Re: Untagged RGB to Lab conversion
Re: Untagged RGB to Lab conversion
- Subject: Re: Untagged RGB to Lab conversion
- From: Robin Myers <email@hidden>
- Date: Tue, 18 Dec 2001 14:17:33 -0800
- Organization: Robin Myers Imaging
David Remington wrote:
>
Measured values are ICC Lab. Captured values are RGB. How can these values be
meaningfully compared?
They cannot be directly compared. One set of values must be converted to the other for
comparison.
>
For that matter, and this is a question no doubt best directed to PhaseOne and Better
Light, how are RAW RGB values determined? Sensor R, G, and B filters as primaries plus
secret sauce?
In the case of Better Light, RAW RGB values are just that, the numeric values from the
analog-to-digital converter (ADC) after converting the voltages from the RGB sensor to
digital values. No tonal reproduction curves, no secret sauce. The three channels may
have been balanced to each other with analog gains to give neutral grays before the
voltages go to the ADC. This may not be the same with other cameras, especially one-shot
cameras, since they do not have enough pixels to give a one-to-one correspondence of
sensor pixels to image pixels. One-shot cameras therefore go through an interpolation
step prior to delivering the "raw" image.
>
Is there any way to convert from RGB to Lab without defining the RGB colorspace?
No. The RGB colorspace also defines the white point and the conversion to Lab requires
the colorspace in additon to the RGB colorimetric values.
>
What I would like to investigate is just where that raw data is starting from. Is it
starting anywhere?
The answer depends on a number of variables, including the type of sensor, its filters,
the type of analog and digital electronics controlling the sensor, the image processing,
etc. Perhaps if you would clarify this a bit more...?
>
Is RGB data completely defined by the box you draw around it?
I do not understand what you mean here? Please clarify.
Robin Myers