lsix

Like "ls", but for images. Shows thumbnails in terminal using sixel graphics.

Usage

lsix [ FILES ... ]

Examples

Basic Usage

Just typing lsix will show images in the current working directory. You can also specify filenames and, of course, use shell wild cards (e.g., lsix *jpg *png).

Because lsix uses ImageMagick pretty much any image format will be supported. However, some may be slow to render (like PDF), so lsix doesn't show them unless you ask specifically. If you want to force a listing of a certain type of image simply specify the filenames or use a wildcard (*.pdf in the example below),.

Expanding GIFs

If you specify a GIF (or actually any file that has multiple images in it) on the command line, all the frames will get expanded and shown in a montage. For example, lsix nyancat.gif shows all the frames. Note that GIF stores some frames as only the pixels that differ from the previous frame.

Terminal background color is detected

You may have noticed that PNGs and SVG files have correct alpha channel for the terminal background. That is because lsix uses terminal escape sequences to try to figure out your foreground and background colors. (Foreground is used for the text fill color.)

In the first example below, after running lsix in a white on black xterm, I sent an escape sequence to swap foreground and background colors. When I ran it again, lsix detected it and changed the background color to white. Of course, you can pick whatever default colors you want (e.g., xterm -bg blue, in the second example below).

Features

• Detects if your terminal can display SIXEL graphics inline using control sequences.

• Works great over ssh. Perfect for manipulating those images on the web server when you can't quite remember what each one was.

• Non-bitmap graphics often work fine (.svg, .eps, .pdf, .xcf).

• Automatically detects if your terminal, like xterm, can increase the number of color registers to improve the image quality and does so.

• Automatically detects terminal's foreground and background colors.

• In terminals that support dtterm WindowOps, the number of tiles per row will adjust appropriately to the window width.

• If there are many images in a directory (>21), lsix will display them one row at a time so you don't need to wait for the entire montage to be created.

• If your filenames are too long, lsix will wrap the text before passing it into ImageMagick's montage. (Without lsix, montage just jumbles long filenames on top of one another.)

• You can easily change things like the width of each tile in the montage, the font family, and point size by editing simple variables at the top of the file. (Tip: try convert -list font to see what fonts you have on your machine.)

• Unicode filenames work fine, as long as your font has the glyphs.

Installation

Just put the lsix file in your path (e.g., /usr/local/bin) and run it. It's just a BASH shell script.

The only prerequisite software is ImageMagick. If you don't have it yet, your OS's package manager will make it easy to get. (E.g., apt-get install imagemagick).

MacOS users may prefer to install lsix using brew install lsix which installs ImageMagick, if necessary.

Your Terminal must support Sixel graphics

I developed this using xterm in vt340 emulation mode, but I believe this should work on any Sixel compatible terminal. You may test your terminal by viewing a single image, like so:

convert  foo.jpg  -geometry 800x480  sixel:- 

XTerm

Note that xterm does not have Sixel mode enabled by default, so you need to either run it like so:

xterm -ti vt340

Or, make vt340 the default terminal type for xterm. Add the following to your .Xresources file and run xrdb -merge .Xresources.

! Allow sixel graphics. (Try: "convert -colors 16 foo.jpg sixel:-").
xterm*decTerminalID :   vt340

Further, some distributions, such as Fedora, appear to not compile xterm with sixel support. In that case, try an alternate terminal, such as foot or mlterm.

SIXEL compatible terminals

• XTerm (tested)
• MLterm (tested)
• foot (tested)
• Wezterm (tested)
• Contour (tested)
• iTerm2 for Apple MacOS (tested)
• Konsole (reported)
• yakuake (reported)
• WSLtty for Microsoft Windows (reported)
• MinTTY for Cygwin (Microsoft Windows) (reported)
• Yaft for Linux framebuffer (tested)
• VTE (special compilation, reported)
• sixel-tmux (fork of tmux, reported)
• ttyd (reported)

SIXEL incompatible terminals

• MacOS Terminal, kitty
• All standard libvte based terminals
  • gnome-terminal
  • terminator
  • lxterm
• Alacritty (might work with a patch)

Configuration

Because lsix is currently designed to be very simple, there are no command line flags, no configuration files, no knobs to twiddle, or frobs to frobnosticate. However, since the script is so simple, if you want to make a change, it's pretty easy to do just by editing the file. Everything is nicely commented with the most common default variables at the top.

Contact the author

I welcome feedback. If you use lsix and like it or have suggestions for how it can be improved, please go ahead and send your thoughts to me @hackerb9 via GitHub.

Bugs

• XTerm's reverse video mode (xterm -rv) is different from specifying the foreground and background explicitly. There is a way to detect the latter, but not the former. That means the background color will be incorrect for folks who use XTerm's reverseVideo resource. (See issue #20).

