Rotation Multiple Displays Density Control Hotplug Color Management

Display Management

Most of this section is written based on arcan using the low level egl-dri platform; the other platforms put you inside a window, so there is not much display management going on.

By default, arcan just picks the display ‘connector’ that comes first in enumeration order - or if you explicitly specify -w 320 -h 200, the first connector that has a display that can accomodate this resolution.

This can be tuned further through the environment variables shown when running ‘arcan’ without any arguments.

Most display changes can be initiated from the Global/Display/Displays menu path where there will be explicitly named monitor references, and the abstract “Current” that refers to the display that has input focus at the moment.

Rotation / Resolution

To switch your current display from horizontal to vertical orientation, simply activate the global/display/displays/current/orientation/toggle HV menu path.

Resolution switching also works in a similar way, the subdirectory global/display/displays/current/resolution will show an estimate of the resolutions that the platform currently supports (estimate because this might actually change in multimonitor settings depending on how you setup your monitors).

Multiple Displays

Each workspace is tied to a display, and each display can have up to 10 workspaces. If a display disappears, the workspace will be re-homed to another display (if possible), otherwise it will be marked as orphaned and adopted when a free slot becomes available somewhere. If the parent display appears, the original workspace migrates back to this display. This behavior can be overridden by explicitly migrating workspaces via the global/workspace/migrate or on a per-window basis with target/window/migrate.

A problem for arcan is to determine how to deal with the synchronization of screen contents transfers. One would naively think that just aligning with the VSYNC of a display would be fine, but but this is not always possible due to driver limitations and the multi-display situations where the individual display synchs do not have a common divisor and buffering is not a workable solution.

All underlying platforms currently lack access to a decent signal-fencing interface for more advanced synchronization. There are two levels of synchronization to work with. One is a “synchronization strategy” which regulates HOW the engine is supposed to prioritize display synchronization. This matters when you have more variable- rate displays like FreeSync or GSync, but also as a power management strategy on mobile devices. The other is the notion of letting a set of primary displays force-synch, and just let the others update whenever they can. Synchronization strategies are found as part of global/display/synchronization and toggle if a single display belong to the primary synch group or not through global/display/displays/current/force synch.

Display Density

Durden should automatically account for variations in display density across multiple displays. If the detection is wrong, which can happen with poor displays that provide the wrong EDID, driver bugs or an interfering KVM switch, you can override the detected density with the global/display/displays/current/density menu path. Note that the density is expressed in pixels per centimeter.

Though the internal UI should reflect density changes instantaneously, connected clients that rely on relative integral scaling factors or are entirely ignorant of output density will not be so lucky. As a band-aid solution, you can try and change the scaling behavior on a per-window basis using the target/video/scaling/ to force-scale the window contents. There are more options for this to explore in the visual section.

Hotplug

By default, there is no active hotplug action. one reason for this is that the lower level APIs do not provide a sane interface for this, but rather expects you to bind to udev, which is on the arcan blacklist. Another reason is that the platform needs to do a full rescan of GPUs and their connectors, which impose a rather heavy (hundreds of miliseconds to seconds) stall on the entire graphics pipeline, with a high chance for driver/kernel crashes if you respond to a hotplug too quickly, particularly with complex display protocols like “display port”.

If you still want automatic hotplug behavior, you can use the IPC facility to hook up the global/display/rescan path to whatever hotplug daemon you are running on your system.

Color Correction

Each display output can have a post-processing on-GPU program (“shader”) applied to it. The default output shader performs no transformation, while-as others can weight the different output shaders or apply some power funtion.

The shaders are scanned at startup, and are presented in the global/display/displays/current/shader path. The individual parameters in that shader will be accessible through the global/display/displays/current/shader weights path that appears when you have picked a shader.

If you want to write your own shader, look in the durden/shaders/display path for code templates to copy and modify.

Future Changes

  • Specify lookup texture to display shader

  • Vector- defined icons for perfect scaling in mixed DPI settings

  • With Arcan 0.5.2 / Durden 0.3, other processes can be put in charge of managing color for one or several displays. This feature is enabled per target basis through target/video/advanced/toggle color sync.