10/12/14 bit FITS Natural Bit Depth

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mAnKiNd
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Re: 10/12/14 bit FITS Natural Bit Depth

Post by mAnKiNd » Sun Jun 24, 2018 4:05 pm

ok so if it's a straight up multiplication by 16, then:

1 (12-bit) now becomes 16
2 -> 32
3 -> 48
4 -> 64
and so on..

....meaning you simply end up with the same histogram but with data gaps of 16 shades between each recorded data point, right? This means you end up with a fragmented looking histogram of the same data when stretching 12-bit to 16-bit, but one that essentially contains the same data.

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Re: 10/12/14 bit FITS Natural Bit Depth

Post by admin » Sun Jun 24, 2018 5:01 pm

Yes, it's just a straight scaling by 16 (for a 12 bit sensor), so the information contained by the data is identical. You would see gaps in the histogram if you zoomed in on it enough (or had some fancy 4K resolution monitor).

There is no real right or wrong choice here, except that some post-processing software prefers one type or the other, so the option is available to choose in SharpCap.

cheers,

Robin

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Re: 10/12/14 bit FITS Natural Bit Depth

Post by mAnKiNd » Sun Jun 24, 2018 5:04 pm

Thanks Robin,

Any comments on the discrepancies for the use of the word "brightness" by ZWO vs. other cameras such as my old QHY? Would this fall under your prior mentioned concerns about changes in these values not scaling back properly if you save as 12-bit?

Cheers
Minos
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
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Re: 10/12/14 bit FITS Natural Bit Depth

Post by admin » Sun Jun 24, 2018 6:54 pm

Brightness/Offset/Black Level are just different terms used by different camera manufacturers for the same thing - namely a constant uplift in the value of all pixels in the image. Because it is purely additive there are usually no ill effects from applying this when capturing (as opposed to gain, white balance, etc).

cheers,

Robin

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Re: 10/12/14 bit FITS Natural Bit Depth

Post by mAnKiNd » Sun Jun 24, 2018 7:01 pm

admin wrote:
Fri Feb 23, 2018 7:16 am

Also you will have to be careful when using this feature as some camera features like brightness/contrast/gamma that are applied in digital space in the driver/SDK may be operating on already stretched 16 bit values. What this means is that the data comes in from the camera in the range 0..4095, then gets stretched in the SDK, multiplying by 16 to get a value in the range 0..65520 but with steps of 16, then gets brightness/gamma/contrast applied (stillin the driver/SDK) which can basically shift all the values around so they are no longer multiples of 16. Then if you ask SharpCap to save without stretch, it tries to undo the 16x scaling by dividing by 16 and is throwing away data in the process. That's why it's marked as an advanced option - if you use it with that sort of digital processing enabled then you degrade your image data.
Thanks Robin, sure I get that, but I was just wanting to make sure that what you said above back in February applies to the brightness/offset/black level slider, whatever the manufacturer chooses to call it.

Cheers
Minos
Celestron AVX & SW Az-GTi (EQ)
SW Esprit 80 & 150PL, Canon 300mm f4 (non-IS)
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Re: 10/12/14 bit FITS Natural Bit Depth

Post by oopfan » Sun Jun 24, 2018 7:50 pm

Hi Robin,

Here is the "Image Controls" panel of my Altair 290M. I've never given it much attention but should I? I always thought that these settings just affected what is displayed in the SharpCap image window. I never thought of the possibility that it could affect what is written to the FITS file.

Brian
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Re: 10/12/14 bit FITS Natural Bit Depth

Post by admin » Sun Jun 24, 2018 8:34 pm

The gamma and contrast are on default, which is fine. You could flip the 'Digital Processing' to 'Off' to disable them entirely if you want.

cheers,

Robin

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