What is the summary of "Letter On Art" by Louis Althusser?

Louis Althusser's "Letter on Art" was a published open letter to Andre Daspré, a literary critic. It was written in response to an article Daspré published in La Nouvelle Critique, a journal of literary criticism, in 1966. Essentially, Althusser takes exception to how Daspré has characterized his position in the article, but the real significance of the letter is that it essentially summarizes Althusser's understanding of the relationship between art and "ideology." Essentially, Daspre has written that Althusser believes art is really nothing more than ideology, which Althusser describes as simply a reflection of the relationship between the state, the means of production, and the subject (or citizen) of that state.
This would suggest that all art is nothing more than a means of getting people to do what the state wants them to do, to accept their place in the world as part of the natural order of things. Althusser states that he does not "rank real art among the ideologies," though art does have a "particular and specific relationship with ideology." What he means, in layman's terms, is that real art (he distinguishes between high art and other forms) does not simply produce this relationship. Rather, it allows us to "see," "perceive," and "feel" the reality of ideology. Art—especially literature, which is really the focus of the letter, does not in itself represent a critique of society, but it rather allows us to see reality.
But he goes on to say that to really understand art, one has to gain a "knowledge" of art, which means reflecting, in a Marxian sense, on the conditions and the "mechanisms" that allowed the art to be produced in the first place. In other words, to understand the relationship between art and ideology, we must understand the material conditions and contexts that underlay the work.
http://www.rlwclarke.net/Theory/SourcesPrimary/AlthusserLetteronArtinReplytoAndreDaspre.pdf

https://plato.stanford.edu/entries/althusser/