[camera] Limit camera preview size to take high quality picture with startImageStream

[comlhj1114]

This is related to the issue, flutter/flutter#46082.
This is an important issue to enable real-time image analysis and take a picture in a Flutter app.

In iOS, I tried to apply head pose estimation and taking a picture in a proper pose automatically, however, when I tried CameraController.startImageStream with ResolutionPreset.max for capture high quality picture, the app crashed unexpectedly. To resolve the problem, I should limit the resolution to ResolutionPreset.high, but in this case, generated pictures also have low resolution (which is 1280x720).

In Android, the preview resolution is limited to 1280x720 (resolutionPreset.high) in the function computeBestPreviewSize in CameraUtils.java. Like this, I can resolve the problem by limiting _previewSize in iOS. So, I propose this PR.

In the future, I recommend to set different resolution for preview and taking a picture by getting some arguments for flexibilities of the camera package.

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[comlhj1114]

@googlebot I fixed it.

[stuartmorgan-g]

Thanks for the contribution!

• All existing and new tests are passing.

This does not appear to be the case; please ensure that all tests pass. This will also need tests covering the change, per Flutter policy.

Based on discussion at #1952 (comment) it appears that the Android limit you are referring to is controversial, so it seems likely that some users would consider this change a regression. Is there a reason not to go directly to providing preview resolution control?

[comlhj1114]

@stuartmorgan Thank you for your comment. I didn't know that discussion. I totally agree with them that someone might need high resolution preview. So, like your opinion, I agree that it is better to go directly to providing preview resolution control.
I will make new PR in near future

[comlhj1114]

And I just found that this modification changes the shape of the preview not actual resolution of the preview.
So, this PR is wrong.