lavu/frame: Improve ROI documentation

Clarify and add examples for the behaviour of the quantisation offset,
and define how multiple ranges should be handled.

libavutil/frame.h

@@ -207,31 +207,58 @@ typedef struct AVFrameSideData {
 } AVFrameSideData;
 
 /**
- * Structure to hold Region Of Interest.
+ * Structure describing a single Region Of Interest.
  *
- * self_size specifies the size of this data structure. This value
- * should be set to sizeof(AVRegionOfInterest). EINVAL is returned if self_size is zero.
+ * When multiple regions are defined in a single side-data block, they
+ * should be ordered from most to least important - some encoders are only
+ * capable of supporting a limited number of distinct regions, so will have
+ * to truncate the list.
  *
- * Number of pixels to discard from the top/bottom/left/right border of
- * the frame to obtain the region of interest of the frame.
- * They are encoder dependent and will be extended internally
- * if the codec requires an alignment.
- * If the regions overlap, the last value in the list will be used.
- *
- * qoffset is quant offset, and base rule here:
- * returns EINVAL if AVRational.den is zero.
- * the value (num/den) range is [-1.0, 1.0], clamp to +-1.0 if out of range.
- * 0 means no picture quality change,
- * negative offset asks for better quality (and the best with value -1.0),
- * positive offset asks for worse quality (and the worst with value 1.0).
- * How to explain/implement the different quilaity requirement is encoder dependent.
+ * When overlapping regions are defined, the first region containing a given
+ * area of the frame applies.
  */
 typedef struct AVRegionOfInterest {
+    /**
+     * Must be set to the size of this data structure (that is,
+     * sizeof(AVRegionOfInterest)).
+     */
     uint32_t self_size;
+    /**
+     * Distance in pixels from the top edge of the frame to the top and
+     * bottom edges and from the left edge of the frame to the left and
+     * right edges of the rectangle defining this region of interest.
+     *
+     * The constraints on a region are encoder dependent, so the region
+     * actually affected may be slightly larger for alignment or other
+     * reasons.
+     */
     int top;
     int bottom;
     int left;
     int right;
+    /**
+     * 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.
+     */
     AVRational qoffset;
 } AVRegionOfInterest;