Linear perspective basically consists of drawing straight lines that extend from the forefront of the painting into the background-lines that seem to be parallel to each other, but which actually converge on a single point in the horizon, called the vanishing point.

that一般是修饰先行词是吗？

 into the background-lines that seem to be parallel to each other

这句话that是修饰背景线的吗？从语义上好像是修整句的吧？