Why set gain > 1 when using auto gain?

[NiteRide]

Hi- there are a number of discussions about setting gain and exposure, but I have a more simplistic question in relation to Pi HQ camera that supports auto exposure and auto gain settings.
Is the gain value setting a lower bound from which auto-gain works? (eg "gain = 4" means autogain but nothing lower than "4"?) Which leads to a follow on of if I choose to let the software automatically determine gain, why would I want to set the lower gain value at anything other than 1 with auto-gain = "Yes?"

I'm not having issues, just curious.

[EricClaeys]

@NiteRide,
Good question.

When in auto-exposure and/or auto-gain mode, the Manual Exposure and Gain settings are only uses as the starting point for their respective values. The actual values used by the auto-exposure / auto-gain algorithm can range from the minimum exposure/gain for a camera to the user-defined maximum values.

For example, assume auto-exposure and auto-gain are enabled and:
Manual Exposure = 5 seconds
Max Auto-Exposure = 60 seconds
The camera's minimum exposure is 10 micro-seconds
Gain = 10
Max Auto-Gain = 16
The camera's minimum gain is 1.
Target Mean = 0.5

If you start Allsky during the day, the first image will be 5 seconds at gain 10 which will result in an extremely over exposed image which will be almost pure white. The Mean value will probably be 0.99.
The auto exposure algorithm will notice that 0.99 > 0.5 so will first decrease the Gain, for example, to 6, but leave the Exposure at 10 and take another image. That image will also be extremely over exposed with a Mean of, for example, 0.98.
The auto exposure algorithm will keep decreasing the Gain until it gets to 1. If the resulting image is STILL overexposed the algorithm will begin decreasing the Exposure until it gets an image whose Mean is close to 0.5.

The above initial settings will work but will probably take 10 or so images to get to a good one. The default daytime Frames To Skip is 5, so the first 5 images will be taken but ignored, i.e., not saved. If we assume it took 5 images for the gain to get to 1, that means the first 5 images took 5 * 5 seconds = 25 seconds to take, plus a second or two of overhead. And at this point NO image has been saved. If you have a daytime Delay of, for example, 30 seconds, Allsky will sleep for 30 seconds after the first five images, then will take another image. At 5 seconds and a Gain of 1, that image will also be overexposed. Let's say it takes 4 more images for the auto-exposure algorithm to find the correct exposure. That means another 4 * 30 seconds = 120 seconds delay plus around 10 seconds for exposure time plus a couple seconds overhead, before the first image is saved.
So, using those initial daytime settings means that if you start Allsky during the day you won't get the first image for a few minutes. After that everything will be fine.
When in auto-exposure / auto-gain mode, we suggest setting the daytime Manual Exposure and Gain settings to what a typical daytime image would be, for example, Exposure=500 (milliseconds) and Gain=1. That way, the algorithm will find the correct exposure/gain very quickly - almost always within the first 5 frames and the first image will appear within a couple seconds.

The same thing applies to nighttime images. Set the Manual Exposure and Gain values to a typical nighttime exposure.

Did this answer your questions?

[NiteRide]

Cool. Yes that totally makes sense. Thanks!

[NiteRide]

Coming back to this…and again out of my curiosity…does this only apply when “starting AllSky” after it has been stopped? (For example if you don’t take images during the day?)
Eg if running continuously I guess the software wouldn’t really need to recalibrate for multiple images to get the right exposure (even at day-to-night or night-to-day transitions) as it could know the reference exposure from the immediately prior image.
I dont save daytime images so have never really thought about the transition time exposures.

[EricClaeys]

@NiteRide,

For example if you don’t take images during the day?

That's a very good question. I don't know and would have to look at the code to find the answer. I am on my phone right now and looking at code on it is next to impossible.
On a normal day-to-night and night-to-day transition, the next image uses the prior image's exposure and gain as a basis, and slowly transitions to the new settings so you don't see anything sudden jump in brightness.
But in the case where the last image was 12 hours ago, it doesn't make sense (I don't think) to use it when determining the next images values.
I'm pretty sure the "skip" images are only done, however, when 0 images have been taken since Allsky started.
No one has ever mentioned there being a problem with exposure after skipping daytime, so I assume it works ok.