• XTerm's screen width is currently limited to 1000px due to a misfeature which causes it to silently show nothing. This limitation will be removed once xterm can handle images greater than 1000x1000. [Last tested with XTerm(344)].

• Filenames that begin with "@" are special to ImageMagick and it'll freak out if you don't prepend a directory. (lsix ./@foo.png) (This is a bug in ImageMagick, not lsix).

• Specifying the empty string "" as a filename makes ImageMagick hang. (This appears to be an ImageMagick bug / misfeature).

• Long filenames are wrapped, but not intelligently. Would it complicate this script too much to make it prefer to wrap on whites space, dashes, underscores, and periods? Maybe.

• Directories specified on the command line are processed as if the user had cd'd to that directory. It wouldn't be hard to implement recursion, but is there actually a need? I'm reluctant to complicate such a simple script with command line flags.

• If you run lsix foo.avi, you're asking for trouble.

Future Issues

• The Sixel standard doesn't appear to have a way to query the size of the graphics screen. Reading the VT340 documentation, it appears your program has to already know the resolution of the device you're rendering on.

  XTerm, as of version 344, has added a control sequence that solves the problem — CSI ? Pi ; Pa ; Pv S — but some terminals, for example mlterm, haven't yet implemented it.

  There is an alternate way to read the window size using the dtterm WindowOps extension but it is not quite the right solution as the geometry of the Sixel graphics screen is not necessarily the same as the window size. (For example, xterm limits the graphics geometry to 1000x1000, even though the window can actually be larger.) To help with terminals such as mlterm, lsix will use the dtterm WindowOps as a fallback.

  If neither solution works, lsix will assume you are on a VT340 (800x480) and can fit only 6 tiles per row.

• The Sixel standard also lacks a way to query the number of color registers available. I used the extensions from xterm to do so, but I do not know how widely implemented they are. If a terminal does not respond, lsix presumes you're on an original vt340 and uses only 16 color registers. (Sorry, 4-gray vt330 users! Time to upgrade. ;-) )

• The Kermit project created a MS-DOS terminal emulator that was popular in the late 1980s/early 1990s. Its sixel implementation is not compatible with lsix because it shows the graphics on a screen separate from the text. However, I noticed one feature in its documentation: an escape sequence to request the current graphics window size and number of colors:

 ESC [ ? 256 n                  Request screen size report

        Report is ESC [ ? 256; Ph; Pw; Pc n     for graphics systems

        where   Ph is screen height in dots
                Pw is screen width in dots
                Pc is number of colors (0, 1 or 16, for none, b/w, ega/vga)

        Report is ESC [ ? 24; 80; 0 n  for pure text mono systems.

Did any other terminal emulators ever use the sequence? Would it be worthwhile to add to lsix?

• libsixel is an excellent project for writing programs that can output optimized Sixel graphics commands. Because I have a lot of respect for the project, I feel I should explain why lsix does not use libsixel.

  • (a) I wanted lsix to work everywhere easily. Bash and imagemagick are ubiquitous, so a shell script is a natural solution.

  • (b) I wanted lsix to be simple enough that it could be easily customized and extended by other people. (Including myself.)

  • (c) ImageMagick has better support for reading different formats than stb_image (the library used by libsixel's img2sixel). (For example: xpm, svg, 16-bit png, and even sixel files are not recognized by img2sixel). Since ImageMagick can read all of those and write sixel output directly, it made sense to use it for both.

  • (d) While libsixel is optimized and would surely be faster than ImageMagick, it's overkill. For a simple directory listing, this is plenty fast enough.

Resources

• XTerm Control Sequences

• ImageMagick

• VT340 Programmer's Reference:

  • Chapter 14. Sixel Graphics.

  • Chapter 16 Difference between Level 1 and Level 2 Sixel implementations.

    Nota bene: this reference has the sense for DECSDM (sixel display mode) reversed! The actual behaviour of the VT340 is that when DECSDM is reset (the default), sixel scrolling is enabled. This can be done by sending Esc[?80l, but lsix does not do so as it would break many current terminal emulators. See issue #41 for details.

• DEC STD 070 Video Systems Reference Manual. A weighty tome which covers nearly everything in exacting detail. I referred mostly to sections 4 (escape sequences) and 9 (sixel programming).

• VT340 Test, a project to document the actual behaviour of the DEC VT340 hardware.

• Digital ANSI-Compliant Printing Protocol: Level 2 Programming Reference Manual, Chapter 5: Sixel Graphics. An excellent and reasonably clear discussion for anyone who wants to generate or parse sixel graphics.