Question

英语有个句子不是很懂，请指教

The best signal depends not only on its brightness but also on how well it contrasts with the background against which it must be seen.
这个句子讲的是关于 动物的光信号
我的问题是against which it must be seen.中的against 后面引导的部分，是与前面哪个部分相对应？？
how well it contrasts with the background against which it must be seen.是什么意思？？
p.s.我主要是against 在这里的句子的逻辑关系不清楚了，请指教

Answer 1

最佳光信号不仅取决于物体的亮度，也取决于物体与其背景对比的好坏程度，当然物体在背景映衬之下是可见的。against是介词，后面的which指代前面的background，也就是The best signal depends not only on its brightness but also on how well it contrasts with the background,and it must be seen against the background。

Answer 2

against 对应的是background,
最好的光信号不只在于它的亮度, 更在于光信号和它发出时的背景反差有多大.
the background against which it must be seen
(it must be seen against the background)

Answer 3

against which it must be seen修饰的是background
最好的信号不仅取决于它的亮度，还取决于它与它必须依靠的背景的对比的强烈程度。

Answer 4

这个很容易啊 指光照要看和背景的对比效果
against which it must be seen 指的是background
最有益的光信号不仅取决于光本身的亮度，也取决于光信号和背景对比的效果
how well it contrasts with the background against which it must be seen 直译是 光信号和它所必须被看到所依赖的背景对比的怎么样

Answer 5

against 后面的which指的是它前面的background，这是一个定语从句，限定前面的background，意思是“衬着这个背景它可以被看见”。
整句的意思是：最好的信号不仅取决于它的亮度，还要看它跟背景的对比度，衬着背景它可以被看见。

Answer 6

最好的光信号不仅取决于它本身的亮度，而且还取决于看见的背景的对比程度。
against 后面接的是定语从句 it 代指背景background。