[Elphel-support] the discussion on the Heptaclops camera

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 >  				 				 				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