Hi, Alex,
Here is another interesting passage from the Rawnalyze help:
The tools of pixel selection are:
* the Black point and the White point values together determine the pixel value range, the black point giving the lower limit and the white point the upper limit of the range.
The current values are shown in the respective input field. The range of possible values for both limits is
o 0 to 16383 if the raw values are limited to 4095,
o 0 to 65535 if the raw values are limited to 16383,
o 0 to 131071 otherwise (this occurs with 16 bit depth, created by some digital backs).
The initial value of the black point is either
o the minimum of the black level correction values, if there are any; these are gained either from the descriptive data of the raw file, or calculated the so-called masked pixels around the image,
o 0 if black level correction does not apply.
The initial value of the white point is either
o the highest of the saturation/non-linearity levels of the pixels, if available,
o the white level specification from the descriptive data of the raw file,
o the maximum possible pixel value according to the bit depth of the sensor.
I haven't completely figured out yet all its treasures.
Here is another:
Black level
Some cameras' raw images include several rows and/or columns of pixels outside the captured image as well; these are called masked pixels. Their values show the effect of the so-called black current, that part of the pixel value, which is not caused by the captured light.
Other cameras record one or more direct values indicating the black levels.
The pixel values of the image have to be corrected by the calculated values, depending on the pixel's position (typically with Canon cameras), or with the recorded correction value. Other cameras do not record any information for this purpose, rather they record the corrected pixel values.
The process described above is called the black level correction.
The initial value of Black point is the minimum of the black level correction values, if there are any, zero otherwise. However, this is only for orientation; when carrying out the black correction, the program always calculates with the pixel-related value, not with the minimum. [Huh?]
The minimum, maximum and average of the correction values are shown with the histogram display. [Labeled "black level".]
The first step of the conversion is substracting the respective black level correction value from the original pixel value. A negative result will be replaced by zero (yes, it can happen that the black level correction is higher than the pixel value).
In case the resulting value is zero, black clipping is said to have occured on raw level. "Clipping" in this sense is not a generally accepted term, but this is analogous to the clipping of highlights in the sense, that the value 0 represents all smaller values, which could not be recorded.
If the Raw clipping checkbox is checked, the black-clipped pixels will be displayed with the RGB value 255, or in case of composite color it contributes to the resulting color with 255. If the raw clipping option is not selected, such pixel values are converted to the RGB value 0, respectively they contribute to the composite color with 0.
Further:
Black point
The result of the black level correction is an absolute pixel value, starting with 0. An adjustment can be applied to either increase the absolute values, or to clip them at a certain level; this adjustment is the black point setting.
If the black point is lower than the minimum of the black levels, then all absolute pixel values will be increased by the difference between the black point and the minimum of black levels. This causes the image to appear lighter; however, the relative differences between the pixels become less, i.,e. the contrast decreases.
Obviously, this can not happen if black level correction does not apply.
If the black point is higher than the minimum of the black levels, then all absolute pixel values will be reduced by the difference between the black point and the minimum of black levels. If black level correction does not apply, then the black point itself is the "clipping point".
This adjustment causes the image to appear darker, and details may be lost, as all pixel values between 0 and the black point will become "equalized", this is induced clipping, as opposed to raw clipping.
If the Show clipping checkbox is checked, then the pixels clipped by the black point but not clipped at the raw level will be displayed with the RGB value 255, or in case of composite color it contributes to the resulting color with 255. If the Show clipping option is not selected, such pixel values are converted to the RGB value 0, respectively they contribute to the composite color with 0.
Another:
Pixel saturation
The pixels of some cameras can reach the maximum value representable with the given bit depth; for example the pixel values of the Canon 20D can reach 4095. Other cameras' pixels may clip at a lower level; the clipping level may depend on the ISO, on other factors, even on the particular copy of the camera model. It often depends on the channel (pixel color); even differences between the two green positions within the color filter array are not seldom. Some cameras' pixels (sometimes only certain channels) do not clip at a single level, but in a level range; this is the range of non-linearity. The pixels have to be regarded as clipped from the lower limit of this range, because the values within the range do not represent meaningful information about the image.
If the original (i.e. not black level corrected) pixel value is equal to or greater than the respective clipping level, then that pixel is said to be raw clipped. If the Raw clipping checkbox is checked, the clipped pixels will be displayed with the RGB value 0, or in case of composite color it contributes to the resulting color with 0. If the Raw clipping option is not selected, such pixel values are converted to the RGB value 255, respectively they contribute to the composite color with 255.
The current version of Rawnalyze contains the clipping levels of most supported cameras hard coded; however, those values may not be accurate with all copies of that particular camera model. This may lead to reporting raw clipping, when it did not occur, or not reporting actual clipping.
And finally (for now):
White point
The white point specification, reduced by the current black point value sets an upper limit to the pixel values after black level correction and the application of the white balance and lightness adjustment factors. Thus the white point induces a clipping based on adjustments, as opposed to the raw clipping.
Reducing the specified white point by the black point is necessary, so that the white point value can be regarded relative to the uncorrected raw pixel values. Thus the white point gets "black point adjusted" in certain sense. If a pixel is not clipped on the raw level and the adjusted pixel value is equal to or higher than the white point and the Show clipping checkbox is checked, the clipped pixels will be displayed with the RGB value 0, or in case of composite color it contributes to the resulting color with 0. If the Show clipping option is not selected, such pixel values are converted to the RGB value 255, respectively they contribute to the composite color with 255.
Thus a pixel can show up as clipped only in response to one of the clipping indication options, not both.
The initial value of the white point is either
* the highest of the saturation/non-linearity levels of the pixels, if available,
* the white level specification from the descriptive data of the raw file,
* the maximum possible pixel value according to the bit depth of the sensor.
Decreasing the white point can induce clipping and it generally increases the contrast and lightness of the displayed image. Increasing the white point causes the image appear darker and less contrasty, but it may be necessary to avoid induced clipping, which can be caused by whute balancing and lightness adjustment. Turning on-off-on-off the clipping indication (by checking-unchecking the respective box) makes the affected pixels appear alternatingly very bright and very dark; this way it is easy to spot the affected areas.
Thus a pixel can show up as clipped only in response to one of the clipping indication options, not both.
I have to go out and run some errands soon, but later today I will contemplate all this raw wisdom (!).
Best regards,
Doug
For further analysis, you can export a .fit file and analyze that further with ImageJ. I personally prefer that workflow.