Constraints

 Constraints limit freedom but generate invention. So, for example, if I had to translate a poem into English without using the letter e, I would be obliged to find inventive solutions instead of translating as I normally would.  

"Let the bride awake

the day of her wedding"

Most of these words have an e.  

Now, let's consider a more normal case: translating a sonnet into a sonnet.  Here the constraint is formal and metrical.  Lines will be metrical, and there will be a rhyme scheme.  I'm still trying to translate the semantic content.  These constraints will generate more semantic slippage. It is unlikely that a literal translation will automatically fall into metrical form! 

For many centuries in the English-speaking world, verse translations were almost always governed by metrical constraints: sonnets became sonnets, romances became ballads.  

This, I would argue, is the main driver of verse translation, not what Venuti says, the translator's invisibility.  

Retirado en la paz de estos desiertos, con pocos, pero doctos libros juntos, vivo en conversación con los difuntos y escucho con mis ojos a los muertos. Si no siempre entendidos, siempre abiertos, o enmiendan, o fecundan mis asuntos; y en músicos callados contrapuntos al sueño de la vida hablan despiertos. Las grandes almas que la muerte ausenta, de injurias de los años, vengadora, libra, ¡oh gran don Josef!, docta la emprenta. En fuga irrevocable huye la hora; pero aquella el mejor cálculo cuenta que en la lección y estudios nos mejora.