Color management Gtk4

Introduction

Inkscape has supported color management for some time using ICC color profiles. ICC color profiles are used three places in Inkscape:

1. For specifying a particular color (see Fill and Stroke dialog). The profile and the color value are stored in the SVG file (see the SPObject based ColorProfile class).
2. For color adjustment of monitor(s).
3. For pre-flighting output.

The latter two uses are controlled by settings in the "Color management" subsection of the Input/Output section of the Preferences dialog.

The current code has some problems, particularly, the monitor adjustment code. These problems include

• Only one profile can be specified even if there are multiple displays that need different corrections.
• Retrieving a profile from a display doesn't appear to work.
• The code for retrieving display profiles relies on X11 functionality that is missing under Wayland or other windowing systems.
• The code was written for Gtk2 which defined a Gdk screen per monitor. In Gtk3, there is only one screen that can have multiple monitors. Gtk4 removes Gdk screen completely.
• Only one color transform can be specified at a time due to the use of global variables.
• The code is quite messy, based on GObject, and is poorly documented.
• The drawing doesn't always update properly.

For per monitor color correction to work under X11, a ICC profile must be attached to each monitor, verify by:

  xprop -display :0.0 -len 14 -root _ICC_PROFILE

Spec: https://www.burtonini.com/computing/x-icc-profiles-spec-latest.html

Darktable has a color management test: darktable-cmstest

A redesigned system

Each canvas should have its own color transform. This color transform should be updated when:

• The user changes a preference.
• The canvas is moved to a different monitor.

The color transform is defined as a cmsHTRANSFORM (void *), this is a pointer to a data structure defined by lcms2. We need to track it, not only to avoid recreating it but also to delete it when replaced or no longer needed.

A test program for examining monitor information can be found at: https://gitlab.com/Tavmjong/gtk_sandbox/-/tree/master/gtk_monitor