[Elphel-support] the discussion on the Heptaclops camera

[Biel Bestué de Luna]

Andrey, about only having 150 gray levels, why does the camera only output
150 gray levels when the image that comes out of the sensor has 4096 output
values? maybe this is because this amount of sampled information contain a
lot of noise?

and, if 8 bits allow 256 shades of gray, does that meant that, while 8 bits
per pixel allows it the camera can't reach the 256 shades of gray? if so,
wouldn't it mean having a lot of banding?

what factors need to change in order to have a camera that at least has the
full 256 colours from 8 bits from the sensor? maybe a sensor with higher
ADC would do the job?

about shot noise, what factors make shot noise optimal for gamma 0.5, can
this factors be changed for other gamma values? like 0.46 (1/2.2)?

2012/10/29 support-list <support-list at support.elphel.com>

>  >
>                      Biel Bestué says:
>  >              October 28, 2012 at 5:31 PM
>  >              damn, that is a lot of info.
>  >  the “GAINR,GAING,GAINB and GAINGB” values in the Elphel 353 is this
> analog gain in the CMOS?
>
> Not directly - autocampars.php -> parsedit.php (or directly with
> parsedit.php) you may see these parameters as well as the sesnor analog
> gain registers. Sesnor has rather larger gaing steps (you may read more in
> the sensor datasheet), while the GAINx parameters are 16-bit values. Camera
> sets the analog gain to the nearest, and then multiplies the digital code
> by a value close to 1.0 to make it possible to balance colors with finer
> steps than the sensor can do that.
>
>  >  what do you mean by “linear bits” versus “2^12 2^14″?
> 12 bit ADC has 4096 output codes. But a sensor with 12-bit ADC while
> having 4096 output values has only say 150 distinct grey levels.
>
> > maybe by  “linear bits” do you mean the colour gradient from black to
> white  resulting from using a gamma value of 1, versus the same colour
> gradient  but with gamma 0.46?
> >  shot noise is the noise in the “well exposed” areas isn’t it? the
>  minimum “vibration” of the colours in the well exposed part of the  range?
>
> Ideal gamma for working with shot noise is 0.5 - so the result function is
> Math.sqrt(). With gamma 1.0 (linear) there will be no meaningful difference
> between 4095 and 4050. having ideal illumination and static objects, the
> same pixel will provide 4000+/-44 (+/- Math.sqrt(8500)*(4095/8500) where
> 8500 is the number of electrons that fits in the pixel and correspond to
> 4095 ADC output).
>
>  >  what part does “Black level” play in this issue? I’ve seen that Black
>  level changes where the gamma curve starts to affect the picture, Black
>  level is an effector after the acquisition, as gamma seems to be, but
>  before the  picture compression? I’ve seen that there are colour
>  deviations closer to the dark tones, like colour getting too green in the
>  dark tones, is this possible that green black level should be displaced
>  further closer so the lowered green could deliver a much cleaner dark
>  colour representation?
> Black level here is just "fat zero". As the random noise on the output can
> be either sign, and cutting it at zero (pixel value should be positive)
> would skew the results. So sesnor is programmed to output small positive
> value for the zero output, default value is 160 (of 4095). You do not
> really need to change it in most cases.
>
> And, by teh way - if you use autocampars.php and open parsedit.php with
> it, each parameter has short explanation - what iis it - it appears as a
> "tooltip" when you keep mouse pointer over it.
>
> Andrey
>
>
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