 to set their signs LDX secondAxis \ Otherwise set X = secondAxis so the next time we reach \ the end of the loop, we take the BEQ branch we just \ passed through LDA #0 \ Set (H G) = (201 0) - (H G) SEC \ SBC G \ starting with the low bytes STA G LDA #201 \ And then the high bytes SBC H STA H STA U \ Set (U G) = (H G) \ \ (U G) and (H G) were set to yawRadians / 2 for the \ first pass through the loop above, so we now have the \ following: \ \ 201 - yawRadians / 2 \ \ PI is represented by 804, as 804 / 256 = 3.14, so 201 \ represents PI/4, so this the same as: \ \ PI/4 - yawRadians / 2 \ \ Given that we expect (U G) to contain half the angle \ we are projecting, this means we are going to find the \ sine of this angle when we jump back to rotm1: \ \ PI/2 - yawRadians \ \ It's a trigonometric identity that: \ \ sin(PI/2 − x) = cos(x) \ \ so jumping back will, in fact, find the cosine of the \ angle JMP rotm1 \ Loop back to set the other variable of sinYawAngle and \ cosYawAngle to the cosine of the angle