Lab 4 geometry: two-link legs (L₁ servo arm + L₂) folding inward, servo angles α solved via the law of cosines. ψz = 0 as assumed in the lab.
Blue segment = first link L₁ (the servo arm, angle α from the horizontal servo plane, amber arc), folding toward the centre column like the real linkage. Gray segment = second link L₂ up to the magnetic spherical joint on the plate. When a joint leaves the reachable annulus |L₁ − L₂| ≤ d ≤ L₁ + L₂, the law of cosines gives |cos β| > 1 and α is undefined; that leg turns red dashed, matching the "Plate Angle Not Valid" case from Task 3. Drag the canvas to orbit; a side view shows the α triangles from Figure 4.11b. Measure lines: "Add measure line" drops a ruler on screen. Drag its middle to move it, drag an endpoint to rotate or stretch it, click empty space to deselect. Delete with the button or the Delete key. Each ruler shows its length (mm in the view plane, valid because the projection is orthographic) and its angle from horizontal. Camera frame {C}: hidden by default, toggled with "Show camera". It starts right next to O so the two frames nearly coincide, making the transform easy to verify before you move it. Drag the teal camera body to move it (it slides in your current view plane, so orbit to reposition in depth), or type exact coordinates in the number boxes. Drag the ringed handle on the zᶜ arrow to aim the camera anywhere; "Point down" and "Aim at T" are quick presets. The optical convention keeps xᶜ horizontal, so camera roll is fixed. Tracking point T (orange) lives on the plate: drag it, or double-click anywhere on the plate to place it there. Both readouts follow p_C = R_CO (p_O − C), the same rigid transform used to convert camera detections of the ball into world coordinates. Click O to pin a tag with its coordinates (and its position in the camera frame when the camera is shown); click the camera body to pin its world coordinates. Click either again to dismiss.