Imagine you have RPi with some target app (e.g. kiosk-mode browser) starting in X session, and want to have a full-screen splash for the whole time that device will be booting, and no console output or getty's of any kind, and no other splash screens in there - only black screen to logo to target app.
In case of average Raspberry Pi boot, there is:
A firmware "color test" splash when device gets powered-on.
Removed with disable_splash=1 in /boot/config.txt.
"Rainbow-colored square" over-current indicator popping up on the right, regardless of PSU or cables, it seems.
avoid_warnings=1 in the same config.txt
As kernel boots - Raspberry Pi logo embedded in it.
logo.nologo to /boot/cmdline.txt.
Replacing that logo with proper splash screen is not really an option, as logos that work there have to be tiny - like 80x80 pixels tiny.
Anything larger than that gives fbcon_init: disable boot-logo (boot-logo bigger than screen), so in-kernel logo isn't that useful, and it's a pain to embed it there anyway (kernel rebuild!).
Lots of console output - from kernel and init both.
cmdline.txt: console=null quiet
Getty showing its login prompt.
systemctl disable getty@tty1
More printk stuff, as various kernel modules get initialized and hardware detected.
console=null in cmdline.txt should've removed all that.
consoleblank=0 loglevel=1 rootfstype=ext4 helps if console=null is not an option, e.g. because "fbi" should set logo there (see below).
Need for "rootfstype" is kinda funny, because messages from kernel trying to mount rootfs as ext2/ext3 seem to be emergency-level or something.
Removing all the stuff above should (finally!) get a peaceful black screen, but what about the actual splash image?
fbi -d /dev/fb0 --once --noverbose\ --autozoom /path/to/image.png </dev/tty1 >/dev/tty1
Or, in early-boot-systemd terms:
[Unit] DefaultDependencies=no After=local-fs.target [Service] StandardInput=tty StandardOutput=tty ExecStart=/usr/bin/fbi -d /dev/fb0\ --once --noverbose --autozoom /path/to/image.png [Install] WantedBy=sysinit.target
"fbi" is a tool from fbida project.
console=null should NOT be in cmdline for this tool to work (see above).
First time you run it, you'll probably get:
ioctl VT_GETSTATE: Inappropriate ioctl for device (not a linux console?)
A lot of people on the internets seem to suggest something like "just run it from Alt + F1 console", which definitely isn't an option for this case, but I/O redirection to /dev/tty (as shown above) seem to work.
Blank black screen and whatever flickering on X startup.
Running X on a different VT from "fbi" seem to have nice effect that if X will have to be restarted for some reason (e.g. whole user session gets restarted due to target app's watchdog + StartLimitAction=), VT will switch back to a nice logo, not some text console.
To fix blackness in X before-and-after WM, there're tools like feh:
feh --bg-scale /path/to/image.png
That's not instant though, as X usually takes its time starting up, so see more on it below.
Target app startup cruft - e.g. browser window without anything loaded yet, or worse - something like window elements being drawn.
There can be some WM tricks to avoid showing unprepared window, including "start minimized, then maximize", switching "virtual desktops", overlay windows, transparency with compositors, etc.
Depends heavily on WM, obviously, and needs one that can be controlled from the script (which is rather common among modern standalone WMs).
Another trick is to start whole X without switching VT - i.e. X -novtswitch vt2 - and switch to that VT later when both X and app signal that they're ready, or just been given enough time.
Until switch happens, splash logo is displayed, courtesy of "fbi" tool.
On Raspberry Pi in particular, there're some direct-to-display VideoCore APIs, which allow to overlay anything on top of whatever Linux or X draw in their VTs while starting-up.
This is actually a cool thing - e.g. starting omxplayer --no-osd --no-keys /path/to/image.png.mp4 (mp4 produced from still image) early on boot (it doesn't need X or anything!) will remove the need for most previous steps, as it will eclipse all the other video output.
"omxplayer" maybe isn't the best tool for the job, as it's not really meant to display still images, but it's fast and liteweight enough.
Better alternative I've found is to use OpenVG API via openvg lib, which has nice Go (golang) version, and wrote an overlay-image.go tool to utilize it for this simple "display image and hang forever" (to be stopped when boot finishes) purpose.
Aforementioned Go tool has "-resize" flag to scale the image to current display size with "convert" and cache it with ".cache-WxH" suffix, and "-bg-color" option to set margins' color otherwise (for e.g. logo centered with solid color around it). Can be built (be sure to set $GOPATH first) with: go get github.com/ajstarks/openvg && go build .
Finally some destination state with target app showing what it's supposed to.
Yay, we got here!
Not a very comprehensive or coherent guide, but might be useful to sweep all the RPi nasties under an exquisite and colorful rug ;)
Update 2015-11-30: Added link to overlay-image.go tool.
Update 2015-11-30: A bit different version (cleaned-up, with build-dep on "github.com/disintegration/gift" instead of optional call to "convert") of this tool has been added to openvg lib repo under "go-client/splash".