这是在网上看到的《全国翻译专业资格（水平）考试 英语二级《笔译实务》试卷》中的一段：

There they come, trudging along, straight upright on stubby legs, shoulders swinging back and forth with each step, coming into focus on the screen just as I’m eating my first bite of popcorn.

请问straight upright on stubby legs在句中是什么成分？ on stubby legs是介宾短语吗？和straight upright是什么关系？coming into focus on the screen just as I’m eating my first bite of popcorn可以翻译成“就像我吃第一口爆米花一样，他们成了我眼中的焦点。”