[scribus] Lighten dark colors

Sat Jul 11 01:44:50 UTC 2020

Hi Bert & Jluc.

Pixel images with only 8-Bit depth will always be at risk of "discontinuities of hue" or "colour breaks", simply because 256 steps is a serious limitation.

If you only have an 8-Bit JPEG image of good quality from your camera and want it to be printed, I suggest you open it in GIMP and then immediately change the precision to 16 Bit and scale it 300 dpi. After that, you should save it in GIMP's native XCF format and then edit it to your likings. Note that GIMP provides a pre-press preview via its settings.

Once you've edited your image file in GIMP, you can export the XCF file as 8-Bit JPG again. The problem with your approach is that JPEG isn't a lossless format -- you'll lose image data everytime you edit and save a JPEG file as such.

If you have a RAW file, I suggest either using one of the magnificent FLOSS RAW editors, like Darktable or RawTherapee. If you're unfamiliar with RAW editing, I suggest you install Darktable alongside GIMP. That way, you can open a RAW file in GIMP via Darktable. GIMP will import this image as a 16-Bit OpenEXR file. Once you've edited the image, you can export it as an 8-Bit JPEG for print.

HTH,
Christoph

> Gesendet: Freitag, 10. Juli 2020 um 13:50 Uhr
> Von: "Bert Driehuis" <driehuis at playbeing.org>
> An: "Scribus User Mailing List" <scribus at lists.scribus.net>
> Cc: scribus at lists.scribus.info
> Betreff: Re: [scribus] Lighten dark colors
>
> 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 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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