+ /**
+ * 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.
+ */