Create rofi-thumbnails.5.markdown

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+# rofi-thumbnails(5)
+
+## NAME
+
+**rofi-thumbnails** - Rofi thumbnails system
+
+## DESCRIPTION
+
+rofi is now able to show thumbnails for all file types where an XDG compatible thumbnailer is present in the system.
+This is done by default in filebrowser and recursivebrowser mode, if rofi is launched with the `-show-icons` argument.
+In a custom user script or dmenu mode, it is possible to produce entry icons using XDG thumbnailers by adding the prefix `thumbnail://` to the filename
+specified after `\0icon\x1f`, for example:
+```bash
+    echo -en "EntryName\0icon\x1fthumbnail://path/to/file\n" | rofi -dmenu -show-icons
+```
+
+### XDG thumbnailers
+
+XDG thumbnailers are files with a ".thumbnailer" suffix and a structure similar to ".desktop" files for launching applications. They are placed in `/usr/share/thumbnailers/` or `$HOME/.local/share/thumbnailers/`, and contain a list of mimetypes, for which is possible to produce the thumbnail image, and a string with the command to create said image. The example below shows the content of `ffmpegthumbnailer.thumbnailer`, a thumbnailer for video files using ffmpeg:
+```
+[Thumbnailer Entry]
+TryExec=ffmpegthumbnailer
+Exec=ffmpegthumbnailer -i %i -o %o -s %s -f
+MimeType=video/jpeg;video/mp4;video/mpeg;video/quicktime;video/x-ms-asf;video/x-ms-wm;video/x-ms-wmv;video/x-ms-asx;video/x-ms-wmx;video/x-ms-wvx;video/x-msvideo;video/x-flv;video/x-matroska;application/mxf;video/3gp;video/3gpp;video/dv;video/divx;video/fli;video/flv;video/mp2t;video/mp4v-es;video/msvideo;video/ogg;video/vivo;video/vnd.divx;video/vnd.mpegurl;video/vnd.rn-realvideo;application/vnd.rn-realmedia;video/vnd.vivo;video/webm;video/x-anim;video/x-avi;video/x-flc;video/x-fli;video/x-flic;video/x-m4v;video/x-mpeg;video/x-mpeg2;video/x-nsv;video/x-ogm+ogg;video/x-theora+ogg
+```
+The images produced are named as the md5sum of the input files and placed, depending on their size, in the XDG thumbnails directories: `$HOME/.cache/thumbnails/{normal,large,x-large,xx-large}`. They are then loaded by rofi as entry icons and can also be used by file managers like Thunar, Caja or KDE Dolphin to show their thumbnails. Additionally, if a thumbnail for a file is found in the thumbnails directories (produced previously by rofi or a file manager), rofi will load it instead of calling the thumbnailer.  
+If a suitable thumbnailer for a given file is not found, rofi will try to use the corresponding mimetype icon from the icon theme. 
+
+### Custom command to create thumbnails
+
+It is possible to use a custom command to generate thumbnails for generic entry names, for example a script that downloads an icon given its url or selects different icons depending on the input. This can be done providing the `-preview-cmd` argument followed by a string with the command to execute, with the following syntax:
+```
+    rofi ... -preview-cmd 'path/to/script_or_cmd "{input}" "{output}" "{size}"'
+```
+rofi will call the script or command substituting `{input}` with the input entry icon name (the string after `\0icon\x1fthumbnail://`), `{output}` with the output filename of the thumbnail and `{size}` with the requested thumbnail size. The script or command is responsible of producing a thumbnail image (if possible respecting the requested size) and saving it in the given `{output}` filename.