Must be in the range -1 to +1. A value of zero indicates no quality
change. A negative value asks for better quality (less quantisation),
while a positive value asks for worse quality (greater quantisation).
The range is calibrated so that the extreme values indicate the
largest possible offset - if the rest of the frame is encoded with the
worst possible quality, an offset of -1 indicates that this region
should be encoded with the best possible quality anyway. Intermediate
values are then interpolated in some codec-dependent way.
For example, in 10-bit H.264 the quantisation parameter varies between
-12 and 51. A typical qoffset value of -1/10 therefore indicates that
this region should be encoded with a QP around one-tenth of the full
range better than the rest of the frame. So, if most of the frame
were to be encoded with a QP of around 30, this region would get a QP
of around 24 (an offset of approximately -1/10 * (51 - -12) = -6.3).
An extreme value of -1 would indicate that this region should be
encoded with the best possible quality regardless of the treatment of
the rest of the frame - that is, should be encoded at a QP of -12.
Quantisation offset.
Must be in the range -1 to +1. A value of zero indicates no quality change. A negative value asks for better quality (less quantisation), while a positive value asks for worse quality (greater quantisation).
The range is calibrated so that the extreme values indicate the largest possible offset - if the rest of the frame is encoded with the worst possible quality, an offset of -1 indicates that this region should be encoded with the best possible quality anyway. Intermediate values are then interpolated in some codec-dependent way.
For example, in 10-bit H.264 the quantisation parameter varies between -12 and 51. A typical qoffset value of -1/10 therefore indicates that this region should be encoded with a QP around one-tenth of the full range better than the rest of the frame. So, if most of the frame were to be encoded with a QP of around 30, this region would get a QP of around 24 (an offset of approximately -1/10 * (51 - -12) = -6.3). An extreme value of -1 would indicate that this region should be encoded with the best possible quality regardless of the treatment of the rest of the frame - that is, should be encoded at a QP of -12.