XELK-AN-010: Capturing framebuffer screenshots
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 This application note has been validated using the kit version in the History table. |
History[edit | edit source]
| Version | Date | Development Kit version |
|---|---|---|
| 1.0.0 | February 2021 | XELK 3.0.1 |
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 well-known ImageMagick suite, this job can be accomplished straightforwardly. This Application Note (AN) shows how to use it on a system running a XELK-based Yocto Linux distribution.
Installing ImageMagick convert[edit | edit source]
Assuming that root file system provides the smart tool and it is set up properly, the ImageMagick package can be installed as follows:
root@sznpc:~# smart install imagemagick
Loading cache...
Updating cache... ###################################################################################### [100%]
Computing transaction...
Installing packages (11):
imagemagick-6.9.2-r0@cortexa9hf_neon libice6-1:1.0.9-r0@cortexa9hf_neon libxau6-1:1.0.8-r0@cortexa9hf_neon libxext6-1:1.3.3-r0@cortexa9hf_neon
lcms-2.7-r0@cortexa9hf_neon libsm6-1:1.2.2-r0@cortexa9hf_neon libxcb1-1.11.1-r0@cortexa9hf_neon libxt6-1:1.1.5-r0@cortexa9hf_neon
libfftw-3.3.4-r0@cortexa9hf_neon libx11-6-1:1.6.3-r0@cortexa9hf_neon libxdmcp6-1:1.1.2-r0@cortexa9hf_neon
3.8MB of package files are needed. 8.7MB will be used.
Confirm changes? (Y/n): y
[ 410.232039] rtc-pcf2127-i2c 1-0051: low voltage detected, check/replace RTC battery.
Fetching packages...
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libxcb1-1.11.1-r0.cortexa9hf_neon.rpm
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libice6-1.0.9-r0.cortexa9hf_neon.rpm
libice6-1.0.9-r0.cortexa9hf_neon.rpm ###################################################################################### [ 9%]
libxcb1-1.11.1-r0.cortexa9hf_neon.rpm ###################################################################################### [ 18%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/lcms-2.7-r0.cortexa9hf_neon.rpm
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libsm6-1.2.2-r0.cortexa9hf_neon.rpm
libsm6-1.2.2-r0.cortexa9hf_neon.rpm ###################################################################################### [ 27%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libxt6-1.1.5-r0.cortexa9hf_neon.rpm
libxt6-1.1.5-r0.cortexa9hf_neon.rpm ###################################################################################### [ 36%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libxext6-1.3.3-r0.cortexa9hf_neon.rpm
libxext6-1.3.3-r0.cortexa9hf_neon.rpm ###################################################################################### [ 45%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libxdmcp6-1.1.2-r0.cortexa9hf_neon.rpm
libxdmcp6-1.1.2-r0.cortexa9hf_neon.rpm ###################################################################################### [ 54%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libxau6-1.0.8-r0.cortexa9hf_neon.rpm
libxau6-1.0.8-r0.cortexa9hf_neon.rpm ###################################################################################### [ 63%]
lcms-2.7-r0.cortexa9hf_neon.rpm ###################################################################################### [ 72%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libx11-6-1.6.3-r0.cortexa9hf_neon.rpm
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/libfftw-3.3.4-r0.cortexa9hf_neon.rpm
libfftw-3.3.4-r0.cortexa9hf_neon.rpm ###################################################################################### [ 81%]
libx11-6-1.6.3-r0.cortexa9hf_neon.rpm ###################################################################################### [ 90%]
-> http://yocto.dave.eu/xelk-3.0.0/cortexa9hf_neon/imagemagick-6.9.2-r0.cortexa9hf_neon.rpm
imagemagick-6.9.2-r0.cortexa9hf_neon.rpm ###################################################################################### [100%]
Committing transaction...
Preparing... ###################################################################################### [ 0%]
1:Installing libice6 ###################################################################################### [ 9%]
2:Installing lcms ###################################################################################### [ 18%]
3:Installing libxdmcp6 ###################################################################################### [ 27%]
4:Installing libfftw ###################################################################################### [ 36%]
5:Installing libxau6 ###################################################################################### [ 45%]
6:Installing libsm6 ###################################################################################### [ 54%]
7:Installing libxcb1 ###################################################################################### [ 63%]
8:Installing libx11-6 ###################################################################################### [ 72%]
9:Installing libxt6 ###################################################################################### [ 81%]
10:Installing libxext6 ###################################################################################### [ 90%]
11:Installing imagemagick ###################################################################################### [100%]
Output from imagemagick-6.9.2-r0@cortexa9hf_neon:
update-alternatives: Linking /usr/bin/animate to /usr/bin/animate.im6
update-alternatives: Linking /usr/bin/compare to /usr/bin/compare.im6
update-alternatives: Linking /usr/bin/composite to /usr/bin/composite.im6
update-alternatives: Linking /usr/bin/conjure to /usr/bin/conjure.im6
update-alternatives: Linking /usr/bin/convert to /usr/bin/convert.im6
update-alternatives: Linking /usr/bin/display to /usr/bin/display.im6
update-alternatives: Linking /usr/bin/identify to /usr/bin/identify.im6
update-alternatives: Linking /usr/bin/import to /usr/bin/import.im6
update-alternatives: Linking /usr/bin/mogrify to /usr/bin/mogrify.im6
update-alternatives: Linking /usr/bin/montage to /usr/bin/montage.im6
update-alternatives: Linking /usr/bin/stream to /usr/bin/stream.im6
Capturing screenshots[edit | edit source]
The following examples refer to a system using a 1024x600 24bpp framebuffer, which is mapped to /dev/fb0.
When the display shows the image to be captured, to grab a "raw" screenshot and save it to file, issue the following command:
root@sznpc:~# dd if=/dev/fb0 of=screenshot.raw 4800+0 records in 4800+0 records out
Then convert the raw data to a PNG image file by running the following command:
root@sznpc:~# convert -size 1024x600 -depth 8 rgba:screenshot.raw screen0.png
Alternatively, if the convert utility can not be installed onto the target's root filesystem, the raw data file screenshot.raw can be copied to a Linux host where convert is available.
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@sznpc:~# fbset
mode "1024x600-67"
# D: 51.201 MHz, H: 43.538 kHz, V: 67.085 Hz
geometry 1024 600 1024 600 32
timings 19531 104 43 24 20 5 5
accel false
rgba 8/16,8/8,8/0,8/24
endmode
The numbers following geometry indicate the resolution (1024x600) and the bits required to represent a pixel (32). The last line tells the pixel format (RGBA). In this case, 8 bits are used for each channel (reg, green, blue, and alpha) resulting in 32 bits combined. That is why the depth parameter of convert was set to 8 and the format was set to rgba.
