Which colourspace for ChartThrob?

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.

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.

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

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.

ColinRH said:

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.

Click to expand...

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!

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.