Y Scale in Rawdigger & Gamma For Histogram

[Marc Hildebrant]

Group,

A discussion about the use of the Gamma function at "scantips.com" indicated that all Raw data from the Camara is encoded with a gamma value of 2.2.
Later, the Monitor applies an inverse function to remove the encoded data and return the original values as produced by the Camara A to D process. The conclusion
is that the eye wants a linear result from the camara's sensor.

Thus, a scale factor of square root of 2 is an option on the Rawdigger scale to show a "roughly" linear amplitude output.

Does this logic make sense?

I know that the curves can be modified to help with mid-tones, etc., but that is not what I am asking here.

Marc

 

[Ysarex]

Group,

A discussion about the use of the Gamma function at "scantips.com" indicated that all Raw data from the Camara is encoded with a gamma value of 2.2.

Raw data has no Gamma value. Output from a digital sensor is linear and the raw data is as such linear.

Later, the Monitor applies an inverse function to remove the encoded data and return the original values as produced by the Camara A to D process. The conclusion
is that the eye wants a linear result from the camara's sensor.

Thus, a scale factor of square root of 2 is an option on the Rawdigger scale to show a "roughly" linear amplitude output.

RawDigger provides two histogram display options. In a) the EV steps are equal in width on the graph. But note the numerical values for each EV step double per step. One stop = twice as much. In b) each EV step is twice as wide on the graph as the previous step -- a strictly linear visualization but less useful because there's no room to display the graph.* But note that the numerical values for each EV are the same as in a) and double per step. One stop = twice as much.

Does this logic make sense?

I know that the curves can be modified to help with mid-tones, etc., but that is not what I am asking here.

Marc

* With a beginning class of students in digital photography I prank them a bit as I describe for them how the sensors in their cameras work. I want them to understand what it means to record the data linearly. So I pull a blank sheet of paper (8.5X11) from the printer and say; "assume this is the first stop of light recorded by your camera sensor." And I place the sheet on the table. "The next stop is twice as much, let's graph it by using these blank sheets of paper." So I leave a little space and lay down two sheets of paper end to end. "Ok, the next stop is twice that last stop." At this point I've handed a couple of them a stack of paper and so I tell them to lay down the next stop. We run out of table and so I tell them to use the floor. It doesn't take long to get to the door and we aren't half finished. So I open the door and point down the 30 foot hallway and ask; "Do you think there's enough room? OK, we can stop now but do you get the point?" This is why RawDigger's b) display option isn't very useful.

 

[Marc Hildebrant]

Would it then be accurate to say that the output of the A to D process is linear, but the raw output data is gamma encoded?

Marc

 

[Ysarex]

Would it then be accurate to say that the output of the A to D process is linear, but the raw output data is gamma encoded?

Marc

No, the output of the sensor is linear, the output of the ADC is linear and that linear output is stored as the raw data which is linear. When the raw data is processed into an RGB image that processed data has a tone curve(s) applied. The JPEG the camera creates is processed to a target Gamma.

 

[Marc Hildebrant]

Ysarex,

Thanks for the reply. The internet source I reference, seems to imply that the Raw Output from the camara has had a gamma curve applied before it is sent.

My interest is how to understand the rawdigger info.

Your class demonstration on the power of two was clever. It reminds me of the classic, "put a penny on a chess square, and double that number on the next and so on...."

Marc

 

[Ysarex]

Ysarex,

Thanks for the reply. The internet source I reference, seems to imply that the Raw Output from the camara has had a gamma curve applied before it is sent.

Gamma, a tone curve is a necessary step to convert linear raw data into RGB photographic data. Maybe your source isn't clear enough about when that happens. It doesn't happen between the sensor and the writing of the raw data into the raw file. The raw file contains linear data.

My interest is how to understand the rawdigger info.

Your class demonstration on the power of two was clever. It reminds me of the classic, "put a penny on a chess square, and double that number on the next and so on...."

It's terrifying to realize how many young (college!!) students don't have a solid grasp of an exponential progression.

Marc

 

[Marc Hildebrant]

Ysarex,

O.K. then. The data stays linear as raw data until the data is processed later.

College students are another matter...

Marc

 

[Marc Hildebrant]

Update.

Re-reading the posting at "scantips.com" confirms that the gamma encoding is for a histogram after it is processed. The histogram in rawdigger is, as stated by Ysarex, linear.

Marc