(Re)introducing screentest

1. blog

I have accidentally rewritten screentest, an old X11/GTK2 program that I was previously using to, well, test screens.

Screentest is dead

It was removed from Debian in May 2023 but had already missed two releases (Debian 11 "bullseye" and 12 "bookworm") due to release critical bugs. The stated reason for removal was:

The package is orphaned and its upstream is no longer developed. It depends on gtk2, has a low popcon and no reverse dependencies.

So I had little hope to see this program back in Debian. The git repository shows little activity, the last being two years ago. Interestingly, I do not quite remember what it was testing, but I do remember it to find dead pixels, confirm native resolution, and various pixel-peeping. Here's a screenshot of one of the screentest screens:

Now, I think it's safe to assume this program is dead and buried, and anyways I'm running wayland now, surely there's something better?

Well, no. Of course not. Someone would know about it and tell me before I go on a random coding spree in a fit of procrastination... riiight? At least, the Debconf video team didn't seem to know of any replacement. They actually suggested I just "invoke gstreamer directly" and "embrace the joy of shell scripting".

Screentest reborn

So, I naively did exactly that and wrote a horrible shell script. Then I realized the next step was to write an command line parser and monitor geometry guessing, and thought "NOPE, THIS IS WHERE THE SHELL STOPS", and rewrote the whole thing in Python.

Now, screentest lives as a ~400-line Python script, half of which is unit test data and command-line parsing.

Why screentest

Some smarty pants is going to complain and ask why the heck one would need something like that (and, well, someone already did), so maybe I can lay down a list of use case:

• testing color output, in broad terms (answering the question of "is it just me or this project really yellow?")

• testing focus and keystone ("this looks blurry, can you find a nice sharp frame in that movie to adjust focus?")

• test for native resolution and sharpness ("does this projector really support 4k for 30$? that sounds like bullcrap")

• looking for dead pixels ("i have a new monitor, i hope it's intact")

What does screentest do?

Screentest displays a series of "patterns" on screen. The list of patterns is actually hardcoded in the script, copy-pasted from this list from the videotestsrc gstreamer plugin, but you can pass any pattern supported by your gstreamer installation with --patterns. A list of patterns relevant to your installation is available with the gst-inspect-1.0 videotestsrc command.

By default, screentest goes through all patterns. Each pattern runs indefinitely until the you close the window, then the next pattern starts.

You can restrict to a subset of patterns, for example this would be a good test for dead pixels:

screentest --patterns black,white,red,green,blue

This would be a good sharpness test:

screentest --patterns pinwheel,spokes,checkers-1,checkers-2,checkers-4,checkers-8

A good generic test is the classic SMPTE color bars and is the first in the list, but you can run only that test with:

screentest --patterns smpte

(I will mention, by the way, that as a system administrator with decades of experience, it is nearly impossible to type SMPTE without first typing SMTP and re-typing it again a few times before I get it right. I fully expect this post to have numerous typos.)

Here's an example of the SMPTE pattern from Wikipedia:

For multi-monitor setups, screentest also supports specifying which output to use as a native resolution, with --output. Failing that, it will try to look at the outputs and use the first it will find. If it fails to find anything, you can specify a resolution with --resolution WIDTHxHEIGHT.

I have tried to make it go full screen by default, but stumbled a bug in Sway that crashes gst-launch. If your Wayland compositor supports it, you can possibly enable full screen with --sink waylandsink fullscreen=true. Otherwise it will create a new window that you will have to make fullscreen yourself.

For completeness, there's also an --audio flag that will emit the classic "drone", a sine wave at 440Hz at 40% volume (the audiotestsrc gstreamer plugin. And there's a --overlay-name option to show the pattern name, in case you get lost and want to start with one of them again.

How this works

Most of the work is done by gstreamer. The script merely generates a pipeline and calls gst-launch to show the output. That both limits what it can do but also makes it much easier to use than figuring out gst-launch.

There might be some additional patterns that could be useful, but I think those are better left to gstreamer. I, for example, am somewhat nostalgic of the Philips circle pattern that used to play for TV stations that were off-air in my area. But that, in my opinion, would be better added to the gstreamer plugin than into a separate thing.

The script shows which command is being ran, so it's a good introduction to gstreamer pipelines. Advanced users (and the video team) will possibly not need screentest and will design their own pipelines with their own tools.

I've previously worked with ffmpeg pipelines (in another such procrastinated coding spree, video-proxy-magic), and I found gstreamer more intuitive, even though it might be slightly less powerful.

In retrospect, I should probably have picked a new name, to avoid crashing the namespace already used by the project, which is now on GitHub. Who knows, it might come back to life after this blog post; it would not be the first time.

For now, the project lives along side the rest of my scripts collection but if there's sufficient interest, I might move it to its own git repositories. Comments, feedback, contributions are as usual welcome. And naturally, if you know something better for this kind of stuff, I'm happy to learn more about your favorite tool!

So now I have finally found something to test my projector, which will likely confirm what I've already known all along: that it's kind of a piece of crap and I need to get a proper one.