[v2,0/4] Simple pipeline: proportional AGC, AWB stats fix, OV2740 tuning
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  • Simple pipeline: proportional AGC, AWB stats fix, OV2740 tuning
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Javier Tia March 4, 2026, 11 p.m. UTC 
This series fixes AGC oscillation and an AWB statistics bug in the
Simple pipeline, and adds the first sensor-specific tuning file for
the SoftISP.

Patch 1 replaces the fixed 10% bang-bang step in the AGC with a
proportional controller where step size scales with the MSV error.
This eliminates the brightness flicker observed on sensors with
non-trivial control latency (tested on OV2740 behind Intel
IPU6/IVSC). Unchanged from v1.

Patch 2 fixes a bit-depth mismatch in the statistics gathering.
The SWSTATS_ACCUMULATE_LINE_STATS() macro divides the luminance value
for the histogram to normalize it to 8-bit range, but does not apply
the same normalization to the RGB sums. For 10-bit and 12-bit
unpacked Bayer formats, the sums are accumulated at native bit depth
while the AWB subtracts an 8-bit black level, under-correcting by 4x
or 16x respectively. This skews the AWB grey world gains, producing a
visible color cast (~9% green on the OV2740 with BLC=16). The fix
right-shifts the RGB sums in finishFrame() to normalize them to 8-bit
scale, matching the histogram and the 8-bit BLC used by AWB.

Patch 3 adds an OV2740 tuning file. With the AWB fix from patch 2,
no CCM is needed - AWB grey world convergence gives R/G ~ 0.98 and
B/G ~ 0.99 under 6500K lighting.

Patch 4 adds the OV2740 black level (4096, i.e. 0x40 at 10-bit) to
CameraSensorHelperOv2740, following the established pattern used by
OV5675, IMX219, and other sensors. This is the canonical location for
sensor calibration data and is used across all pipeline handlers.

Changes since v1:
- Patch 2: New. Fixes the AWB/BLC bit-depth mismatch for >8-bit
  sensors, eliminating the green cast without needing a CCM.
- Patch 3 (was patch 2 in v1): Removed CCM - no longer needed with
  the AWB fix. Removed explicit blackLevel from the tuning file;
  the value is now provided by CameraSensorHelperOv2740 (patch 4)
  as suggested by Robert Mader.
- Patch 4 (was patch 3 in v1): Unchanged.

Tested on ThinkPad X1 Carbon Gen 10 (Alder Lake, OV2740, IPU6 ISYS,
CachyOS kernel 6.19.3, libcamera 0.7.0).

Test results (converged frames, 6500K lighting, no CCM):

  With AWB fix (v2):    R/G=0.984  B/G=0.985  (~1.5% residual)
  Without fix (v1):     R/G=0.910  B/G=0.904  (~9% green cast)
  BLC=0 control:        R/G=1.000  B/G=1.001  (proves the mismatch)

Javier Tia (4):
  ipa: simple: agc: Replace bang-bang controller with proportional
  libcamera: software_isp: Normalize statistics sums to 8-bit
  ipa: simple: data: Add OV2740 tuning file
  ipa: libipa: camera_sensor_helper: Add OV2740 black level

 .../internal/software_isp/swstats_cpu.h       |  1 +
 src/ipa/libipa/camera_sensor_helper.cpp       |  2 +
 src/ipa/simple/algorithms/agc.cpp             | 73 +++++++++++++------
 src/ipa/simple/data/meson.build               |  1 +
 src/ipa/simple/data/ov2740.yaml               | 13 ++++
 src/libcamera/software_isp/swstats_cpu.cpp    |  9 +++
 6 files changed, 75 insertions(+), 24 deletions(-)
 create mode 100644 src/ipa/simple/data/ov2740.yaml