[scribus] Lighten dark colors

[JLuc]

Le 10/07/2020 à 13:50, Bert Driehuis a écrit :
> Actually, Fogra39 and ISO coated v2 shouldn't be that different, as
> ISO Coated V2 is the successor of Fogra39. The ISO profile promises a
> bit of improvement in faithfully rendering greys, it should not affect
> the color darkness that much.

I tested both profiles with "display out of gamut" scribus option
on some test images that proved critical during my last print.

FYI see IsoCoated out of gamut areas :
https://covid19.framadrop.org/lufi/r/fDAUkVtDgb#LWwKwzh0GjLxxrSyPyd+P4nlM+f2BTxLH4+9A6AR9AQ=

and fogra39 out of gamut areas :
https://covid19.framadrop.org/lufi/r/L6pnmxnXCQ#NVLQTDCHufebFQR99koI4UiHl8UCF3QRK+L28O2kcfE=

The fogra39L's outofgamut area is about 15% smaller so i hoped this would improve the rendering.
I also compared the produced PDFs but they look allmost the same :
depending on the picture, FOGRA39 or isocoated look better but the difference is hardly noticeable,
and in any case they both look *much* nicer on screen then in the printed book :-(
("of course" my monitor has no profile)

Thanks for you explanations about imagemagick quality.
Setting the opacity to 75% will increase light
but it will grey all black and will reduce the contrast :-/

My hope when using "convert -level b,w,gamma" is to use a gamma correction curve
that keeps some black parts and hopefully a better contrast
as described here http://www.imagemagick.org/Usage/color_mods/
I've began to check that and it probably improves printing for some dark images
but it still degrades contrast on other dark images.

JL

> I never figured out how the color profile of the images used in
> Scribus affects the outcome, so maybe someone on the list can clarify
> that.
> 
> It's not ImageMagick's fault that one loses quality when converting
> jpg to jpg. By modifying the exposure, the three input color channels
> of the JPG that originally span 256 values of lightness each get
> reduced to less than 256 values. That can cause a "stepped" look in
> color nuances. Also, it is likely that the JPEG color table will be
> recomputed, which can also cause some quality issues degradation. If
> you want to turn the brightness knob in Scribus with less quality
> degradation, you could look into Scribuses image properties, for
> example, by setting Opacity of the image to 75%.
> 
> Of course, PDF with opacity can cause its own issues when printing.
> 
> 
> With kind regards,
> 
> Bert
> 
> On Fri, Jul 10, 2020 at 12:39 PM JLuc <jluc at no-log.org> wrote:
>>
>> Le 10/07/2020 à 12:00, Bert Driehuis a écrit :
>>> If your printer is an offset outfit, chances are they are using
>>> Fogra39.
>>
>> Yes they are !
>> I could talk to a pre-press person and learned that the ICC is Fogra39.
>>
>> He said that "Offset paper increases red chanel by 2 or 3%".
>> That could contribute to darkening effect.
>>
>>> You probably don't have a cheap way to do a trial print, do you ?
>>
>> Printer proposes Cromalin or Chrome-alun.
>> It's 200km away... So maybe i'll go there and also have a splash in the closeby mediterranee.
>>
>>> Don't know an answer to your question, but using ImgeMagick to fix up
>>> jpg colors will significantly reduce image qsuchuality,
>>
>> Ah :-/
>> What sort of quality is reduced with ImageMagick ?
>> Even with that quality loss, maybe lessening that bad "dark area flattening and darkening" effect
>> would be worth some imagemagick quality loss ?
>> I could apply it selectively.
>>
>>   > so figuring out the ICC profile is your best bet.
>>
>> Until now i used "ISO Coated V2 300%" becauses this was the ICC for the previous printer.
>> Now i'll switch to FOGRA.
>> But even before, I didnt achieve perfect photo rendering except on specific photos.
>>
>>
>> JL
>>
>>
>>> On Fri, Jul 10, 2020 at 10:09 AM JLuc <jluc at no-log.org> wrote:
>>>>
>>>> Le 10/07/2020 à 09:56, JLuc a écrit :
>>>>> My new printer didnt provide an ICC profile file when asked (said it's not required afaicr).
>>>>>
>>>>> Printed photos are not too bad but are quite often too contrasted
>>>>> and dark areas appear flattened and shifted to the very dark.
>>>>> Sometimes also the light areas appear also flattened and shifted to the very light
>>>>> but most issues are with darker areas.
>>>>>
>>>>> I've asked some advices to the printer but i'm afraid i wont get usefull answer.
>>>>> (Do you sometime get any usefull answers from your printers ?)
>>>>>
>>>>> As for now i'm thinking of bash-editing all images so as to curve the dark colors lighter.
>>>>> How could i best bash achieve this on linux ?
>>>>
>>>> After trying a few parameters it looks like this command could do :
>>>> convert cheval.jpg -level -0%,100%,1.5 cheval-level0,100,1.5.jpg
>>>> My other questions remain open.
>>>>
>>>>> What should i care do to avoid worsening the print ?
>>>>
>>>> JL
>>>>
>>>>
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>>
>>
>>
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