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

[Quibbledoot]

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 #967

[...]
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!

[davidplowman]

Hi, thanks for the questions, I'll make a couple of suggestions and comments.

You should be able either to edit the tuning file in situ on the disk, or by loading it in Python. When I edit it on the disk, I always like to check I've got the right tuning file by typing a couple of characters of garbage at the very top and checking that the camera doesn't start. This has saved be significant amounts of head-scratching on occasion!

Secondly, I don't believe we've actually plumbed through the "custom" metering mode. Probably we should have done, but I get a warning when I try to use it telling me that it isn't supported. Instead, I would recommend overriding one of the other modes, either "spot" or "matrix" that you aren't going to use, and then you can go with that instead.

I normally like to experiment with this sort of thing interactively. That is, open a preview window when you start the camera (picam2.start(show_preview=True) instead of just picam2.start()), and then you can type controls in interactively and see what they do.

One thing to note about your script is that, once the camera is running, controls take several frames to apply. So capturing a frame right after a picam2.set_controls(...) will probably show no change. You could either wait a second before capturing, or you could set the control before starting the camera, in which case it takes effect immediately, on the very first camera frame.

Hope that helps!