[HOW-TO] Set exposure measurement area with Raspberry Pi Camera

Start by reproducing the posted Picamera2 script with imx477.json and the custom AeMeteringMode setup, then inspect the rpi.agc metering_modes configuration and the approach from issue #967. Compare the captured exposure when each side of the image is covered; done means confirming whether the custom weights are applied and documenting the required configuration or code change.

Hi there :)

I am trying to take images with a raspberry pi 2018 HQ Camera V1.0 / raspberry Pi 5 combination and I want to manually set the auto exposure metering so that it only meters the right hand side of the camera. As far as I know the tuning file for this camera is the imx477.json.

My question is how can I ensure that my custom metering array is used?

So far I have tried two approaches:

Approach 1:

I did try to directly edit the tuning file like this

"rpi.agc":
{
    "channels": [
        {
            "comment": "Channel 0 is normal AGC",
            "metering_modes":
            {
                "centre-weighted":
                {
                    "weights":
                    [
                        *Array*
                    ]
                },
                "spot":
                {
                    "weights":
                    [
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 1, 2, 1, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 1, 2, 3, 2, 1, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 1, 2, 1, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0

                    ]
                },
                "matrix":
                {
                    "weights":
                    [
                        *Array*
                    ]
                }
                "custom":
                {
                    "weights":
                    [
                        0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 4, 3, 2,
                        0, 0, 0, 0, 0, 0, 0, 2, 3, 4, 5, 6, 5, 4, 3,
                        0, 0, 0, 0, 0, 0, 0, 3, 4, 5, 6, 7, 6, 5, 4,
                        0, 0, 0, 0, 0, 0, 0, 4, 5, 6, 7, 8, 7, 6, 5,
                        0, 0, 0, 0, 0, 0, 0, 5, 6, 7, 8, 9, 8, 7, 6,
                        0, 0, 0, 0, 0, 0, 0, 6, 7, 8, 9, 9, 9, 8, 7,
                        0, 0, 0, 0, 0, 0, 0, 7, 8, 9, 9, 9, 9, 9, 8,
                        0, 0, 0, 0, 0, 0, 0, 8, 9, 9, 9, 9, 9, 9, 8,
                        0, 0, 0, 0, 0, 0, 0, 7, 8, 9, 9, 9, 9, 9, 8,
                        0, 0, 0, 0, 0, 0, 0, 6, 7, 8, 9, 9, 9, 8, 7,
                        0, 0, 0, 0, 0, 0, 0, 5, 6, 7, 8, 9, 8, 7, 6,
                        0, 0, 0, 0, 0, 0, 0, 4, 5, 6, 7, 8, 7, 6, 5,
                        0, 0, 0, 0, 0, 0, 0, 3, 4, 5, 6, 7, 6, 5, 4,
                        0, 0, 0, 0, 0, 0, 0, 2, 3, 4, 5, 6, 5, 4, 3,
                        0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 4, 3, 2
                    ]
                }
            },

And then take an image like this, where I select the AeMeteringMode to Custom

from picamera2 import Picamera2
from libcamera import controls
from PIL import Image
from datetime import datetime

tuning = Picamera2.load_tuning_file("imx477_edited.json")
picam2 = Picamera2(tuning=tuning)

picam2.configure(picam2.create_still_configuration())
picam2.start()

picam2.set_controls({"AeMeteringMode": controls.AeMeteringModeEnum.Custom})
img = picam2.capture_array()
Image.fromarray(img).save(f"capture_{datetime.now().strftime('%Y%m%d_%H%M%S')}.jpg")
picam2.stop()

and while this resulted in images being taken, the images did not suggest, that it was only metering the right hand side of the image.

Approach 2:

I then tried to set the array in code similar to the solution that I saw in issue https://github.com/raspberrypi/picamera2/issues/967?new_signup=true

[...]
tuning = Picamera2.load_tuning_file("imx477.json")
algo = Picamera2.find_tuning_algo(tuning, "rpi.agc")
algo["channels"][0]["metering_modes"]["custom"] = {
    "weights": [0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 4, 3, 2, 0, 0, 0, 0, 0, 0, 0, 2, 3, 4, 5, 6, 5, 4, 3, 0, 0, 0, 0, 0, 0, 0, 3, 4, 5, 6, 7, 6, 5, 4, 0, 0, 0, 0, 0, 0, 0, 4, 5, 6, 7, 8, 7, 6, 5, 0, 0, 0, 0, 0, 0, 0, 5, 6, 7, 8, 9, 8, 7, 6, 0, 0, 0, 0, 0, 0, 0, 6, 7, 8, 9, 9, 9, 8, 7, 0, 0, 0, 0, 0, 0, 0, 7, 8, 9, 9, 9, 9, 9, 8, 0, 0, 0, 0, 0, 0, 0, 8, 9, 9, 9, 9, 9, 9, 8, 0, 0, 0, 0, 0, 0, 0, 7, 8, 9, 9, 9, 9, 9, 8, 0, 0, 0, 0, 0, 0, 0, 6, 7, 8, 9, 9, 9, 8, 7, 0, 0, 0, 0, 0, 0, 0, 5, 6, 7, 8, 9, 8, 7, 6, 0, 0, 0, 0, 0, 0, 0, 4, 5, 6, 7, 8, 7, 6, 5, 0, 0, 0, 0, 0, 0, 0, 3, 4, 5, 6, 7, 6, 5, 4, 0, 0, 0, 0, 0, 0, 0, 2, 3, 4, 5, 6, 5, 4, 3, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 4, 3, 2]
}
picam2 = Picamera2(tuning=tuning)
[...

While here I again got no errors and images where taken these are not what I expect given my custom metering array.

Images

These are examples of the images that I took with these approaches.
For this one the left side was covered and here I expect a relatively normal exposure since the right hand side should anyway only be metered.

For this image, however, where the right hand side is covered I would expect a lot of overexposure on the left side, since the metering should only be done for the right hand side.

To me it seems like it then just defaults to a centre-weighted metering mode. How can I ensure that my custom metering array is used?

Any help is greatly appreciated!