I'm new to the forum and been working my way up the Qimage learning curve.

My dSLR is a Sigma SD14 and I'm trying to achieve a high degree of image accuracy. I have a X-Rite ColorChecker which I have shot RAW in full Sun light with a Sunlight white balance.

Within Qimage, I am trying to create an accurate Selective Color Table. The problem that I'm having is the SD14 recorded the Red and Magenta squares Saturated, 165 0 58 and 194 0 161 respectively - Completely lacking anything in the Green channel. The sRGB target values for the X-Rite ColorChecker are RED: 175 54 60 and Magenta: 187 86 149, not what I'd call saturated colors.

Since the operation of the Selective Color Table is a pure multiplication, How do I add an appropriate Green bias selectively or to the overall image so I can have Green values to work with ?

The other X-Rite ColorChecker primary color squares, Blue, Green, Yellow, and Cyan each have Red, Green, and Blue channel values, so they are no problem to accurately alter with the Selective Color Table.

I know the SD14 tends to be Red sensitive, sacrificing the Green channel in the process. I also know the SD14 characteristically records intense, saturated, colors, but my goal with the Selective Color Table is to achieve a high correlation between the image and the actual scene.

As an aside, Why does the Embedded Image Information screen list the Depth for a RAW X3F file as 8 Bits when the SD14 uses 12 bit Analog to Digital converters ? Does this imply that Qimage lost some data along the way when the file was opened ?

Thanks for the quick response Terry.

First of all, I do have my monitor calibrated, which I update about every other month.

I am not trying to set the Selective Color Table values by eye.

My work flow consists of:

Opening the image in Qimage with Create Associative Filter => Full Edit

I then use to cursor to examine each of the X-Rite ColorChecker primaries, recording the values that Qimage lists as "Curr. Color", which I'm assuming are the file values and not what XP sends to the monitor post color management.

I then do a bunch of calculations, dividing the X-Rite ColorChecker sRGB values by the measurements from Qimage to derive the Selective Color Table values.

For example, Qimage reports the "Curr. Color" of the Green square as [ 96, 146, 87 ], The ColorChecker target values are [ 70, 148, 73 ], so the conversion values needed by the Selective Color Table become [ 70 / 96, 148 / 146, 73 / 87 ] or [ 0.729167, 1.013699, 0.839081 ]

Now the problem comes when the reported "Curr. Color" contains a 0, Red being [ 164, 0, 56 ], and the ColorChecker target value is [ 175, 54, 60 ]. Calculating the Selective Color Table value ends up being [ 175 / 164, 54 / 0, 60 / 56 ] leaving the value for the Green channel effectively undefined. Tentatively, I've used the values of [ 1.067073, 1.0, 1.071429 ] knowing that this will leave the Red saturated.

This is a problem where Qimage uses the formula:

< Filtered Pixel Value > = < Selective Color Table term > * < Initial Image Pixel Value >

and not the formula:

< Filtered Pixel Value > = < Selective Color Table term > * ( < Initial Image Pixel Value > + < A Bias Value > )

If I could add a Green bias, lets say 10, across the whole image, I could create a Selective Color Table which would compensate and the math would work for the Red and Magenta to be on target.

Afterwards, I have no problem accepting that display or printing of an image will have the reproduced colors limited by the gamut mapping.

I think this is a wrong assumption but someone more expert than me would need to confirm or otherwise. I think that what the cursor reads is the screen value, therefore it depends very much on your monitor profile.

Your approach seems very unconventional and a lot of hard work, that is why I said about colour management. The conversion from one space to another is done by clever algorithms and the rendering intent selected.
If you are using Qimage for raw conversion, are you using a custom camera profile, well worth the few $ and you'll get the best possible correlation of colour.
Terry.

It's the current pixel value displayed on screen.  So if you want the numbers without (monitor) color management, you need to turn Mntr ICC off on the main window first.

Mike