这位画家在油画下面留下了细致的底稿，为最终作品提供了精确的轮廓。

The painter left detailed ___underdrawing beneath the oil painting to provide precise outlines for the final artwork.

X光检测显示，这幅油画下面有一层精细的底稿。

X-ray examination revealed a delicate ___underdrawing beneath the oil painting.