lightposition behaves weirdly

lightposition behaves similarly for both mesh3d and surface, but it's nearly unusable. Generally x is left (-) to right (+) and y is bottom (-) to top(+), but z has effects that defy easy categorization other than z=1 being "special". Sometimes x and y flip randomly, and anyway they don't seem to have the same scales, ie x=y does not put the light at 45 degrees.

首先使用 mesh3d 和 surface traces 重现 lightposition，检查 x、y 和 z 的值，包括 z=1。将得到的光线方向和坐标轴比例与文档中记录的坐标预期进行比较。当行为一致，且 x=y 产生预期的 45 度关系，并且为报告的案例提供回归覆盖时，即表示完成。