ipa: ipu3: Rename IspStatsRegion to Accumulator

The IspStatsRegion structure was introduced as an attempt to prepare for
a generic AWB algorithm structure. The structure name by itself is not
explicit and it is too optimistic to try and make a generic one for now.

Its role is to accumulate the pixels in a given zone. Rename it to
accumulator, and remove the uncounted field at the same time. It is
always possible to know how many pixels are not relevant for the
algorithm by calculating total-counted. The uncounted field was only
declared and not used. Amend the documentation accordingly.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

2 files changed, 62 insertions, 21 deletions

diff --git a/src/ipa/ipu3/algorithms/awb.cpp b/src/ipa/ipu3/algorithms/awb.cpp
index e05647c9..964d6995 100644
--- a/src/ipa/ipu3/algorithms/awb.cpp
+++ b/src/ipa/ipu3/algorithms/awb.cpp
@@ -21,26 +21,69 @@ static constexpr uint32_t kMinZonesCounted = 16;
 static constexpr uint32_t kMinGreenLevelInZone = 32;
 
 /**
- * \struct IspStatsRegion
- * \brief RGB statistics for a given region
+ * \struct Accumulator
+ * \brief RGB statistics for a given zone
  *
- * The IspStatsRegion structure is intended to abstract the ISP specific
- * statistics and use an agnostic algorithm to compute AWB.
+ * - Cells are defined in Pixels
+ * - Zones are defined in Cells
  *
- * \var IspStatsRegion::counted
- * \brief Number of pixels used to calculate the sums
+ *                             80 cells
+ *            /───────────── 1280 pixels ───────────\
+ *                             16 zones
+ *             16
+ *           ┌────┬────┬────┬────┬────┬─  ──────┬────┐   \
+ *           │Cell│    │    │    │    │    |    │    │   │
+ *        16 │ px │    │    │    │    │    |    │    │   │
+ *           ├────┼────┼────┼────┼────┼─  ──────┼────┤   │
+ *           │    │    │    │    │    │    |    │    │
+ *           │    │    │    │    │    │    |    │    │   7
+ *           │ ── │ ── │ ── │ ── │ ── │ ──  ── ─┤ ── │ 1 2 4
+ *           │    │    │    │    │    │    |    │    │ 2 0 5
  *
- * \var IspStatsRegion::uncounted
- * \brief Remaining number of pixels in the region
+ *           │    │    │    │    │    │    |    │    │ z p c
+ *           ├────┼────┼────┼────┼────┼─  ──────┼────┤ o i e
+ *           │    │    │    │    │    │    |    │    │ n x l
+ *           │                        │    |    │    │ e e l
+ *           ├───                  ───┼─  ──────┼────┤ s l s
+ *           │                        │    |    │    │   s
+ *           │                        │    |    │    │
+ *           ├───   Zone of Cells  ───┼─  ──────┼────┤   │
+ *           │        (5 x 4)         │    |    │    │   │
+ *           │                        │    |    │    │   │
+ *           ├──                   ───┼─  ──────┼────┤   │
+ *           │                   │    │    |    │    │   │
+ *           │    │    │    │    │    │    |    │    │   │
+ *           └────┴────┴────┴────┴────┴─  ──────┴────┘   /
  *
- * \var IspStatsRegion::rSum
- * \brief Sum of the red values in the region
  *
- * \var IspStatsRegion::gSum
- * \brief Sum of the green values in the region
+ * The algorithm works with a fixed number of zones \a kAwbStatsSizeX x
+ * \a kAwbStatsSizeY. For example, a frame of 1280x720 is divided into 80x45
+ * cells of [16x16] pixels. In the case of \a kAwbStatsSizeX=16 and
+ * \a kAwbStatsSizeY=12 the zones are made of [5x4] cells. The cells are
+ * left-aligned and calculated by IPAIPU3::calculateBdsGrid().
  *
- * \var IspStatsRegion::bSum
- * \brief Sum of the blue values in the region
+ * Each statistics cell represents the average value of the pixels in that cell
+ * split by colour components.
+ *
+ * The Accumulator structure stores the sum of the average of each cell in a
+ * zone of the image, as well as the number of cells which were unsaturated and
+ * therefore included in the average.
+ * \todo move this description and structure into a common header
+ *
+ * Cells which are saturated beyond the threshold defined in
+ * ipu3_uapi_awb_config_s are not included in the average.
+ *
+ * \var Accumulator::counted
+ * \brief Number of unsaturated cells used to calculate the sums
+ *
+ * \var Accumulator::rSum
+ * \brief Sum of the average red values of each unsaturated cell in the zone
+ *
+ * \var Accumulator::gSum
+ * \brief Sum of the average green values of each unsaturated cell in the zone
+ *
+ * \var Accumulator::bSum
+ * \brief Sum of the average blue values of each unsaturated cell in the zone
  */
 
 /**
@@ -157,7 +200,7 @@ uint32_t Awb::estimateCCT(double red, double green, double blue)
 	return 449 * n * n * n + 3525 * n * n + 6823.3 * n + 5520.33;
 }
 
-/* Generate an RGB vector with the average values for each region */
+/* Generate an RGB vector with the average values for each zone */
 void Awb::generateZones(std::vector<RGB> &zones)
 {
 	for (unsigned int i = 0; i < kAwbStatsSizeX * kAwbStatsSizeY; i++) {
@@ -174,7 +217,7 @@ void Awb::generateZones(std::vector<RGB> &zones)
 	}
 }
 
-/* Translate the IPU3 statistics into the default statistics region array */
+/* Translate the IPU3 statistics into the default statistics zone array */
 void Awb::generateAwbStats(const ipu3_uapi_stats_3a *stats,
 			   const ipu3_uapi_grid_config &grid)
 {
@@ -215,7 +258,6 @@ void Awb::clearAwbStats()
 		awbStats_[i].rSum = 0;
 		awbStats_[i].gSum = 0;
 		awbStats_[i].counted = 0;
-		awbStats_[i].uncounted = 0;
 	}
 }
 
@@ -304,7 +346,7 @@ void Awb::prepare(IPAContext &context, ipu3_uapi_params *params)
 
 	/*
 	 * Optical center is column start (respectively row start) of the
-	 * region of interest minus its X center (respectively Y center).
+	 * cell of interest minus its X center (respectively Y center).
 	 *
 	 * For the moment use BDS as a first approximation, but it should
 	 * be calculated based on Shading (SHD) parameters.
diff --git a/src/ipa/ipu3/algorithms/awb.h b/src/ipa/ipu3/algorithms/awb.h
index cc848060..ac8ccc84 100644
--- a/src/ipa/ipu3/algorithms/awb.h
+++ b/src/ipa/ipu3/algorithms/awb.h
@@ -33,9 +33,8 @@ struct Ipu3AwbCell {
 	unsigned char padding[3];
 } __attribute__((packed));
 
-struct IspStatsRegion {
+struct Accumulator {
 	unsigned int counted;
-	unsigned int uncounted;
 	unsigned long long rSum;
 	unsigned long long gSum;
 	unsigned long long bSum;
@@ -82,7 +81,7 @@ private:
 	uint32_t estimateCCT(double red, double green, double blue);
 
 	std::vector<RGB> zones_;
-	IspStatsRegion awbStats_[kAwbStatsSizeX * kAwbStatsSizeY];
+	Accumulator awbStats_[kAwbStatsSizeX * kAwbStatsSizeY];
 	AwbStatus asyncResults_;
 };