From the specification,
void *glCopyBufferSubData(enum readtarget, enum writetarget, intptr readoffset, intptr writeoffset, sizeiptr size);
glCopyBufferSubData copies part of the data store attached to readtarget to the
data store attached to writetarget. The number of basic machine units indicated by size
is copied from the source, at offset readoffset to the destination at writeoffset,
also in basic machine units.
It's not very clear, however, what's the meaning if readoffset, writeoffset and size, and how should them be used.
glBindFramebuffer(GL_COPY_READ_BUFFER, fboSrc); glBindFramebuffer(GL_COPY_WRITE_BUFFER, fboDst); glCopyBufferSubData(GL_COPY_READ_BUFFER, GL_COPY_WRITE_BUFFER, ?, ?, ?);