Oboe/english Horn Glissandi

Hi, as I understand glissandi on the oboe and english horn are nearly impossible, but I've also heard that they can be accomplished at least to a semitone by manipulating the embouchure. Is this true? Can it be done in both directions? Would these glissandi be possible on the english horn?

Not a wind player myself, but I have had heard oboe glissandi done in some pieces. I know Mahler used this technique in several symphonies, the 3rd and 9th come to mind. Not sure about the english horn...I'm guessing semi-tone glissandi would be ok. 

Embouchure gliss in reeds will be always down, but I'd say maximum 1 semitone, maybe less, you can hear it in Symphonie Fantastique, beginning of last movement, Berlioz wrote a whole 8va gliss, which is never possible, (and he was an Orchestration "Expert" :facepalm: ) but I have always heard it like that, less than a semitone, and if its possible in oboe it must be possible in english horn, but in both it might be possible in some notes and others not, I'd guess very low it won't work

check video at 44:04

Penderecki's Capriccio for Oboe has lot of unusual effects including wider glissandos but I don't have the score, I don't know how did he notate those.

I don't play oboe but in clarinet you can do gliss in notes are not keys but holes, and you slide the finger to open them, but I don't know which notes are in oboe.