I have a very good ear, and can sing anything that's given to me (without ever hearing it before, if it wasn't obvious). However, the aspect of reading music from a staff instead of commands which indicate which interval or scale degree to sing, is a lot harder - as it barely gives me any information about the music - I don't know what the tonic is until I start sight singing using the intervals, and because of the irregular notation of intervals - sight singing using the intervals is very hard (and the fact that key signatures change the intervals just makes it a lot harder).
Right now I can only think of one solution - and that is to read the absolute pitch (in pitch class) and calculate the intervals - however, that makes it a lot harder to use scale degrees.
How do you go about sight singing (extremely different from sight reading for instruments)? Is the approach that I suggested the only one or are there more?
Thanks in advance.