[libcamera-devel] Suboptimal resolution for digital zoom (controls::ScalerCrop)

[Elias Naur]

Hi,

I'd like to squeeze out as much resolution as possible from my
Raspberry Pi camera v1
for scanning QR codes. The camera  has a fixed focus at ~50cm from the
lens, so the
resolution gain from moving the QR codes closer is lost to blur from
the loss of focus.

My current strategy is to scan codes at a distance, but use the
considerable resolution
of the sensor (~2500x1900) to gain enough resolution for successful scans.

However, I fail to find any way to achieve that with libcamera. I
tried three options, all
suboptimal:

1. Configure the camera for full resolution and crop to a smaller
section in my program.
That works surprisingly well, except that the white-balance etc.
algorithms run on the full
image and not just the cropped part.
2. Configure the camera for full resolution and set
controls::ScalerCrop to the area
of interest. This seems to fix the white-balance algorithms, but
results in the cropped
area being *upscaled* to full resolution when I receive them. Scanning
for QR codes
at full resolution is way too slow on a RPi Zero.
3. Configure the camera for the same resolution of the area of interest, and set
controls::ScalerCrop. This is efficient, but the sensor is configured
for the low
resolution and thus ruins the resolution gain.

Questions:
1. Is there another way?
2. The following hack works for me,

diff --git a/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
b/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
index 00600441..6ed81dc1 100644
--- a/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
+++ b/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
@@ -842,7 +842,7 @@ int PipelineHandlerRPi::configure(Camera *camera,
CameraConfiguration *config)
         * request, and apply it to the sensor with the cached transform, if
         * any.
         */
-       V4L2SubdeviceFormat sensorFormat =
findBestFormat(data->sensorFormats_, rawStream ? sensorSize : maxSize,
bitDepth);
+       V4L2SubdeviceFormat sensorFormat =
findBestFormat(data->sensorFormats_, Size{2592,1944}, bitDepth);
        const RPiCameraConfiguration *rpiConfig = static_cast<const
RPiCameraConfiguration *>(config);
        ret = data->sensor_->setFormat(&sensorFormat,
rpiConfig->combinedTransform_);
        if (ret)

Would a similar change that *always* uses the full sensor resolution
be acceptable?
I can't discern any performance issues between full resolution and
lower resolution.

3. If the above fix is unacceptable, what is the right way to fix this
issue in libcamera?

Thanks,
Elias