Which colourspace for ChartThrob?

[ColinRH]

I have Windows 7 – PS CS5 - and using ChartThrob V1.15



When I analyse my RGB cyanotype chart with the C/T I get a curve to use. However as the future images will be in greyscale, I tried to analyse in greyscale but C/T advises me to convert to RGB.

If I use RGB but desaturate, or even make the scan itself a greyscale I keep being told to return to RGB. That doesn’t make sense to me when the future prepared PS images will be greyscale and I would have thought any resulting curve should be greyscale biased.

So exactly which colourspace should it all be done in? Kevin states that C/T should accept scans in RGB, greyscale or CMYK but my experience is that all have different outcomes and for me, it certainly does not like greyscale.

 

[nmp]

It shouldn't matter as long as you are consistent.

Print the image negative in the same manner as the ChartThrobe negative (greyscale, RGB or CMYK) with the same printer parameters.

It's been a while since I did it myself, but I guess you can simply input the raw scan in RGB to get the correction curve. If you desaturate first so R=G=B but still in RGB mode, that should work too (not sure I understand why it wouldn't, after all that is just like a very neutral print.) Perhaps there are some subtle differences in the exact correction curves arrived at the two different ways, but I would think the basic tonal relationships shouldn't be too far apart. So the ultimate digital negative would be fine.

:Niranjan.

 

[Andrew O'Neill]

I greyscale the the negative, then convert it back to RGB. That always worked for me when using ChartThrob.

 

[ColinRH]

Thanks Andrew. Makes absolute sense, make image in greyscale, convert to RGB apply Curve do the rest and print. I'll need to make a small Action otherwise I'll forget something! Thanks.

 

[Andrew O'Neill]

Thanks Andrew. Makes absolute sense, make image in greyscale, convert to RGB apply Curve do the rest and print. I'll need to make a small Action otherwise I'll forget something! Thanks.

Yes, that is exactly how I do it. Then invert to a negative. I never thought about making an Action, though... I'll have to try that. Thanks!

 

[Dan Pavel]

It has been a while since I've looked into the CT code but, as far as I remember, it converts the scanned RGB values into gray values before computing the curve. The curve generated by CT is a simple grays-only curve and not a RGB one. It should work similar both in RGB and Grayscale.