KillDash9 KillDash9 -4 years ago 90
Android Question

Camera2 API - How to set long exposure times

I'm trying to capture images with 30 seconds exposure times in my app (I know it's possible since the stock camera allows it).

But SENSOR_INFO_EXPOSURE_TIME_RANGE (which it's supposed to be in nanoseconds) gives me the range :

13272 - 869661901

in seconds it would be just

0.000013272 - 0.869661901

Which obviously is less than a second.

How can I use longer exposure times?

Thanks in advance!.

Answer Source

The answer to your question:

You can't. You checked exactly the right information and interpreted it correctly. Any value you set for the exposure time longer than that will be clipped to that max amount.

The answer you want:

You can still get what you want, though, by faking it. You want 30 continuous seconds' worth of photons falling on the sensor, which you can't get. But you can get something (virtually) indistinguishable from it by accumulating 30 seconds' worth of photons with tiny missing intervals interspersed.

At a high level, what you need to do is create a List of CaptureRequests and pass it to CameraCaptureSession.captureBurst(...). This will take the shots with as minimal an interstitial time as possible. When each frame of image data is available, pass it to some new buffer somewhere and accumulate the information (simple point-wise addition). This is probably most properly done with an Allocation as the output Surface and some RenderScript.

Notes on data format:

  • The right way to do this is to use the RAW_SENSOR output format if you can. That way the accumulated output really is directly proportional to the light that was incident to the sensor over the whole 30s.

  • If you can't use that, for some reason, I would recommend using YUV_420_888 output, and make sure you set the tone map curve to be linear (unfortunately you have to do this manually by creating a curve with two points). Otherwise the non-linearity introduced will ruin our scheme. (Although I'm not sure simple addition is exactly right in a linear YUV space, but it's a first approach at least.) Whether you use this approach or RAW_SENSOR, you'll probably want to apply your own gamma curve/tone map after accumulation to make it "look right."

  • For the love of Pete don't use JPEG output, for many reasons, not the least of which is that this will most likely add a LOT of interstitial time between exposures, thereby ruining our approximation of 30s on continuous exposure.

Note on exposure equivalence:

This will produce almost exactly the exposure you want, but not quite. It differs in two ways.

  1. There will be small missing periods of photon information in the middle of this chunk of exposure time. But on the time scale you are talking about (30s), missing a few milliseconds of light here and there is trivial.

  2. The image will be slightly nosier than if you had taken a true single exposure of 30s. This is because each time you read out the pixel values from the actual sensor, a little electronic noise gets added to the information. So in the end you'll have 35 times as much of this additive noise (from the 35 exposures for your specific problem) as a single exposure would. There's no way around this, sorry, but it might not even be noticeable- this is usually fairly small relative to the meaningful photographic signal. It depends on the camera sensor quality (and ISO, but I imagine for this application you need that to be high.)

  3. (Bonus!) This exposure will actually be superior in one way: Areas that might have been saturated (pure white) in a 30s exposure will still retain definition in these far shorter exposures, so you're basically guaranteed not to lose your high end details. :-)

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