qt4 - Compressing and Serializing QImage over the Network and knowing compressed size in advance -

i sending several messagepacket , screenpacket on network. packets followed commonheaderpacket tells type, size, id, time, , checksum of upcoming packet. need know size of packet before sending.
screenpacket has _image:qimage (jpeg) compressed , serialized on network. can through qimagewriter while sending on socket. before sending screenpacket need send commonheaderpacket includes size. how can know size in advance ? qimage has no setquality() or setcompression() method.
dont want write in temporary stream know size in advance. else way know size in advance.

summary

qimage has no setcompression or setquality method. qimagewriter has. need know bytesize before writing. , byte size depends on compression , quality.
e.g. need qimage saved persistent storable object change in quality, compression. without disk overhead.

would use qimagewriter write qbuffer? way avoid disk overhead , can access qbytearray size , data push directly network stream. have encode jpeg still once.