colour shift on b/w with adjustments

[DavidGB]

MacOS. 10.15.7     Affinity Publisher 1.10.4

I am preparing a book of black and white photos to be printed in four color B/W. The picture files are RGB placed into my Affinity doc set up in cmyk. Images which I adjust (levels, brightness/contrast etc.) in Affinity Publisher all show a colour shift. Most of the time the shift is very subtle and barely noticeable in Publisher but when I export to PDF the shift becomes very obvious. The greater the adjustment, the greater the colour shift usually towards blue. I have used the same method to publish full colour images with adjustments giving no adverse effect.

Is this a bug or am I doing something wrong?

See the attached screengrab

[lacerto]

Cyan is typically overpowering after converting RGB gray to CMYK. You can eliminate the color cast if you apply e.g. HSL Adjustment as the lowest adjustment layer saturation turned off.

[DavidGB]

Thanks for that.  Recolour set to zero appears to work best.

[lacerto]

Ok, I probably misunderstood. Note though that HSL saturation off keeps the image four-color (CMYK) job when exported while Recolor saturation off would make it K-only. But that might depend on the export options used:

a) HSL adjustment saturation off applied to a CMYK image:

 

b) Recolor adjustment saturation off applied to a CMYK image:

 

Note that even if you have equal CMY values when turning off saturation, you are likely to get "color cast" in output (typically brownish warmth, especially if you print on uncoated paper), so it is possible or even likely that you get more "neutral" results when you simply just convert from RGB grays to CMYK using the correct profile (even if you get a slight cyan cast on the screen). The screen preview is always an approximation, and the way the printed colors look is naturally also affected greatly by the viewing conditions. 

[DavidGB]

This is all at the edge of my knowledge and experience. I will have to look at it more closely.

Thanks for your help.

[lacerto]

On 11/6/2021 at 7:33 PM, DavidGB said:

This is all at the edge of my knowledge and experience. I will have to look at it more closely.

The attached video might be helpful. It shows a procedure of having initially an RGB image, then using Black & White Adjustment to make it grayscale in a selective way (accentuating e.g. reds, yellows and greens, and getting the background blue a bit stronger). Simple grayscale conversion could directly have been made, as well, but this way you can have certain parts of the color image better represented in black and white.

When the image with the B & W adjustment is rasterized, it is sill an RGB image but has equal values in R, G and B channels. I assumed that you have your images initially imported in this state, but I might have been wrong. This image could directly have been converted to CMYK to show that a neutral conversion would not have equal amounts of C, M, Y and K inks (and not even C, M and Y inks). But I converted the RGB image to grayscale to show the same effect. 

So when you have a pure grayscale image and then convert that to CMYK, the way C, M, Y and K inks are distributed to maintain the initial neutrality of a grayscale image, will be dependent on your CMYK target profile, and would be done differently e.g. for coated and uncoated paper. If your goal is to simply just produce neutral grayscale photos but in four colors to have deeper blacks and richness in the image, I would do mere profile conversion, but possibly starting with the kind of B/W adjustment processing to have the initial RGB images selectively grayscaled. This kind of procedure will cause what you call "color shift": the C, M, Y (and especially not K) channels do have different values, and should have, too.

In the end of the video it is shown how differently HSL and Recolor Adjustments process desaturation when applied in CMYK color mode (in RGB mode they behave identically): the former evens out CMY channels, and the latter zeroes them and would render everything in black ink. In RGB color mode desaturation would cause very flat grays so it would not work there, at all, but in CMYK mode it could be used to equalize CMY channels and keep the black, which would give warm brownish tones. But if you aim at neutrality, you should work with color profile based conversions. As a general note, using colors to produce tones of "gray" requires the proper target color profile and a well calibrated display.