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I was looking at one of the example sentences in a kanji learning app I use, and the particle choice struck me as strange. In the sentence "青空を背景に木々を描く。", I can't understand why を is used after 青空. Assuming the phrase could be translated as 'the background of the blue sky' (as the app's provided translation suggested) why is を used instead of の? 木々 is the direct object, 青空 is neither 'a point of departure' nor 'the route of motion' (as described here: https://nihongoichiban.com/home/japanese-grammar-particles/), so is this a typo or just an unfamiliar usage of the particle?