[Scribus] getting started with colour management

Hal V. Engel hvengel
Sun Oct 28 01:28:37 CEST 2007

On Saturday 27 October 2007 15:46:09 Craig Ringer wrote:

> In general, avoid altering the contrast or sharpness settings of LCD
> displays. It's not necessary and rarely useful.

For CRTs the "contrast" control is really a gain control and it actually 
increases/decreases the luminosity or white level of the display.  That is 
this sets how bright the whites are on the display.  For an LCD display 
luminosity is controlled by the "brightness" control.  On a CRT 
the "brightness" control actually sets the displays black level (IE. how 
bright or dark the darkest tones will be).  On an LCD you normally do not 
have a way to set the black level since this is a function of the white 
level.   That is on an LCD the ratio between the white level and black level 
is always the same.

Most LCD displays have a max white level of at least 200 Cd/m^2 with some 
going as high as 500 Cd/m^2.   Most fall into the 250 Cd/m^2 to 300 Cd/m^2 
range.  Where as CRTs are typically only capable of 120 Cd/m^2 or less when 
new.  Many older CRTs max out at 80 Cd/m^2 or less.  For working in a 
darkened work environment like the sRGB standard work environment a white 
level of around 90 Cd/m^2 is about right.  As you can see turning the CRT 
contrast all the way up will get the white level about where it should be for 
this type of work and that is why the standard advice is to turn the contrast 
all the way up.  But this is only valid for CRTs.

> > Another thing I've never understood is why, with colour management
> > turned off, graphics have been looking quite different in
> > Gimp 2.2 as compared with Scribus 1.3.  For example, I have some
> > TIFF files with a red that shows very bright in Gimp but
> > considerably darker in Scribus.  I'm *guessing* that Gimp and
> > Scribus have been implicitly defaulting to quite different
> > colour profiles, but I don't even know whether that's a sensible
> > way of phrasing it.
> Assuming it's an RGB TIFF, with colour management turned off, both
> should be sending the exact same pixel values to the window system, and
> through to the display. Are you *SURE* colour management is off in Scribus?
> If the TIFF is not RGB, all bets are off. There is no meaningful
> transformation from CMYK to RGB without some information about what
> those colours *mean* on the source and target devices. Applications must
> use horrible quick and dirty fixed transforms, and they generally work
> very badly. Scribus and the GIMP probably use different ones. They're
> both bad, and really can't be anything else. The only app I've ever seen
> do this vaguely sanely was early versions of Photoshop, which basically
> hard coded a non-linear CMYK <> RGB translation table for a small set of
> presses. In essence, it hard coded profiles and conversion routines for
> sRGB <> SWOP etc. It was bad, but not as bad as what the GIMP and
> Scribus use when no colour management engine is available.
> > I've got the vague idea that most software assumes the monitor
> > profile would be some flavour of sRGB, with *which* flavour
> > being application dependent.
> Nope. Without colour management it assumes nothing at all - it just
> sends raw RGB values through, and murders CMYK colours horribly. Avoid
> at all costs.
> > If I knew what Scribus assumes,
> > then I'd know how to choose a "no-op" monitor profile matching
> > the default assumption.
> There really isn't any such thing. The closest to a "no op" profile
> setup is where your inputs are in the same colour space as your monitor.
> This is often the case if you've told your apps that your monitor is
> sRGB, and your images are tagged as sRGB. You should in this case see
> on-screen results practically the same as if colour management is
> disabled. Of course, anything involving CMYK works differently, since
> Scribus will use the CMS engine to transform the colours according to
> the profile rather than using a fixed (terrible) transform function.
> Even with an inaccurate generic display and printer profile, you'll
> still get better results from Scribus using colour management than
> without it. You won't get on-screen images looking like they'll print,
> but you'll get a better printed result and a closer approximation of
> CMYK colours on screen.
> --
> Craig Ringer
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