DESK-MX6-L-AN-0004: Capturing framebuffer screenshots
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Contents
History[edit | edit source]
| Date | Development Kit version |
|---|---|
| August 2023 | DESK-MX6L-L 4.0.1 |
Introduction[edit | edit source]
In certain situations, it is necessary and/or convenient to grab a screenshot of the full display from the framebuffer directly, for example for debugging purposes or documentation. By using the fbgrab (framebuffer device grabber), this job can be accomplished straightforwardly. This Application Note (AN) shows how to use it on a system running a DESK-MX6-L-based Yocto Linux distribution.
Installing fbgrab[edit | edit source]
Assuming that root file system provides the dnf tool and it is set up properly, the fbgrab package can be installed as follows:
root@desk-mx6:~# dnf install fbgrab desk-mx6l-cortexa9hf 379 kB/s | 76 kB 00:00 desk-mx6l-cortexa9t2hf_neon 5.1 MB/s | 7.3 MB 00:01 desk-mx6l-cortexa9t2hf_neon_mx6qdl 2.0 MB/s | 675 kB 00:00 desk-mx6l-noarch 1.2 MB/s | 275 kB 00:00 Dependencies resolved. ======================================================================================================================================================================= Package Architecture Version Repository Size ======================================================================================================================================================================= Installing: fbgrab cortexa9t2hf_neon 1.5-r0 cortexa9t2hf_neon 12 k Transaction Summary ======================================================================================================================================================================= Install 1 Package Total download size: 12 k Installed size: 13 k Is this ok [y/N]: y Downloading Packages: fbgrab-1.5-r0.cortexa9t2hf_neon.rpm 191 kB/s | 12 kB 00:00 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- Total 165 kB/s | 12 kB 00:00 Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running transaction Preparing : 1/1 Installing : fbgrab-1.5-r0.cortexa9t2hf_neon 1/1 Verifying : fbgrab-1.5-r0.cortexa9t2hf_neon 1/1 Installed: fbgrab-1.5-r0.cortexa9t2hf_neon Complete! root@desk-mx6:~#
Capturing screenshots[edit | edit source]
The following examples refer to a system using the 800x480 16bpp framebuffer, as in the SBC AXEL Evaluation Kit, which is mapped to /dev/fb0.
When the display shows the image to be captured, to grab a screenshot and save it to a PNG file, issue the following command:
root@desk-mx6:~# fbgrab -d /dev/fb0 screen.png Resolution: 800x480 depth 16 Converting image from 16 Now writing PNG file (compression -1) root@desk-mx6:~#
It is also worth remembering that, if the screen parameters such as resolution or pixel depth are unknown, the fbset tool can be used to display them:
root@desk-mx6:~# fbset
mode "800x480-59"
# D: 33.260 MHz, H: 31.201 kHz, V: 59.205 Hz
geometry 800 480 800 480 16
timings 30066 128 128 15 30 10 2
accel false
rgba 5/11,6/5,5/0,0/0
endmode
root@desk-mx6:~#
The numbers following geometry indicate the resolution (800x480) and the bits required to represent a pixel (16). The last line tells the pixel format (RGBA). In this case, the RGB565 format (reg, green, blue, and alpha) resulting in 16 bits combined.
Displaying the capture screenshot[edit | edit source]
The capture PNG screenshot can now be displayed:
root@desk-mx6:~# ls -la total 28 drwx------ 2 root root 4096 Aug 25 09:38 . drwxr-xr-x 3 root root 4096 Mar 9 2018 .. -rw------- 1 root root 976 Aug 25 09:35 .viminfo -rw-r--r-- 1 root root 15698 Aug 25 09:38 screen.png root@desk-mx6:~#
For this purposes, the fbi utility - already installed in the root file system - can be used:
root@desk-mx6:~# fbi --help
This program displays images using the Linux framebuffer device.
Supported formats: PhotoCD, jpeg, ppm, gif, tiff, xwd, bmp, png,
webp. It tries to use ImageMagick's convert for unknown file formats.
usage: fbi [ options ] file1 file2 ... fileN
-h -help print this help text
-V -version print fbi version number
-i -info print device info
-store write cmd line args to config file
-l -list <arg> read image filelist from file <arg>
-P -text switch into text reading mode
-a -autozoom automagically pick useful zoom factor
-(no)autoup like the above, but upscale only
-(no)autodown like the above, but downscale only
-w -(no)fitwidth use width only for autoscaling
-v -(no)verbose show filenames all the time
-u -(no)random show files in a random order
-1 -(no)once don't loop (for use with -t)
-(no)comments display image comments
-e -(no)edit enable editing commands (see man page)
-(no)backup create backup files when editing
-(no)preserve preserve timestamps when editing
-(no)readahead read ahead images into cache
-cachemem <arg> image cache size in megabytes
-blend <arg> image blend time in miliseconds
-T -vt <arg> start on virtual console <arg>
-s -scroll <arg> scroll image by <arg> pixels
-t -timeout <arg> load next image after <arg> sec without user input
-r -resolution <arg> pick PhotoCD resolution (1..5)
-f -font <arg> use font <arg> (anything fontconfig accepts)
-d -device <arg> use framebuffer/drm device <arg>
-o -output <arg> use drm output <arg> (try -info for a list)
-(no)interactive allow keyboard interaction
-m -mode <arg> use video mode <arg> (from /etc/fb.modes)
Large images can be scrolled using the cursor keys. Zoom in/out
works with '+' and '-'. Use ESC or 'q' to quit. Space and PgDn
show the next, PgUp shows the previous image. Jumping to a image
works with <i>g. Return acts like Space but additionally prints
prints the filename of the currently displayed image to stdout.
root@desk-mx6:~#
As a first step, before displaying the PNG image, clean the framebuffer using dd:
root@desk-mx6:~# dd if=/dev/zero of=/dev/fb0 dd: writing to '/dev/fb0': No space left on device 4097+0 records in 4096+0 records out 2097152 bytes (2.1 MB, 2.0 MiB) copied, 0.022475 s, 93.3 MB/s root@desk-mx6:~#
then, display the PNG screenshot:
root@desk-mx6:~# fbi --noverbose -T 1 screen.png using "DejaVu Sans Mono-16", pixelsize=16.67 file=/usr/share/fonts/truetype/DejaVuSansMono.ttf trying drm: /dev/dri/card0 ... drm: no dumb buffer support root@desk-mx6:~#
Please note the --noverbose parameter avoiding to display the PNG file info (in case of the pure image should be displayed).
