The word "semiopaque" refers to an object or substance that is partly translucent or not completely transparent. Antonyms for this word would include terms such as transparent, clear, see-through, and translucent. Other contrasting terms might include opaque, cloudy, dense, or obscure. In certain contexts, the opposite of "semiopaque" may be described as luminous, radiant, glowing, or shining. When describing a material or surface that allows light to pass through easily, words like crystal-clear, diaphanous, or gossamer might be used instead. Ultimately, the choice of antonym will depend on the specific context and intended meaning of the word "semiopaque.