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libcamera: ipa: raspberrypi: agc: Improve centre-weighted luminance calucation

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Previously the calculation computed Y for each region before returning
the weighted average, which "baked in" the over-importance of small
statistics regions. The revised calculation will treat all pixels
equally when the region weights are the same, making it easier to
use. With the previous scheme, proper "average" metering was difficult
to implement.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
This commit is contained in:
David Plowman 2020-11-23 07:37:58 +00:00 • committed by Kieran Bingham
parent 6af665992d
commit 9db94a3635
1 changed files with 15 additions and 10 deletions
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25
src/ipa/raspberrypi/controller/rpi/agc.cpp
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@ -385,18 +385,23 @@ static double compute_initial_Y(bcm2835_isp_stats *stats, Metadata *image_metada
awb.gain_r = awb.gain_g = awb.gain_b = 1.0; // in case no metadata
if (image_metadata->Get("awb.status", awb) != 0)
LOG(RPiAgc, Warning) << "Agc: no AWB status found";
double Y_sum = 0, weight_sum = 0;
// Note how the calculation below means that equal weights give you
// "average" metering (i.e. all pixels equally important).
double R_sum = 0, G_sum = 0, B_sum = 0, pixel_sum = 0;
for (int i = 0; i < AGC_STATS_SIZE; i++) {
if (regions[i].counted == 0)
continue;
weight_sum += weights[i];
double Y = regions[i].r_sum * awb.gain_r * .299 +
regions[i].g_sum * awb.gain_g * .587 +
regions[i].b_sum * awb.gain_b * .114;
Y /= regions[i].counted;
Y_sum += Y * weights[i];
R_sum += regions[i].r_sum * weights[i];
G_sum += regions[i].g_sum * weights[i];
B_sum += regions[i].b_sum * weights[i];
pixel_sum += regions[i].counted * weights[i];
}
return Y_sum / weight_sum / (1 << PIPELINE_BITS);
if (pixel_sum == 0.0) {
LOG(RPiAgc, Warning) << "compute_initial_Y: pixel_sum is zero";
return 0;
}
double Y_sum = R_sum * awb.gain_r * .299 +
G_sum * awb.gain_g * .587 +
B_sum * awb.gain_b * .114;
return Y_sum / pixel_sum / (1 << PIPELINE_BITS);
}
// We handle extra gain through EV by adjusting our Y targets. However, you